when did the mammal fauna of the british isles arrive?

Mammal Review Volume 12. Number 1 March 1982

When did the mammal fauna of the British Isles arrive? 1 D . W . YALDEN

Department of Zoology. The University of Manchester. Manchester MI 3 9PL

CONTENTS

Introduction . . . . . . . Terminology . . . . . . . Linesofevidence . . . . . .

(i) Carbon 14dating . . . . (ii) Pollen . . . . . . (iii) Beetle faunas . . . . (iv) Mollusc faunas . . . . (v) Climaticevidence . . . (vi) Sea levels . . . . .

History ofthemammals . . . . (i) Fossil and sub-fossil records . (ii) Problems of distribution . . (iii) Genetic studies . . . .

Aworkinghypothesis . . . . . Outstanding problems . . . .

Awkward fossil occurrences . . The Lusitanian element . . .

Furtherprogress . . . . . . Archaeological evidence . . . Geneticstudies . . . . . Geological problems . . . .

Summary . . . . . . . . Acknowledgments . . . . . References . . . . . . . Postscript . . . . . . . .

Page . . . . . . . . . . . . . . . . . . . 1 . . . . . . . . . . . . . . . . . . . 4 . . . . . . . . . . . . . . . . . . . 5 . . . . . . . . . . . . . . . . . . . 5 . . . . . . . . . . . . . . . . . . . 5 . . . . . . . . . . . . . . . . . . . 10 . . . . . . . . . . . . . . . . . . . 10 . . . . . . . . . . . . . . . . . . . 12 . . . . . . . . . . . . . . . . . . . 15 . . . . . . . . . . . . . . . . . . . 22 . . . . . . . . . . . . . . . . . . . 23 . . . . . . . . . . . . . . . . . . . 30 . . . . . . . . . . . . . . . . . . . 32 . . . . . . . . . . . . . . . . . . . 39 . . . . . . . . . . . . . . . . . . . 42 . . . . . . . . . . . . . . . . . . . 42 . . . . . . . . . . . . . . . . . . . 45 . . . . . . . . . . . . . . . . . . . 49 . . . . . . . . . . . . . . . . . . . 49 . . . . . . . . . . . . . . . . . . . 50 . . . . . . . . . . . . . . . . . . . 51 . . . . . . . . . . . . . . . . . . . 51 . . . . . . . . . . . . . . . . . . . 52 . . . . . . . . . . . . . . . . . . . 52 . . . . . . . . . . . . . . . . . . . 56

INTRODUCTION The origin of the present fauna of the British Isles (a term which I take to be a geographical rather than a political entity. to include Ireland and Man). has been the subject of innumer- able papers and at least two influential books . While the evidence used has come from many groups of animals and. indeed. from plants. the mammals have been an important group in speculation about the presence and distribution of our fauna . The numerous island races. especially of the Wood mouse Apodemus sylvaticus. and the erratic distribution of some species and subspecies (for example. Microtus arualis. the common Field vole in Europe. occurs only in Orkney and on Guernsey). provided a fertile source for speculation .

Impressed by the geological evidence for the Ice Ages and by the apparent differentiation of many island forms (accorded the status. in many cases. of full species). Hinton elaborated

0305-1838/82/0300-0001 802.00 0 1982 Blackwell Scientific Publications

2 D. W. Yalden

a theory in which early immigrants to the British Isles were isolated on various islands by the advance of the ice sheets, surviving in glacial refugia, and were replaced in the main islands, as the ice subsequently retreated, by new forms immigrating from Europe. Fossil forms from various Pleistocene sites were regarded as evidence for the former widespread Occurrence of the forms which became glacial relicts (Hinton, 1910; Barrett-Hamilton & Hinton, 1913; Hinton, 1926). Hinton was a mono-glacialist, believing that there was only one glacial period, but as the evidence for a sequence of glacial and interglacial periods over the last million or so years became overwhelming, more complicated explanations of mammal distributions became possible, and were provided by Beirne (1947, 1952). For example, the hare populations of the British Isles were regarded by him as the result of three separate invasions; an earlier one by hares ancestral to the Irish hare Lepus timidus hibernicus, one by ancestors of Scottish mountain hares Lepus timidus scoticus, and the latest one of Brown hares Lepus europaeus (L . capensis).

Matthews (1952) repeated Beirne's hypotheses, though he expressed considerable reservations about their validity; the severity of the last glaciation was by then more clearly established, the possibility of accidental human introduction of some (at least) island forms had been canvassed, and the degree of distinctiveness of the island forms was being questioned. Matthews presented the arguments in favour of different explanations very fairly, but did not present any synthesis of the most likely course of events himself.

A major clarification of the likely course of events, and a major theoretical advance, came from the clear advocacy by Corbet (1961) of the role of accidental human introduction of at least the island forms of small mammals. Corbet pointed out that, on geological evidence, the survival of island forms or their ancestors through the last glaciation was extremely unlikely, and the depth of the channels separating various islands (particularly Orkney, Shetland and St Kilda) was so great that they were never likely to have been joined to the mainland in late- or post-Pleistocene times. He also pointed out that the characters which were supposed to distinguish the island forms had been somewhat exaggerated; for the most part, they just represented the extremes of variation present in mainland populations. Finally he pointed out that if the island forms were the result of natural recolonization then the Field vole Microtus agrestis, which occurs furthest north in Scandinavia (to 70"N), should Occur on most of the islands; the Bank vole Clethrionomys glareolus, which extends north to the Arctic Circle, should be next most widespread, and the Wood mouse Apodemus sylvaticus, which only occurs in southern Scandinavia up to 61"N, should be least widespread. In fact, the Wood mouse has the most widespread distribution, occurring in Shetland, in Ireland and, indeed, in Iceland, while the Bank vole has the most restricted island distribution. In the face of these arguments, Corbet concluded that many island populations of small mammals had resulted from accidental introduction by man, at any time since the Mesolithic, and he proceeded to argue each case in detail. He pointed out that island economies are impoverished, with the smallest islands having the greatest deficits, and carriage of livestock, fodder, human food and building material would provide ample opportunity for some small mammals, at least, to be transported.

This general thesis has not been challenged. Though one might argue with some of the detailed evidence, the overwhelming majority of subsequent evidence has confirmed most aspects of it. Moreover, there is now much more geological and biological evidence to give general support to the thesis than when Corbet wrote, and it seems worthwhile to present a general review of this topic. This seems doubly useful since, on the one hand, there is no general review of the origin of the present fauna and, on the other, such a review highlights the area where a search for evidence that is currently missing might most usefully be undertaken.

Ongin of the British Mammals 3

Table 1 Correlation table for the various lines of evidence relevant to this review. The most extensive evidence comes from pollen analysis, and the Pollen Zones contribute the basic framework. Radio-carbon dates

allow the various lines of evidence to be correlated

4C DATES years b.p.)

1,000 -

2.000 -

3,000 -

4,000 -

5,000 -

6,000 -

7,000 -

8.000 -

9.000 -

10,000 -

11,000 -

12,000 -

13.000 -

-elm decline -

VllA

V I

V

II

I -

ATLANTIC

)ak-elm-aldel

BOREAL

hazel-pine

PRE-BOREAL birch

YOUNGER DRYAS

LLERldD rlTERSTADIA

birch

OLDER DRYAS

- IEETLE AUNAS

warm, uoodlanc

warm, open

arctic

- -

-

cool

- warm, open

arctic

dOLLUSC !ONES

f

e

d

C

b

a

2

_ - - -

M A M M A L S

R ABBl T?

BLACK RAT

H O U S E M O U S E

_ - - - - .AND BRlDGl '0 IRELA_ND! . - -

THATCH AM STAR CARR

REINDEER - - - - -

MEGA CEROS I N IRELAND

ARCHAEOLOGY

MEDIEVAL NORMAN

ANGLO-SAXON

ROMAN

IRON AGE 5 BRONZE

NEOLITHIC

/ MESOLITHIC

PALEOLITHIC

4 D. W. Yulden

TERMINOLOGY Any discussion of this field requires a clear understanding of the terminology applied to the periods of time. These terms are essentially those of the geologists. The last 2 million years or so constitute the geological period known as the Pleistocene. Geologically, this is characterized by the sequence of ice ages, the glacial periods, and the fist indication of a lowering of the world's temperature, revealed by oxygen isotope measurements (see below), is taken to mark the start of the Pleistocene; formally, the appearance of the northern bivalve mollusc Arrica islandica in the Mediterranean is taken to show the start of the Pleistocene, an event which happened about 2.4 million years ago (West, 1977).

Initially, geologists thought that there was only one glacial period, but it became clear at the beginning of the century that at least four glacial periods, termed the Gunz, Mindel, Riss and (most recent) Wurm glaciations, could be recognized in the Alps. In northern Europe too, evidence of at least four glaciations, termed there the Elster, Holstein, Saale and Weichsel, was found, while in North America, a similar sequence of four glaciations is termed Nebraskan, Kansan, Illinoan and Wisconsin. In Britain, there is clear evidence, so far, of only three glacial periods, which indicates why the geologists insist on using local name stages. In Britain the three glacial periods are referred to as the Anglian, Wolstonian and Devensian; obviously these are likely to be contemporaneous with three of the glaciations recorded elsewhere, but the synchroneity of the periods may not be absolute. There seems no doubt that the last glacial period, the Wurm in the Alps, Weichsel in Scandinavia, Wisconsin in North America and Devensian in Britain, was broadly contemporaneous; it lasted from about 70,000 years ago (b.p.-before present) until it ended quite abruptly about 10,000 b.p.

The glacial periods were separated by warm periods, Interglacials, when conditions were at least as warm as now, and which lasted long enough for the establishment of forest cover in Britain. The period since the end of the last glaciation has been regarded as post-Pleistocene time, and called either the Holocene or Post-Glacial; the geologists argue that we are just in another interglacial period, albeit an uncompleted one, and refer to it as the Flandrian Interglacial.

In addition to the Interglacials, it is possible to recognize shorter warm interludes, Interstadials, within the Glacial Periods, which were however not long enough to allow the establishment of forest cover. One of these, marked by the presence of birch scrub, occurred near the end of the Last Glacial (Devensian), and is recognized as the Allered Interstadial (after an archaeological site in Denmark where it was first noticed). At some sites, a more complicated zonation is suggested by some lines of evidence, and an earlier interstadial, the Balling Interstadial, may be recognized. An alternative view regards the whole period, including both Belling and Allered Interstadials, as one long but fluctuating interstadial, to which the name Windermere Interstadial may be applied (Pennington, 1977; Coope, 1977).

So far as this review is concerned, as will be made clear, only the last 13,000 years or so, covering the end of the Devensian Glaciation and the Flandrian Interglacial, are relevant; the period 13,000 to 10,OOO b.p., referred to as the Late Glacial, is particularly important.

Biologically, the Pleistocene of Europe is characterized by the appearance of, among others, modern horses Equus, cattle Bos, elephants and, particularly, man. Human history, especially in Flandrian times, has been plotted by a series of cultural zones-Palaeolithic, Mesolithic, Neolithic, Bronze Age, Iron Age, Roman, etc.-which were, until the application of radio-carbon dating (see below), rather poorly related to the other zonation schemes (Table 1).

origin of the Bdish Mammals 5

LINES OF EVIDENCE It seems convenient to recognize six areas which provide the background material for this synthesis: (i) Carbon 14 dating (ii) Pollen analysis (iii) Beetle faunas (iv) Mollusc faunas (v) Climatic evidence, from general geological work but especially from oxygen 18 studies (vi) Sea levels.

(i) Carbon 14 dating Carbon exists naturally as three isotopes; the most frequent, stable, 12C contributes nearly 99% of atmospheric carbon, the rarer 13C, also stable, makes up 1%, and the very much rarer, radioactive, 14C contributes only 1 x 10-10 %. However, it is the last, rarest, fraction which is important here; it decays radio-actively with a half-life of, conventionally, 5568 years; that is, of any amount present at any one time, half has lost its radioactivity after that number of years. The different isotopes of carbon are incorporated by plants, during photosynthesis, and by animals when they eat plants, in proportion to their abundance in the atmosphere. At death, however, the radio-active 14C starts to decay; if the amount of radio-activity which remains in a sub-fossil tree trunk, peat deposit or, indeed, bone can be measured, it therefore gives a measure of how long has elapsed since the original tree, plant or animal died. In practice, measuring such small amounts of I4C becomes increasingly difficult with age, but the method works well back to about 35,000 year b.p., and this is quite sufficient for the present review.

The annual growth rings laid down by trees can be counted back, in material from living or recently dead trees, to give absolute ages. Moreover, the width of the rings varies from year to year; good growing seasons result in wide rings, whereas droughts or severe cater- pillar attacks result in narrow rings. The patterns of variation in ring width are quite distinctive, and can be correlated between timber samples from any local area. Recognition of these patterns of variation combined with direct counts of the annual rings provided an absolute tree-ring dating (dendrochronology) back to about 7000 b.p. However, the annual rings in these old trees can also be carefully dated by radio-carbon methods. Such comparisons have shown that a radio-carbon date of 6000 b.p. is actually about 900 years too young (Renfrew, 1974). This has led to a convention where ‘b.p.’ and ‘b.c.’ are used for radio- carbon dates, while ‘B.P.’ and ‘B.C.’ are used for calendar (and tree ring) dates. At present, there is insufficient evidence from tree-rings to calibrate earlier radio-carbon dates, and the radio-carbon chronology is used throughout this account.

The period of interest for this review commences at about 13,000 b.p. (1 1,000 b.c.), with the retreat of ice toward the end of the last glacial period. Most archaeological sites, and important records from pollen analysis, beetle, mollusc and indeed mammal faunas, can now be assigned radio-carbon dates. Even with the possible inaccuracy mentioned above, this provision of an absolute time scale for the events discussed here is vital to understanding and interpreting them. There are other problems, of a technical nature, which can affect dates obtained for particular specimens; in general, however, the available radio-carbon dates provide a self-consistent sequence which is clearly reliable.

A general review of the techniques and problems is given by West (1977, chapt. 9), and Renfrew (1974) gives a useful compilation of dates for British archaeological sites.

(ii) Pollen Pollen grains have a rather hard outer coat which preserves well in acid substrates, particularly in peat bogs. Wind-pollinated plants produce large quantities of pollen, and analysis of the pollen flora in a sequence through a column of peat provides a valuable account of the changes in the general vegetation which occurred in the area during the period

6 D. W. Yalden

that the peat was forming. The best peat columns may provide a continuous record of the vegetation over 12,000 years, though often only a short period of time is represented. However, the changes in vegetation revealed by pollen analysis are so great that a number of very distinct pollen zones can be recognized within the last 12,000 years. Thus, even a short sequence of peat can, by pollen analysis, be assigned a date within a pollen zone, which may be only 500 years or up to 3000 years long. The pollen zones have themselves been given radio-carbon dates as well.

There are a number of limitations which need to be borne in mind. Firstly, the pollen rain is, as mentioned above, dominated by pollen from wind-pollinated species, especially trees and shrubs. Thus, hazel Corylus, oak Quercus, pine finus, alder Alnus and elm Ulmus, plus the pollens of grasses Gramineae and sedges Cyperaceae predominate-all these trees have catkins; conversely lime Tilia and willows Salk are rather poorly represented, though they may well have been very common (indeed, macrofossils-leaves, fruits, etc.-of Salk are often very common in peat). Because of this predominance, pollen curves are often presented calibrated as '% AF-percentage of arboreal pollen, usually excluding Corylus. 'NAP-non-arboreal pollen-is another frequent abbreviation. A second restriction is that

HERBS, SEDGES 8 GRASSES

10-

ALDER 10-

w' 10- ELM

BEECH 2 50- P I Recent <p 6 - Planting

1

1

HAZEL

50-

I II 1 1 1 IV v VI VllA VllB Vll l I 1 I I 1 I I I I I

1 4 1 3 1 2 11 10 9 8 7 6 5 4 3 2 1 0 THOUSAND YEARS b.p

Pollen diagram for the British Isles, covering the past 14,000 years. This is a somewhat schematic diagram, but is based heavily on the curve for Littleton Bog, Co. Tipperary (Mitchell, 1965) with additions from L. Windermere for the early part of the diagram (Low Wray Bay, from Pennington, 1977). The diagram opens with three peaks of open, herbaceous, vegetation (Oldest Dryas, Older Dryas, Younger Dryas) separated by peaks of juniper/birch scrub and birch scrub (Belling and Allered Interstadials). The Flandrian warming is indicated by the succession, birch - hazel+pine - hazel+oak+elm - hazel+oak+elm+alder. The elm decline, marking the boundary between Pollen Zones VIIA and VIIB, also coincides with the rise of the curve for ash. Note the erratic but persistent rise of herbaceous pollen over the last 4000 years, the very abrupt slump in hazel, and the general decline in tree pollens, reflecting increased forest clearance and farming. The small increase in pine and the appearance of beech reflect planting activity from the eighteenth century.

Fig. 1.

h g t n of the British Mammals 7

pollen is usually only identifiable to genus, and sometimes only to family or subfamily. This is not too serious a restriction for the British post-Pleistocene (Flandrian), since we have only one or two species of trees (as native species) in the major genera (one alder, Alnusglutinosa, one hazel Corylus avellana, one pine Pinus sylvestris, two oaks Quercus robur and Q. petraea, etc.); however, grass and sedge pollen can each only be identified to family, and the large daisy family Compositae, for example, can only be divided into two large groups. Hence pollen analysis provides a very good picture of the general vegetation of an area, but usually without allowing the flora to be detailed; woodland, scrub, and open vegetation can be differentiated very clearly, but the distinction between acid grassland and chalk grassland is much less evident.

The major synthesis of this work by Godwin (1975) lists 890 sites which had, up to 1970, yielded information on the history of the British flora in the last 13,000 years; pollen analysis proceeds at such a rate that at least 100 more sites could now be added to the list. The pattern of vegetation change revealed by this work, though it shows local and regional variation, is well established, and reasonably consistent; nine pollen zones (Table l), identified by roman numerals I-VIII, but including VIIa and VIIb, can usually be recognized in a complete pollen record (Fig. 1). These pollen zones have been dated by radio-carbon analysis at many sites, but especially at Scaleby Moss in Cumberland (Godwin, Walker & Willis, 1957) and Red Moss in Lancashire (Hibbert, Swiftsur & West, 1971); these two have been taken as the standard sites, in Britain, for establishing this relationship. In fact, because plants were immigrating from the south, the zones were not synchronous throughout Britain; in the south of Britain, the zones are about 1000 years earlier than in southern Scotland (Smith & Pilcher, 1973). The zones have conventional names (Table l), which are frequently also used in the literature.

In the earliest three pollen zones, a fluctuation from tundra-type open vegetation, (zone I) to birch scrub (zone 11) and back to tundra-type vegetation (zone 111) is regularly reported. The two tundra-type zones are referred to as the Older Dryas and Younger Dryas, from archaeological sites in Denmark where the Mountain Avens Dryas octopetala was a common plant at those times (Fig. 2). The warmer phase of birch scrub is referred to as the Allered Interstadial. At some sites, a more complicated pollen zonation may be recognized; at Lake Windermere, for example, there are two peaks of birch pollen (Pennington, 1977). This sequence is also recorded in Danish sites, and Pollen Zone 1 has there been subdivided into Zones la, lb, and lc, termed the Oldest Dryas, Belling Interstadial, and Older Dryas respectively. This more complicated sequence is not shown at sites in south-east England, and further south in Europe even the reversion to more open vegetation in Younger Dryas times may not be evident. In both Older and Younger Dryas, the pollen record in Britain is dominated by grasses and sedges, while other indicators of open vegetation such as rock rose Helianthemum are also present. The general warming up indicated by the spread of birch (tree birch, either Betula pubescens or B. pendula, and not the Dwarf birch Betula nana) also saw juniper Juniperus communis established as a common component, while in Ireland crowberry Empetrum was also important. The return to open vegetation in zone I11 must represent a deterioration in climate and is, indeed, correlated with a resurgence of the local ice cap in western Scotland known to geologists as the Loch Lomond Readvance. This represents the final phase of the last glacial period, and pollen zones 1-111 are referred to as the Late Glacial, Late Devensian, or Late Weichsel; the end of zone I11 time, dated to 10,300 b.p. is thus the beginning of the post-glacial or Flandrian warm period in which we are living.

In the pollen record, the first indication of warmer times is a resurgence of birch (zone IVY Pre-Boreal); this is quickly followed by the appearance of hazel and pine Pinus indicating

8 D. W. Yalden

I DRYAS

Fig. 2. Distribution maps for (A) Dryas octopetala Mountain avens. Characteristic of open ground, either montane or tundra, at the present day. Widespread in the Late-Glacial throughout western Europe (fossil sites marked +) (after Godwin, 1975). (B) Betula nana Dwarf birch. Also a plant of high altitude and high latitude at present, though with some relict sites (0) on heaths elsewhere. Also widespread in the Late Glacial across western Europe. (fossil sites indicated +) (after Godwin, 1975). (C) Corylus aoellana Hazel. A warmth-loving tree with a somewhat southern and coastal distribution in Scandinavia. At the thermal maximum, it occurred further north in Sweden than it does now (fossil sites, +) (after Godwin, 1975; Jalas & Suominen, 1976). (D) Emys orbicularis Pond Terrapin. A reptile which requires high summer sunshine and temperatures to hatch its eggs. At the Flandrian thermal maximum, it occurred in Sweden, Denmark and England, where it is now extinct (+) (after Stuart, 1979).

zone V, the Boreal period, with hazel-pine woodland. Herbaceous vegetation declines to very low levels by the start of zone V, so that this clearly indicates closed woodland. The first appearance of deciduous forest trees, oak Quercus and elm Urnus, dates the start of zone VI, but they become dominant elements of the pollen flora from 7500 b.p., indicating the start of pollen zone VII, the Atlantic. Pine is also an important component in zone VI, but its pollen drops to negligible levels in southern Britain by the start of zone VII; in Scotland north of the Central Lowlands, pine remained the dominant tree through to the present. There is some suggestion that the climate during Atlantic times was somewhat warmer than it is now; trees grew higher up mountains than they do now, hazel grew further north in Scandinavia than it does at present, and some other warmth-loving plants were also more widespread.

At about 5000 b.p., there is almost everywhere a sharp decline in the representation of elm pollen, and this ‘elm decline’ is used to divide zone VIIa, the Atlantic, from zone VIIb, the Sub-Boreal. This decline was originally thought to indicate a deterioration in the climate,


but it is now clear that it represents the first widespread indication of human interference with the vegetation; the first appearances of cereal pollen, and also of pollen of obvious weeds of cultivation such as plantain Plantugo, coincide with the elm decline. Neolithic farmers probably found the light soils, on which elm grows, the easiest to cultivate, and elm is also selectively eaten by sheep and cattle for its high phosphate content.

From the time of the elm decline, the representation of tree pollens becomes, erratically but inexorably, less and less, and herbaceous pollens (grass, including cereals, and weed species in particular) become more and more prominent. The appearance of ash Fruxinus becomes more regular through the Sub-Boreal, and in southern Britain hornbeam Curpinus and beech Fagus become noteable contributors to the tree pollen rain from about 2500 b.p. (500 b.c.), which marks the start of pollen zone VIII, the Sub-Atlantic. The spread of heath and moorland in some parts of Britain is evidenced by the increase in pollen of Ericaceae from 5000 b.p. onwards. The minimal representation of tree and scrub pollen (hazel pollen is quite late in declining) was reached around 300 b.p., and shortly thereafter evidence of

BOREAPHILUS HENNINGIANUS

Fin. 3. Distribution maos for some beetles. (A)

BEMBlDlON OCTOMACULATUM

loreaDhilus henninaianus. a staohvlinid with a - northern distribhion at present, which'was abkdant in Younger' Dryas' times in S.W. Scotland and at St. Bees Head, Cumbria (after Coope & Joachim, 1980). (B) h c h e i l a arctica, a carabid with a tundra distribution at present, which was also numerous in Younger Dryas times in Britain (after Bishop & Coope, 1977). (C) Bembidion ocromucularum, a carabid with a predominantly southern distribution at present, which was abundant at St Bees, Cumbria during Windermere Interstadial (?Balling Interstadial) times (aftei Coope & Joachim, 1980). (D) Bembidion cullosurn, a southern European carabid at present, which was present in S.W. Scotland during Windermere Interstadial times (after Bishop & Coope, 1977).

10 D. W. Yalden

replanting shows in the pollen record; for example beech pollen appears in the Irish pollen record, and beech is not a native tree in Ireland (Mitchell, 1965).

In general, the pollen record thus outlined suggests an orderly progression of vegetation types since the end of the last glaciation, reflecting a general increase in temperature; birch, the most cold-tolerant of our trees, then hazel and pine, then the deciduous forest trees, with the most warmth demanding, such as lime Tilia and beech, being slowest to colonize. The general pollen record makes it quite clear that most of Britain was covered with high forest from about 7500 b.p. until human interference started to disrupt it; the pollen of herbaceous plants declines to negligible levels throughout that period in most sites. Only on the higher mountains, for example in the Peak District (Tallis, 1964) and at Teesdale (Turner, 1978), in northern Scotland (Birks, 1977), and in a few specialized sites such as the peat mosses of Lancashire (Hibbert et al., 1971) did open vegetation persist through this period.

(iii) Beetle faunas Compared with pollen analysis, which started in the 1930s, the study of fossil beetle faunas is a relatively recent field which began in the 1960s. The evidence they provide corroborates much of the general picture given by pollen analysis, yet contradicts some of the detail. Late- Glacial beetle faunas suggest a sequence from cold-temperateold, similar to that suggested by pollen analysis. Various sites with 14C dates between 14,000 b.p. and 13,000 b.p. have an arctic type fauna including such species as Boreaphilus henningianus and Bembidion hasti (Fig. 3). From 13,000 to 12,200 b.p., there was a warm temperate beetle fauna which would not have been out of place now (Coope & Joachim, 1980). Then rather abruptly, the warmth- loving forms are replaced by a cool fauna, and from 11,000 b.p. to 10,000 b.p. by a tundra fauna, with such species as h c h e i l a arctica and Olophrum boreale (Coope, 1977; Bishop & Coope, 1977). This sequence is reminiscent of the Older Dryas-Allered-Younger Dryas alternation in the pollen record, but comparison of the radio-carbon dates and, indeed direct comparison of the pollen and beetle evidence from the same deposits, when they are both available (e.g. in Lake Windermere, Coope, 1977; Pennington, 1977) shows that the evidence is discrepant; the beetle faunas imply a warming up for the interstadial from 1,000 or 1,500 years earlier than does the pollen evidence. However, the two lines of evidence concur in indicating an arctic severity for the climate during the period 1 1,000-10,000 b.p., the Younger Dryas or zone I11 time, which is perhaps the most significant fact so far as interpreting the mammals is concerned.

The beetle evidence suggests that the post-glacial began very suddenly; at West Bromwich, the tundra fauna at 10,025 b.p. was replaced at 9970 b.p. by a temperate fauna which would not be out of place now (Osborne, 1980). Woodland species appear from about 9500 b.p., some 500 years in advance of the date suggested by pollen analysis for the appearance of woodland; Osborne suggests that these beetles were managing to survive on birch and willow (Osborne, 1974). Then, from about 9000 b.p., beetles characteristic of forest trees appear, and this is concordant with the evidence from pollen. During the period when forest cover was extensive, several species occurred which are now absent from Britain; Buckland & Kenward (1973) list five species from Thorne Moor, Yorkshire, from an oak log dated to 3090 b.p., and mention a longhorn beetle from Fenland in the same category. Extinction of these beetles presumably resulted from the extensive clearance of woodland by man which the pollen record documents.

(iv) Mollusc faunas Pollen and beetle remains are best preserved in peat, and therefore recorded most often from sites in the north and west of Britain. Mollusc shells, in contrast, are best preserved in

origin of the Bniish Mammals 11

COLUMELLA

- --

Fig. 4. Distribution maps for some molluscs (from sources indicated and Kerney, pers. comm.). (A) Cofumeffa cofumeffa, a montane/tundra species at present in Europe, which was present in southern England during Younger Dryas times (after Kerney & Cameron, 1979). (B) Vemgo genesii, a montanehndra species at present, known in Britain only from Upper Teesdale, which was present in southern England during Younger Dryas times (after Kerney & Cameron, 1979). (C) Porntius eleguns, a snail with a rather southern distribution, in Britain and Europe, which was rather late to arrive in Britain, occurring as a fossil late in Mollusc Zone d. The scatter of apparently relict sites (0) well north of its main range suggests that it extended further north at the time of the climatic optimum, but has since retreated southwards (after Kerney & Cameron, 1979). (D) Discus ruderatus, a snail which was common in southern Britain during Mollusc Zone b times, but was displaced during Mollusc Zone c times by its relative D. rorundatus, and is now confined largely to northern and eastern Europe (after Kerney, 1977).

lime-rich areas; most of the important sites are in southern and south-eastern Britain, and therefore provide a story which is largely complementary. The recent paper by Kerney, Preece & Turner (1980) is a particularly useful one because it provides a complete record of mollusc faunas from the Late-Glacial to Recent times, and it is also directly related to a pollen record from the same sites.

Seven major mollusc zones are recognized in this and other studies (Table 1). Zone z, a late glacial zone, contains molluscs typical today of open or tundra conditions such as Catinella arenaria, Columella columella, Vertigo genesii, Abida secale, Pupilla muscorum, and abundant Trichia hispida. Of these, Columella columella has a boreo-alpine distribution (Fig. 4) and is extinct in Britain, Vertigo genesii has a similar distribution but has recently been discovered at Teesdale (Coles & Colville, 1980) and Cm'nella arenaria has a peculiar

12 D. W. Yalden

distribution, being found, rarely, in various coastal localities, more commonly in the Swiss and Norwegian mountains, and in Central Ireland (see the maps in Kerney & Cameron, 1979). The other species have fairly wide distributions in north-western Europe, though Abida secale does not occur in Scandinavia. This mollusc fauna seems to be contemporary with the Younger Dryas, Pollen Zone 111.

The next Mollusc Zone, a, is characterized by a group of species, referred to as the catholic ‘Group A species’, which have at present wide distributions, all ranging from the Pyrenees at least to southern Scandinavia, mostly to the Arctic Circle or beyond. They include Cochlicopa, Columella, Vertigo substriutu, Punctum pygmaeum, Vizrina pellucida, Vitraea, Nesovitraea, Deroceras/Limax, Euconulus fulvus and Arianta/Cepaea. Climatically, this group could be quite typical of Britain today, but it was contemporaneous with Pollen Zone IV.

The next Mollusc Zone, by is characterized by the appearance of ‘Group B species’, characteristic of deciduous woodland. At the present, they are widespread from the Pyrenees north to southern Scandinavia; Carychium mdentatum, Aconthinula aculeata, Ena, Aegopinella, Helicogona lapicida and the Clausiliidae belong here. One additional interesting member of this group is Discus ruderatus, a species now absent from Britain and showing a somewhat continental, eastern, range in Europe (Fig. 4). This mollusc zone is roughly contemporaneous, in Kent, with Pollen Zone V (Kerney et al., 1980).

Mollusc Zone c is characterized by the dominance of ‘Group By molluscs, the loss of open ground species such as Pupilla muscorum and Abida secale, and the replacement of DISCUS ruderatus by D. rotundatus, the species which exists now in Britain. This mollusc zone represents the development of complete woodland cover, and is contemporaneous with Pollen Zone VI.

As with the plants, there is some indication from the molluscs of a climatic optimum. Mollusc Zone d continues with a woodland type fauna, but Oxychilus cellarius and Pomarias elegans are important additional members. Pomatius seems to have spread somewhat beyond its present range during the climatic optimum, for there are now a number of isolated, apparently relict colonies well beyond its main area of distribution, both in Britain and Europe (Fig. 4); so far, however, the fossil record to support this thesis is lacking (Kerney, 1968). This Mollusc Zone is contemporaneous with Pollen Zone VIIa, the Atlantic.

Just as the elm decline marks the effect of human interference in the pollen record, so Mollusc Zone e is characterized by the re-appearance and increasing importance of open ground species such as Pupilla muscorum. Valloniu costata, V. excentrica and Helicella uala, characteristic of grassland, also become important (Evans, 1968). Mollusc Zone f marks the era of human introduction, with the garden snail Helix aspersa, introduced by the Romans, entering the fauna.

By comparison with the pollen record, the mollusc faunas imply that the Younger Dryas was not so severe as to extinguish the fauna, and the rapid re-appearance of the ‘Group A’ species in Mollusc Zone a is about loo0 years before the first appearance of deciduous forest trees. Similarly, the ‘Group B’ woodland species first appear along with the ‘Group A’ forms, and another 750 years elapsed before full woodland conditions, evidenced both by the exclusion of open ground molluscs in Zone c and from pollen analysis, had developed.

(v) Climatic evidence The direct geological evidence for glacial conditions, the extent of glacial tills, the lines of terminal moraines and also, outside the glaciated regions, evidence of periglacial activity has been well summarized many times; West (1977) provides a recent and available account. At the maximum of the last (Weichsel in Europe, Devensian in Britain) glaciation, around

Origin of the British Mammals 13

W

Fig. 5. The maximum extent of the ice sheet in the Weichsel (Devensian) Glaciation, 20-15,OOO years ago (W); and in the Loch Lomond Readvance in Younger Dryas times (111). There is some uncertainty over the Weichselian line in N.E. Scotland (?). The 100 m submarine contour to the S.W. indicates the likely extent of dry land there at the Weichsel maximum glaciation (after West, 1977; Gray & Lowe, 1977).

20-18,000 b.p., the ice front reached as far as the south Wales coast, covered all of Scotland and stretched down the east coast of England (Fig. 5); south of this, permafrost conditions must have existed for such periglacial features as ice wedges and stone polygons occur even in Devon and Cornwall. Much of Ireland was also covered by ice. Just on the geological evidence, of pingos, ice wedges, and ice polygons, Watson (1977) estimated that the mean annual temperature in southern England and Wales during the full glacial was probably - 16 or - 17°C.

On geological evidence accompanied by 14C dating, the ice started to melt rapidly from about 13,000 b.p. Even Scotland was free of ice in the period 13,000-1 1,500 b.p. (Gray & Lowe, 1977). This warming is obviously related to the Allered Interstadial, Pollen Zone 11, of the pollen analysis, yet not totally contemporaneous with it, being rather earlier. From about 12,000 b.p. however, the temperature declined, and glaciers started to redevelop in the Western Highlands; the small ice cap which developed is demarcated by a line of moraines which, at one point, reaches Loch Lomond, and this period is referred to geologically as the

14 D. W. Yalden

SWEDEN Ill

Loch Lomond Readvance. It obviously correlates with the Younger Dryas, Pollen Zone 111; on geological evidence, Watson (1977) proposes a mean annual temperature in southern Britain of - 4 to - 5°C for this period.

A more precise record of fluctuating temperature over at least the last 10,OOO years has been provided by the analysis of 1 8 0 / 1 6 0 ratios. The isotope 180 forms only about 0.2% of the atmospheric oxygen; 1 6 0 , the usual isotope, forms 99.8%. Both these isotopes are stable (that is, are not radlo-active, as is I%), but the relative solubility of 1 8 0 compared with 1 6 0 is dependent on temperature; lower proportions of 180 indicate lower temperatures. Initially this technique was applied to the shells of foraminiferans obtained from deep sea cores, and this allowed the construction of a general paleotemperature curve extending back to Cretaceous times. More recently, it has been applied to ice cores from Greenland and Antarctica (Robin, 1977), and to carbonates from lake deposits in Sweden (Morner & Wallin, 1977) and France (Eicher, Siegenthaler & Wegmiiller, 1981); these provide a more detailed paleotemperature curve through the later part of the Last Glaciation, and in Post-Glacial times. Because the fall of snow which contributes to the ice caps occurs throughout the year, the temperature curve from the ice caps reflects mean annual temperatures; the lake sediments comprise carbonates produced by living organisms, and therefore reflect the temperatures during the growing season, i.e. the summer. The pattern of changes is, however, quite consistent between the two records (Fig. 6).

In the deep sea cores which have been analysed for l8OPO ratios, the foraminiferan faunas themselves have proved to provide an excellent indication of the changes in sea water temperature; some species are characteristic of polar waters, others of tropical waters. Ruddiman, Sancetta & McIntyre (1977) present interesting results from this type of analysis for the North Atlantic (Fig. 19).

IV V VI VllA VllB Vlll ENGLAND I I I I V V VI VllA VllB Vlll

Onen of the British Mammals 15

At the height of the last glaciation, at about 20,000 b.p., the mean annual temperature in Antarctica was 6 8 ° C colder than it is now. Both the Greenland and Antarctic ice cores suggest a very rapid rise of temperature at about 10,000 b.p. In the North Atlantic, the initial warming began about 13,500 b.p., and a cold peak was recorded at about 10,200 b.p.; clearly this represents Younger Dryas times (Ruddiman et ul., 1977). There was then a quite sudden warming to about 9300 b.p. when temperatures similar to today’s were reached. There is some evidence that temperatures reached a maximum around 5000 b.p., before declining; this maximum was a modest 1-2°C warmer than at present.

The 180/L60 ratios from lake sites concur in suggesting a warming up around 13,500 b.p. (Eicher et al., 1981), a cooling at around 11,000 b.p., and a rather abrupt warming up from 9500 to 9300 b.p. (Morner & Wallin, 1977). These sites too suggest a thermal maximum, perhaps 2°C warmer than present temperatures, from about 8000 to 6000 b.p. (Fig. 6).

This evidence apparently conflicts somewhat with the pollen evidence, particularly during the early part of the Late Glacial; the tundra-type vegetation suggested for the Older Dryas, Zone I, is not matched by cold temperatures. On the other hand, beetle faunas do confirm the palaeoclimatic indications of a warm period then. All lines of evidence suggest a very cold period during Younger Dryas times, but the warming up subsequently, seen to be rapid in the oxygen isotope curves and in beetle faunas, was apparently slow according to pollen analysis.

(vi) Sea levels The possible timing of mammal immigration to the British Isles depended on the changing relationship between the improving climate and habitat, the relative sea level, and the depths of the channels which separated the islands from one another and from Europe. At the height of the Last Glacial so much water was locked up in the ice sheets that the general sea level, throughout the world, was much lower than now. Geological evidence, for example of former river valleys which extend across the sea bed, suggests that sea level was 80 m below its current levels, and calculations of the amount of water locked up in the ice caps concur, with suggestions that the theoretical lowering should have been 80 to 150 m (West, 1977). (If the present ice caps were to melt, similar calculations suggest that sea levels would rise by 40 to 60 m.) Such changes, due to melting and freezing, are known as eustatic changes of sea level; if they were the only changes, given also the present depths of the channels, it would be fairly easy to work out when various islands were cut off. However, the problem is complicated because the considerable weight of ice which accumulated over northern Britain and Scandinavia depressed the land relative to the sea; such depressions and subsequent recoveries are termed isostatic movements. At the maximum, Scotland may have been depressed by 250-300 m (Gray & Lowe, 1977). Certainly, the Late Glacial period in the Forth-Tay area saw the formation of a shore-line, at about 13,500 b.p., which is now 20-30 m above sea level (Ordnance Datum); if the eustatically controlled sea level was then 60 m below its present level, this implies an isostatic uplift since then of around 80 m for that part of Scotland (Cullingford, 1977). The western Highlands are still rising at 3 m d y e a r , and in the north of the Gulf of Bothnia, the rise is nearly 1 cm/year. However, southern Britain, which was not directly covered by ice, is actually sinking at a rate of about 2 m d y e a r ; Britain as a whole is behaving rather like a see-saw, with the axis of the swing running through Cumbria and Northumberland.

The most useful information on the relative level of the land and sea comes from the examination of coastal peats; these can be dated by both I4C and pollen analysis, and their present height above or below sea level recorded. Often they lie on top of or are covered by marine clays which indicate earlier or later transgressions of the sea over their locations (peats

16 D. W. Yalden

I II I I I I V V VI VllA VllB Vlll 1 I

only form in fresh water, of course). In southern Britain, in the southern North Sea, and particularly in Holland, sequences of such peats have enabled an accumulative curve of rising sea level to be plotted; from a depth of - 40 m at 11 ,000 b.p., sea level rose rapidly to - 20 m at 8500 b.p. and then at a slower rate to reach present sea level by 5500 b.p. (Mitchell, 1977). This curve matches quite closely similar curves from other parts of the world, not affected by the weight of ice, and therefore reflects largely the eustatic rise of sea level. In the Straits of Malacca, for example, a low sea of -70 m at 10,000 b.p. (Fig. 7), rose very rapidly to - 20 m at 8500 b.p., present sea level at 6500 b.p., actually rose + 5 m above current levels from 5000 to 4000 b.p., and then slipped back to its present level (Geyh, Kudress & Streif, 1979). Evidence from Britain also shows that sea level 'overshot' its present height by about +4 m during the period between 6000 and 4000 b.p. (Mitchell, 1977).

Along the Lancashire coast, almost at the hinge of the isostatic tilting, Tooley (1976) plots the rising sea level from -20 m at 9000 b.p. through a steep rise to - 5 m at 7500 b.p. and present sea level at about 3500 b.p. These are mean tide levels, though, and he shows that high tides, about 4 m above mean tide level, transgressed the present shore lines on a number of occasions.

In central Scotland, depressed much more by the weight of ice, sea levels were about + 12 m above present sea level at 10,000 b.p., and, with isostatic uplift apparently proceeding faster than the eustatic rise, decreased to only +6 rn at about 8000 b.p. This represented a minimum sea level, however, for sea level then rose to about + 15 m in the period 7oo(M000 b.p., before declining toward its present level (Donner, 1970). In south- west Scotland, somewhat less affected by the weight of ice, the levels were slightly lower; about + 5 m at 10,000 b.p. and only + 2 m at about 9000 b.p. before a rise to + 12 m or more

+30] y\ /S W SCOTLAND

40-

S L

7 -10- Fronce - Jersey

- _1 W 5 -30- A

4 % -50- Scotland-lslay- Ireland

W 1

THOUSAND YEARS hp.

Fig. 7. Changes in relative sea level in Late-Glacial and Flandrian times. The general rise in the world's sea level, the eustatic curve due to water returning to the Oceans as the ice cap melted, is based on data from the Straits of Malacca (Geyh, et ul.,1979); the eustatic curve for the Late Glacial (14-10,OOO b.p.) is uncertain, though at some stage a sea level of - 100 m seems probable. The curve for N.W. England (dotted line, from Tooley 1976, 1978) is somewhat less regular but follows a similar trend, as do curves from elsewhere (see West, 1977). In S.W. Scotland however, the land was depressed relative to the sea by the weight of ice, producing a high relative sea level of + 20 m at 12,000 b.p.; the subsequent curve (from Donner, 1970, with additions from Cullingford et ul., 1980) reflects the balance between the eustatic rise of sea level and the isostatic recovery of the land. The depths of some of the postulated land bridges are shown against the eustatic curve; these indicate the times at which those land bridges might have been cut by the rising sea if the land in question was not suffering isostatic depression, and if the channels have not subsequently been scoured.

ENGLAND

Fig. 8. % s

I ,

Submarine contours in the region of two postulated land bridges. (A) The narrows between Scotland and Ireland. The shallowest region at present, between - 50 m and - 100 m, is the ridge from Scotland through Islay to Ireland. Further south, a narrow channel more than 100 m deep extends south to the English Channel. At the glacial maximum, however, this region was blocked by ice, and the very deep (greater than 200 m, heavy stipple) submarine valleys were presumably scoured as ‘tunnel valleys’ by glacier melt-water. What morainic ridges may have existed, and how long they persisted, is not known. (B) The eastern English Channel. The shortest route between England and France is interrupted by a channel more than 50 m deep which is also a ‘tunnel valley’ scoured by melt-water, of an earlier glaciation than the last one (Kellaway a al., 1975). Both north and south of this, shallower seas, between - 25 and - 50 m deep, would have been dry land in Late-Glacial and early Flandrian times. Much of the southern North Sea, in particular, between England and Denmark, was dry land, and therefore allowed extensive immigration of terrestrial animals

!$

c

(see also Fig. 20). 4

18 D. W. Yalden

Table 2

Apparent depths of sea channels between various islands at present; if sea level dropped to the lower level indicated a landbridge would be produced. Whether this would equally have applied in late or post-glacial times depends not only on eustatic lowering of sea levels but also on whether the land was at that time isostatically depressed by the weight of ice, and also on whether the sea poor has been

subsequentlygouged out by tidal action or built up in sand banks

Depth at present (in) Channels

0-20 France-Jersey; Scotland-Skye; Scotland-Mull. 20-50 France-England; England-Isle of Man; Scotland-Arran; France or Jersey-

Islay-Ireland; Scotland-Jura; Scotland-Orkney; Cornwall-Scilly Isles. Inner Hebrides-Outer Hebrides; Outer Hebrides-St Kilda; Orkney-

Rest of Channel Isles. 50-100

,> 100 Shetland.

in the period 8000 to 6000 b.p. The short period of a relatively low sea level has been confirmed by Cullingford, Caseldine & Gotts (1980); they date it, more precisely, to the period between 8300 and 7500 b.p., probably 8000 to 7800 b.p. This has, of course, rather important implications for the possibilities of mammals, and other biota, getting to the various islands and, especially, to Ireland.

The depths of the channels separating various islands can be obtained from a good atlas (Fig. 8). At present, a fall in sea level of 18 m would join Jersey to France; a fall to 37 m (20 fathoms) would join the rest of the Channel Isles to France, and also join England to France, exposing not only the floor of the Channel at the Straits of Dover, but also a large area of the southern North Sea, including the Dogger Bank (Table 2). A drop in sea level of 40 m would join the Isle of Man to Britain, but a lowering to 55 m would be needed to join Ireland, through Islay, to Scotland. The channels between Orkney and Shetland, the Inner and Outer Hebrides, and Outer Hebrides and St. Kilda are all deeper than 100 m.

The relevance of these depths to the curves of rising sea level, even ignoring for the moment any additional isostatic depression of the land, assumes that the channels have not been scoured out, since the sea flooded them. If such scouring has occurred, then the land bridges might have been much higher than the present depths imply. There is, unfortunately, little evidence on this topic for any of the channels except the English Channel, and that is the least contentious of the land bridges anyway. Kellaway et al. (1975) demonstrate that the English Channel floor shows features characteristic of glaciation, which was presumably the earlier, Saale, glaciation and certainly not the last (Weichsel) glaciation. The floor also shows river valleys characteristic of melting water in periglacial regions, which do date from the Weichsel glaciation. Kellaway et al. (1975) conclude that the present conformation of the floor of the English Channel owes its form to glacial and periglacial activity and owes very little to marine erosion subsequent to the rise of sea level. From the eustatic curve (Fig. 7), sea level rose to -37 my which would have cut England off from Europe, by about 9500 b.p. This would coincide with the beginning of Pollen Zone V, a result corroborated by the finding of peat samples (‘moorlog’), dredged from the floor of the North Sea by trawlers, which have been dated to Pollen Zones IVY V and VI. However, radio-carbon dates for such samples of around 8500 b.p. (Godwin, 1975; Mitchell, 1977) are somewhat later, and it may be that the land connection to Europe persisted somewhat later than suggested above. It might be argued conversely that, just because some parts of the


present floor of the North Sea were exposed land at the later dates, it does not follow that the whole of the land bridge was still exposed. Alternatively, the floor of the southern North Sea has apparently sunk somewhat since that time, so that the present depth of water may not be an accurate datum for the relative heights of land and sea at that time.

This argument seems even more pertinent in the case of the putative land bridge between Scotland and Ireland. Given the isostatic depression of Scotland, it is not conceivable that relative sea level was ever lowered, in the Late- or Post-Glacial, by the 55 m needed to expose a land bridge, on the present conformation of the sea bed, between the two. As noted above, sea level in S.W. Scotland was actually 5 m above its present level at 10,000 b.p., and the short phase of lowered sea level around 8000 b.p. was at a level just above present O.D. The implication must be that if there was a land bridge it was very low-lying, short-lived, and, moreover, scouring by the sea through the channel(s) between Scotland and Ireland has since erased it.

It further seems certain that the various northern channels that are now deeper than 100 m have never been dry land; St. Kilda, Shetland and the Outer Hebrides, and probably Orkney too, have never been connected to Scotland in Late-Glacial or Post-Glacial times.

ResumC All the evidence concurs that around 15,000 b.p., with the ice sheet covering northern Britain, even southern Britain, experiencing severe periglacial, permafrost conditions, was quite inhospitable to animal and plant life (Fig. 9). If there was any vegetation, it was very sparse, consisting of grasses, sedges, mosses, and dwarf willows (Pennington, 1977) with an appropriate sparse, high arctic, beetle fauna (Coope, 1977). There was a marked warming at

18

3 1 8

GLISH CHANNEL

1 4 1 3 1 2 1 1 1 0 9 8 7 6 5 4 3 2 1 0 THOUSAND YEARS bp

Fig. 9. Correlation between the curves of temperature, rising sea level and pollen (Based on Figs 6, 7 and 1). This suggests that modern temperatures were attained only 500 years or so before the English Channel was cut, so stopping further natural immigration of terrestrial animals. This seems to have occurred in Pollen Zone V or at the V/VI boundary, about 9000 b.p. Deciduous trees had spread to Britain, but had not achieved dominance by that time. (B, birch, H, hazel; J, juniper; P, pine; -22 herbaceous pollens-herbs, grasses and sedges; black, thermophilous trees-oak, elm, alder, ash).

Table 3 A list ojthe terrestrial mammals oj the Brirish Isles, 13,000 b.p. to present. (Bats, seals, whales, also domestic species, omitted). The listing is based on Corbet

& Southern, 1977and Stuart, 1977, with additions from Bramwell1976, Savage, 1966; nomenclature follows Corbet, 1978, The probable status is indicated by N native; I introduced; E extinct; ? native status uncertain. The sequence N, E, I implies a native form, becoming extinct, and then reintroduced. The latitudinal range in Europe is given as an indication of how early or late in post-glacial times the species might have arrived

(based on maps in Corbet, 1978); Gibraltar is c. 36"N, North Cape is c. 71"N.

Lat. OdY range

Ireland elsewhere (ON) Gt. Britain

Marsupialia

Insectivora

Macropus rujogriseus

Erinaceus europaeus Sorex minutus Sorex araneus Neomys fodiens Crocidura suaveolens Crocidura russula Talpa europaea

Ochotona pusilla Lepus capensis Lepus timidus Oryctolagw cuniculus

Sciurus vulgaris Sciurus carolinensis Castor fiber Dicrostonyx torquatus Lemrnus lemmus Clethrionomys glareolus Awicola terrestris Ondatra zibethica Microtus arvalis Microtus agrestis Microtus oeconomus Microtus gregalis

Lagomorpha

Rodentia

Red-necked wallaby

Hedgehog Pigmy shrew Common shrew Water shrew Lsr white-toothed shrew G u white-toothed shrew Mole

Steppe pika Brown hare Mountain hare Rabbit

Red squirrel Grey squirrel Beaver Arctic lemming Norway lemming Bank vole Water vole Muskrat Common vole Field vole Root vole Northern vole

- Stilly, Jersey etc.

Guernsey etc. -

-

-

Orkney, Guernsey -

3 6 4 1 37-7 1 37-7 1 40-7 1 36-52 36-53 -

(45-55) 36-63 51-71* 36-58

36-7 1

4 3 4 6 63-71 60-7 1 40-7 1 40-7 1

40-62 40-7 1 48-7 1 42-64

-

-

Apodemus Javicollis Apodemus sylvaricus Rattus ranus Rattus norvegicus Mus musculus Glis glis Muscardinus avellanarius Myocastor coypus

Canis lupus Alopex lagopus Vulpes vulpes Ursus arctos Mustela erminea Mustela nivalis Mustela vison Mustela putorius Manes martes

Meles meles Lutra lutra Felis silvestris Felis lynx

Perissodactyla Equus jerus

Artiodactyla Sus scroja Munnacus reevesi Cervus dama Cervus nippon Cervus elaphus Megaceros giga rueus Alces alces Rangiyer tarandus Hydropotes inermis Capreolus capreolus Bos primigenius

Carnivora

Gulo gulo

Yellow-necked mouse Wood mouse Black rat Brown rat House mouse Edible dormouse Hazel dormouse COYPU

Wolf Arctic fox Red fox Brown bear Stoat Weasel American mink Polecat Pine marten Glutton Badger Otter Wild cat Lynx

Tarpan (Wild Horse)

Wild boar Reeve's muntjac Fallow deer Sika Red deer Irish elk Moose Reindeer Chinese water-deer Roe deer Aurochs

42-63 36-63 36-60 36-7 1 36-7 1

37-60 :37-58)

-

36-7 1 62-7 1 36-7 1 40-7 1 42-7 1 36-7 1

36-66 37-68 60-7 1 36-66 36-7 1 36-58 37-70

-

?

36-60

36-60

36-64

54-7 1 60-70

38-66 ?

-

-

-

-

*Also a relict population in the Alps, 4648"N.

22 D. W. Yalden

about 13,500 b.p. (shown both by deep sea cores and oxygen isotope levels) and the beetle faunas evidently responded rapidly to this; at St. Bees, Cumbria (Coope & Joachim, 1980) and in S.W. Scotland (Bishop & Coope, 1977), beetles now characteristic of southern or central Europe (including Bembidion callosum, now found in S. France and Spain-Fig. 3) occur at about 13,000 b.p. The vegetation, however, was still of an open, treeless type, and it seems that the more mobile animals reacted to the improved climate more quickly than the plants; this conflict between beetle and plant evidence has led to the suggestion that the pollen zones I and 11, Older Dryas and Allered Interstadial, give a false impression of events, and that the whole period was in fact an Interstadial, to be termed the Windermere Interstadial (Coope, 1977). There is some evidence of a cooler episode from 12,000 to 11,000 b.p., yet birch scrub developed at this time. Then, all lines of evidence concur in recognizing a short, sharp, cool episode, from about 10,500 to 10,000 b.p., the period of the tundra, Younger Dryas, on pollen evidence, the time of the Loch Lomond Readvance for the geologists. Oxygen isotope evidence, beetle faunas and mollusc faunas all suggest a rapid improvement in climate between 10,OOO and 9500 b.p., so that warm temperate faunas, comparable to those of the present day, were established in Britain by 9300 b.p. The vegetation, however, was again rather slower to respond to the improvement, and closed deciduous forest vegetation was not established until perhaps 1000 years later than, on climatic grounds, it could have survived here. Various lines of evidence-beetle faunas, mollusc faunas, and pollen analysis itself-oncur in suggesting a phase from about 8000 to 5000 b.p. when the forest cover was so complete than animals and plants characteristic of open, sunny, habitats were rare or absent from southern Britain. Some mountain tops (e.g. Teesdale, Peak District), possibly coastal areas such as duneland and the mosslands of Lancashire and Cheshire (Tooley, 1978), and the limited areas near Mesolithic camp sites (Seagrief, 1959, at Wareham) show the persistence of heather, particularly, through this period, but these are clearly rather exceptional sites. Various lines of evidence also agree in suggesting a thermal maximum at about 5000 b.p.; sea level was, world-wide, some 5 m higher than now, implying a reduction of the ice caps compared with the present time, hazel and other trees occurred further north in Europe than now (Godwin, 1975) while the pond tortoise Emys orbicularis reached both southern Sweden and East Anglia (Stuart, 1979), though it is now extinct in Sweden and Britain (Fig. 2).

HISTORY OF THE MAMMALS The most direct evidence of mammalian history in Britain comes from fossil and sub-fossil remains. Much of the relevant material is from archaeological sites, with the added benefit of dating by radio-carbon analysis and pollen zonation; on the other hand, this material is reported in a scattered literature, often in appendices to archaeological reports, and the small mammals have been badly neglected. Unfortunately, much of the best fossil material comes from cave sites which were excavated last century or early in this century, before the complexity of Pleistocene climatic history was appreciated, and long before modern dating techniques became available; even now, cave sites are often difficult to date because they are often in limestone areas, where pollen preserves poorly. Fortunately the (more expensive) method of radio-carbon dating can be applied directly to bone, though generally the bones have to be large ones (deer, for instance) and, since they are destroyed in the process, unimportant ones.

Indirect evidence, from genetic studies and indeed from considering the present or recent distributions of the species, is also still of considerable interest and value, despite the difficulties of interpreting it. The species of concern are listed in Table 3.

Or@n ojthe British Mammals 23

(i) Fossil and sub-fossil records The likelihood must be that at the maximum of the last glaciation, with the majority of both Great Britain and Ireland covered by ice, none of the present mammalian fauna was present. There is little direct evidence of any mammal fauna at that time, but if there was one, it would have contained only such high arctic species as Arctic fox Alopex lagopus, reindeer Rangifer tarandus, perhaps musk ox Ovibos moschatus, and polar bear Thalarctos maritimus. If any of the present day mammals co-existed with such a fauna, they would have been the Mountain hare Lepus timrdus, stoat Mustela erminea, and possibly the weasel M. nivalis; none of the other species range, at present, far enough north.

In the period covered by the Windermere Interstadial (i.e. Belling Interstadial +Older Dryas+Allered Interstadial), there is some direct evidence of the mammal fauna (Campbell, 1977; Stuart, 1977). Near Blackpool, the skeleton of a moose Alces alces found in peat was bracketed by *4C dates of 12,200 and 11,700 b.p. (Hallam et al., 1973) and at Flixton 2, the remains of three horses, Equusjerus came from deposits of Pollen Zone I1 (Moore, 1954; Fraser & King, 1954). In Ireland, the famous site at Ballybetagh near Dublin, which yielded more than 60 giant deer (Megaceros giganteus) skulls, has been studied extensively by palynologists, and they have shown that an Allered, Pollen Zone 11, date applies. Numerous other records of Megaceros giganteus, in Ireland and in the Isle of Man, also belong to this period, and radio-carbon dates have been obtained for three examples: 12,850 b.p. for one from Brandesburton, Yorkshire; 12,180 b.p. for one from Kent’s Cavern, Devon; and 11,310 b.p. for one from Knocknacran, Co. Monaghan (Campbell, 1977; Mitchell, 1969; Mitchell & Parkes, 1949; Watts, 1977). The reindeer Rangifer tarandus is associated with Megaceros gzgunteus at a number of Zone I1 sites in Ireland (Mitchell, 1941) as it is also in Denmark (Degerbel, 1964). Several cave sites in Ireland, and some in England, also contain remains of Rangzyer, Megaceros, or both, and with them a rich fauna of other mammals. In Ireland for example, Castlepook Cave, Co. Clare, yielded both Rangifer and Megaceros, together with both lemmings, Lemmus lemmus and Lhcrostonyx torquaus, wolf Cank lupus, Red and Arctic foxes Vulpes vulpes and Alopex lagopus, hyena Crocuta crocuta, Brown bear Ursus arctos and mammoth Mammuthusprimigenius. Unfortunately, for this and many similar sites, there is no reliable dating; the mammals may well belong to much earlier in the Weichsel glaciation, or to a mixture of early Weichsel, Late-Glacial and Flandrian ages. In fact, a mammoth bone from Castlepook Cave has been given a 14C date of about 33,500 b.p., emphasizing that that mammoth, at least, was a much earlier inhabitant (Stuart, 1977). One cave site which does seem clearly to have a fauna of Allered age is Sun Hole, Somerset; Brown bear Ursus arctos bone has been dated to 12,378 b.p., and the associated fauna included Lemmus lemmus and Dicrostonyx torquaus, Microtus agrestis, M . oeconomus (‘M. rmceps’), Arvicola terrestris, Vulpes vulpes, Rangifer tarandus and Lepus timldus (Campbell, 1977). A carefully stratified excavation at Cat Hole, Glamorgan, which was dated by pollen analysis suggests that the wood mouse Apodemus syIvmCus occurred with a fauna of Dicrostonyx torquaus, Microtus agrestis, Clethrionomys glareolus and Sorex araneus in the Bslling Interstadial, and that it was present again in the Allered Interstadial with a fauna of Lemmus lemmus, Microtus oeconomus and M. agrestis (Campbell, 1977). The lynx has been found at a few cave sites of Palaeolithic age, but there are no firm dates for it.

So far as is known, the colder conditions of the Younger Dryas caused the extinction of Megaceros in Britain and Ireland; no reliably Younger Dryas or Post-Glacial remains of it have been found. However, the reindeer did persist into Younger Dryas times, when it was apparently common and formed an important item of diet for Upper Palaeolithic hunters. It was common, too, in Denmark at this time (Degerbel, 1964). At Ossum’s Cave, Staffordshire, the fauna included Lemmus lemmus, Dicrostonyx torquatus, Arvicola terrestris,

24 0. W. Yalden

EMMUS

Fig. 10. Distribution maps for some northern mammals (from Corbet, 1978, Corbet & Ovenden 1980). (A) Dicrostonyx twquatus Varying lemming, confined in Europe to the tundra of Russia, and with a circum-polar distribution. Occurs frequently in faunas of Younger Dryas age in Britain. (B) Lemmus lemmus Norway lemming, occurs in the tundra and montane regions of Scandinavia, while the closely related L. &ticus replaces it east of the White Sea (dotted). Frequent in Younger Dryas age faunas in Britain. (C) Microzusgregalis Northern vole, also has a tundra and (in Asia) high steppe distribution, and occurs in several Late-Glacial faunas with lemmings. (D) Microtus oeconomur Root vole, has a broader distribution in the boreal zone, with relict colonies in Holland and elsewhere. Occurs in Late-Glacial faunas, and persisted in the Scilly Isles to Bronze Age times (Pernetta & Handford, 1970). (E) Alces alces Moose (Elk), now widespread in the boreal

Clethrionomys sp., Microtus oeconomus, M. agrestis, Vulpes vulpes, Mustela erminea, M. navalis, Equus ferus, Cmus elaphus and Lepus timidus, as well as Rangifer tarandus; a radio- carbon date of 10,590 b.p. on reindeer bone places this well in Younger Dryas, Zone 111, times (Bramwell, 1977). Other cave sites in the Peak District which have yielded faunas containing Gulo, Alopex, Rangifer, Dtcrostonyx, Lemmus and Microtus oeconomus ( M . ratticeps) such as Elder Bush Cave (Bramwell, 1964) and Wetton Mill Rock Shelter (Bramwell, 1976) are likely to be contemporaneous, as are similar sites elsewhere in Britain (Savage, 1969)) but the dating of these faunas is rarely precise. One which is, Robin Hood’s Cave, Derbyshire (Campbell, 1977)) has two radio-carbon dates on wild horse of 10,590 b.p. and 10,390 b.p.; these clearly belong in Younger Dryas time, Pollen Zone 111, though Campbell inexplicaby assigns the fauna to a series of pollen zones from Zone I to Zone 11. Taking the radio-carbon dating as both correct and relevant, the fauna of Lepus timidus, Lemmus lemmus, Dicrostonyx torquatus, Microtus agrestis and M. oeconomus suggests tundra

Ongin of the Bniish Mammals 25

A SYLVATICUS I

zone, occurred in Britain during both the Allered Interstadial and in Flandrian (Pre- Boreal, Pollen Zone IV) times. It was also present in Denmark, but seems not to have reached Ireland. (F) Ran& torandus Reindeer, has a largely tundra distribution (and was introduced to Iceland). In Late-Glacial times, occurred as far south as the Pyrenees; was abundant in Great Britain and Ireland during the Allered Interstadial, and in Great Britain at least in Younger Dryas times (fossil sites after Clark, 1952). (G) Mkrofus apestis Field vole, is widespread throughout north-west Europe, including Scandinavia, but absent from Ireland. (H) Apodemu syloufuu Wood mouse, has a more southerly distribution, occurring throughout the Mediterranean and only in southern Scandinavia, but does occur both in Ireland and Iceland, presumably in both cases as the result of accidental human introduction.

conditions, and the interesting presence of pika Ochotona pusilla in the fauna concurs with the open vegetation suggested by the pollen analysis. Surprisingly, however, Apodemus sylvatkus is again present in the same layers as Lemmus and Dicrostonyx. Another fauna of Zone I11 is that from Nazeing, Essex, dated from the associated pollen stratigraphy; this contained Dicrostonyx, Arvicola terrestris, Microtus oeconomus and M . gregalis, as well as frog Rana temporaria, toad Bufo bufo and lizard Lacerta vivipara (Hinton, in Allison, Godwin & Warren, 1952); Lemmus lemmus and M . oeconomus were also present in a layer earlier than the Zone I11 horizon. At present, Dicrostonyx and Arvicola only overlap in a limited region of northern Russia, east of the White Sea; M. oeconomus, M. gregalis and Lemmus also occur in that region (Fig. 10). If this gives any indication of climatic conditions at Nazeing, and elsewhere, at that time, then they must have been of high arctic, tundra-type, which is what all the other lines of evidence suggest. In that case, the occurrence of Apodemus seems quite incongruous.

26 D. W. Yalden

Table 4 Mammals (minimum number of individuals) recorded at the important Mesolithic sites of Star Carr, Yorks. and Thatcham, Berks. (after Fraser & King, 1954; King, 1962; with modifcations from Degerbel, 1961; Mayhew, 1975; nomenclature follows Corbet, 1978). Thatcham, though the earlier

site, belongs to rather later pollen zones, indicating the earlier immigration of hazel, etc., in the south

Site Thatcham Star Carr Radio-carbon date 10,050-9600 b.p. 9500 b.p.

Pollen zone IV-v-VI IV

Carnivora Felis silvesnis Martes martes Meles meles Vulpes vulpes Canis lupus Canis jamiliaris

Erinaceus europaeus Talpa europaea Sorex araneus

Castor fiber Arvicola terrestris

Lepus ?timidus Oryctolagus cuniculus

Perissodact yla Equus jerus

Artiodactyla Capreolus capreolus Cervw elaphus Alces alces Sus scroja Bos primigenius

Insectivora

Rodentia

Lagmorpha

Wild cat Pine marten Badger Red fox Wolf Dog

Hedgehog Mole Common shrew

Beaver Water vole

Mountain hare Rabbit

Horse

Roe deer Red deer Moose (elk) Wild boar Aurochs

6 2

-

1

1

6 8 1 7 2

48 -

- 2 1 2

2 -

1 -

8 -

1 -

33 80 11

5 9

155

There is some evidence that reindeer, at least, lingered on into the Post-Glacial; one from Roddan’s Port, Co. Down, with a 14C date of 10,250 b.p. is just at the transition to Post- Glacial times, but three others from Dead Man’s Cave, Yorkshire, dated to 9940, 9850, and 9750 b.p. belong in Pollen Zone IV, Pre-Boreal, times (Campbell, 1977). The wild horse, too, lingered into Mesolithic times, at least in the Peak District (Bramwell, 1973). These species suggest the persistence of open vegetation at least on high ground into Pre-Boreal times.

However, these times were characterized by the rapid spread of birch woodland and then, in Boreal (Zone V) times by pine and hazel. Documenting the mammal fauna of these improved times, we have the important Mesolithic archaeological sites of Star Carr in Yorkshire and Thatcham, Berkshire (Table 4). At Star Carr, a 14C date of 9500 b.p. matches the pollen data which are dominated by birch and belong to Pollen Zone IV (Walker & Godwin, 1954); the fauna included Pine marten Martes martes, Red fox Vulpes vulpes, dog

Onpn of the British Mammals 27

Canis familiaris, badger Meles meles, hedgehog Erinaceus europaeus, Roe deer Capreolus capreolus, Red deer Cervus elaphus, moose Alces alces, Wild boar Sus scrofa, aurochs Bos primigenius, beaver Castorfiber and hare Lepus sp. (Fraser & King, 1954). The hare, a single tibia, was considered on size to be probably L. capensis (‘L. europaeus’), Brown hare, by Fraser & King (1954), but Mayhew (1975) notes that there is cline of size in the Mountain hare L. timidus, with largest animals occurring furthest north; after re-examining the Star Carr specimen, he feels that it is at least as likely to be a large Mountain hare as a Brown hare, on size, and more likely to be so on climatic and other grounds. At Thatcham, the occupation of the site extended, on radio-carbon dating, from about 10,050 b.p. to 9600 b.p., and it is evident that the fauna reported from the site also accumulated over this period. The pollen-zonation, however, extends from Zone IV, birch dominant, through Zone V, with birch and pine, into Zone VI, with hazel and pine; although contemporaneous, in its later stages at least, with Star Carr, the more southern site has a ‘later’ vegetation (Churchill, 1962). More accurately, the two sites demonstrate that, as expected, the plants were spreading back into Britain from Europe in waves, and naturally established themselves more rapidly in the south. The fauna at Thatcham includes Common shrew Sorex araneus, mole Talpa europaea, hedgehog, Pine marten, badger, Wild cat Felis silvestris, fox, dog, wolf, Water vole, beaver, rabbit Oryctolugus cunkulus, horse, moose, Red deer, Roe deer, aurochs and boar. It is clear that, at an early post-glacial date, an essentially modern, warm- temperate, fauna had already established itself in Britain. It is rather unfortunate that at neither site were the remains of small mammals collected, but since the mole and Common shrew were present at Thatcham, and such a characteristic member of the deciduous woodland fauna as the hedgehog was present at both sites, all our common rodents (Microcus agrestis, Clethrionomys glareolus, Apodemus sylvaticus) must also have been present. Possibly the dormouse, Muscardinus avellanarius, would not have been present until hazel established itself (in Pollen Zone V), and the Yellow-necked mouse Apodemus flaokollis, which is apparently characteristic of old deciduous woodland (Montgomery, 1978), might also have still been absent. One remarkable presence in the Thatcham list needs comment, since the rabbit is usually regarded as an introduction by the Normans (certainly it is notable for its absence in Neolithic, Iron Age, Roman and Anglo-Saxon sites). There is no doubt that the identification of the Thatcham bones (a pelvis and tibia) is correct (King, 1962; Mayhew, 1975), and apparently no doubt that they were a genuine part of the fauna (rather than a later intrusion). The rabbit is characteristic of open country, and it is possible that it was unable to survive the development of closed woodland during the Atlantic period, Pollen Zone VII. It was evidently common in France during the Mesolithic; Jarman (1972) lists 13 sites there where it is recorded, as well as one each from Italy and Spain. More records of its presence in the Mesolithic of Britain are needed to confirm its status here.

The subsequent history of the British mammal fauna is poorly documented, and the record for Ireland is particularly tantalising. Grigson (1978) has assembled what worthwhile records exist, from archaeological sites, for the large ungulates; aurochs, Red deer, Roe deer and Wild boar are regularly present at sites of Mesolithic, Neolithic, and even later dates. However, reindeer have not been reported from any sites later than Pre-boreal, and it seems increasingly unlikely that it really survived (as argued on the basis of Norse sagas) into the 10th century (A.D.) in Scotland (cf. Corbet, 1974; Perry, 1978). Similarly, the moose (elk) Alces alces is not recorded from sites later than early Mesolithic (Pollen Zone IV/V boundary); while it may have lingered longer in Scotland than in England (where most of the Mesolithic sites are located) the possibility of its survival into near-historical times (Perry, 1978) seems very unlikely. The evidence for Wild horse is much more equivocal. There are a number of well-dated records through to at least Atlantic times (Pollen Zone VIIA). If it is

28 D. W. Yalden

true that the horse was not domesticated, nor brought to Britain, until Neolithic times, then these seem to represent late surviving Wild horses. However, it is not possible to distinguish Wild and domestic horse remains, so the problem cannot readily be solved.

A more extensive review of mammals from archaeological sites in prehistoric Britain, also compiled largely by Grigson, is given in Simmons 8c Tooley (1981). For the ungulates, this confirms what has been written above. Among the interesting records for other mammals, Brown bear is reported for various sites through to Bronze Age times, when a bear femur has been dated by 1% to 2673 b.p. The beaver survived through at least to Bronze Age times; it is reported from at least four Neolithic sites, and it was sufficiently numerous in Fenland in Bronze Age times for Mayhew (1978) to attempt a reconstruction of its population structure. Many of the other British mammals are recorded from Neolithic or Bronze Age sites (Pigmy shrew, polecat, Wood mouse, Field vole, Red squirrel, Brown hare) but these dates are too late to tell us when these species first arrived. Apodemusflawicollis has been reported from the Neolithic of Dowell Cave and the Mesolithic of Etches’ Cave, both in Derbyshire, but details to support the identifications are needed. The abundance of Bechsteins’s bat Myotis bechsteini in the Neolithic site of Grime’s Graves, Norfolk (Clarke, 1963) is noteworthy, since the species is now one of the rarest in Britain; its decline is probably related to the destruction of forests, since it seems to be a woodland species.

The record of mammals from Irish archaeological sites has been most valuably reviewed by Wijngaarden-Bakker (1974), who had the problems posed by this review very much in mind. She argues fairly convincingly that the domestic cat was not introduced into north-west Europe until Roman times, and that remains of Felis from Neolithic and other sites in Ireland represent Wild cat; careful measurements of the bones seem to confirm this. She also presents evidence that Brown bear, Wild boar and Red deer were present in Neolithic (Beaker) times, and can therefore be accepted as native species. Hare bones (presumably Lepus timidus) are also recorded from several archaeological sites, but the possibility that they were intrusive cannot be excluded. However, the more recent excavation of the Irish

Distribution of small mammals in the British Isles and Channel Isles (After Corbet, 1961; Corbet, Table 5

Common vole Field vole Bank vole Microtus Microtus Clethrionomys arvalis agrestis glareolus

Shetland, St. Kilda Orkney Ireland, Man Lewis, Barra, (O.H.) N. & S. Uist (O.H.) Eigg, Muck (I.H.) Raasay (I.H.) Mull, Bute (LH.) Skye, Islay, Jura, Gigha, Arran (I.H.) Skomer Scillies Jersey Sark Alderney, Herm Guernsey

lAmold (1978) included single records of Sorex araneus from Lewis and Clethrionomysglareolus from Islay. These ’Strictly, the Common shrew on Jersey is Surex cmonatus (see Meylan & Hawser, 1978).

origin of the Bn2ish Mammals 29

Mesolithic (Larnian) site of Mount Sandel Upper (with a 14C date of 8725 b.p.) apparently shows that Lepus timidus was the principal mammalian prey, though Red deer and Wild boar were also present (Woodman, 1978). The scarcity of Red deer and Wild boar at this and other Irish sites (as well as the undoubted absence of moose, aurochs and Roe deer from Ireland) has led to the suggestion that ungulates were genuinely scarce there in Mesolithic times; it seems more likely that this was a cultural difference.

One other species which might have been native to Ireland in Flandrian times is the Wild horse; as with the cat there are several sites where horse bones are present, at dates which are much earlier than the supposed introduction of domestic horses (Wijngaarden-Bakker, 1974).

For other islands, there are rather few useful records of mammals in archaeological contexts. However, in Orkney, remains of Orkney vole Microtus arvalis orcadensis have been found at the archaeological site of Skara Brae, at about 4000 b.p. (Berry & Rose, 1975); by Neolithic times they had (already?) acquired the large size which is one of their distinguishing features (Corbet, 1979; Bramwell, 1980). Apodemus was also present in Orkney in Neolithic times (Corbet, 1979), and Red deer are reported from several archaeological sites there, including Skara Brae. The northern vole Microtus oeconomus, which was still present in Boreal times at Nazeing (Allison et al., 1952) survived to Bronze Age times in the Isles of Scilly (Pernetta & Handford, 1970).

The introduction of various mammals seems to be very poorly documented in the archaeological record (or has perhaps been hidden in the literature). The House mouse, Mus musculus or Mus domesticus, was presumably introduced in Neolithic times; it was certainly present in pre-Roman times, being recorded from the Iron Age site of Gussage All Saints (Corbet, 1974) and in a Peak District cave (Yalden, 1977). The Black rat Ru#us rmus is generally supposed to have been introduced to Britain in Mediaeval times, perhaps by returning Crusaders (Corbet, 1974). However, two skulls from a well in York, dated to 1840 b.p. (1 10 A.D.) suggest that it was brought by the Romans (Rackham, 1979). (If indeed

1971; Arnold, 1978). (O.H., Outer Hebrides; I. H., Inner Hebrides; see Fig. 14)

Wood Mouse Common shrew Pigmy shrew White-toothed shrews Apodemus Sorex Sorex Crocidura Crocidura sylvaticus araneus minutus suaveolens russula

+ + + + + + + + + + + + + + +

need confirmation before they can be accepted.

30 D. W. Yalden

plague was responsible for ravaging Ireland in 664 A.D.-the 'Plague of Cadwalder's Time', Twigg, 1978-then rats must certainly have been present before Mediaeval times).

These snippets of information, interesting though they certainly are, do not provide any clear and systematic account of the history of the mammal fauna in the British Isles. There is an enormous archaeological literature, and undoubtedly it contains further information of relevance to this review. Archaeologists have concentrated on bones from animals with an obvious economic relevance, as tools, food remains or sources of clothing, for example. The small mammals, especially the rodents, which would yield far more information on the habitat, have been neglected; and it is the small mammals which pose the major questions for our understanding of mammalian distribution and history.

(ii) Problems of distribution Some of these problems have already been mentioned in the introduction and elsewhere; they include both the general balance of the island faunas and the specific absences and presences of certain species on certain islands (Table 5).

Ireland has, or had, most of the large mammals which were present at Star Carr in the Mesolithic: Pine marten, badger, Red fox, Red deer, wolf, Wild boar, and horse apparently got to Ireland, though the moose, aurochs, Roe deer, and beaver apparently did not (Savage, 1966). The Irish fauna includes also the Mountain hare, stoat, Pigmy shrew and Red squirrel, which are presumably native, as well as the Wood mouse and hedgehog whose status is equivocal. There are also a number of species which are quite clearly human introductions-House mouse, Brown rat Ranus norvegicus, Grey squirrel Sciurus carolinesis, Brown hare Lepus capensis, Fallow deer Cervus duma, Sika Cervus nippon, and Bank vole. Of these, the Wood mouse is the most difficult to explain, since it is in fact regularly recorded from Irish caves; since the caves regularly contain refuse of such obvious domestic (Neolithic or later) introductions as sheep and cattle, as well as extinct forms such as lemmings, the cave faunas do not help to decide when the Wood mouse reached Ireland (Savage, 1966), and there are good reasons to suppose that it did not do so naturally (Berry, 1969).

The absence from Ireland of Field vole, Water vole, Common shrew, Water shrew, mole, and weasel and (until recently) Bank vole is equally noteworthy. The Field vole, Water vole and Common shrew all occur, in Scandinavia, to about 70"N (Table 3), and one might have expected them to be early colonizers of Ireland. The absence of the weasel may simply result from the absence of the Field vole, its principal prey. There is in fact, a single record of a Microtus skull from Ireland, from a cave site at Kilgreany, Co. Waterford, but Savage (1966) has argued convincingly that it is recent, not fossil or sub-fossil, and had almost certainly been dropped by a passing owl; Corbet (1971) drew attention to a rather similar record of three skulls from a Barn owl ( T ~ o alba) pellet on the Isle of Man (where Microtus agrescis is similarly absent). The presence of the Bank vole in Ireland was only discovered in 1964; Fairley (1971) delimited its distribution in 1970, and Smal & Fairley (1978), by plotting its rate of spread to 1975, were able to suggest that it had arrived in about 1950.

The Isle of Man, like Ireland, has only three small (terrestrial) mammals, the House mouse, Wood mouse and Pigmy shrew, while the only carnivores are the stoat and the polecat-ferret. Both Brown hare and Mountain hare (as well as rabbit) are now present, but the Mountain hare is known to be a recent introduction, and it is probable that the Brown hare, too, was introduced. Conversely, Red deer, Roe deer, and Red fox (as well as Meguceros giganteus, mentioned before) are known from sub-fossil remains to have reached the island in Late- or Post-Glacial times, and it seems likely that some other species (Wild cat, Pine marten, badger, Red squirrel, moose, for example) did so too. Their subsequent


extinction was presumably due to a combination of direct persecution and habitat destruction (Garrad, 1972).

Shetland and St Kilda have Apodemus and Mus (the latter now extinct on St Kilda) but no other small mammals; the stoat has been introduced to Shetland, as has the hedgehog, in recent times-the 17th century and about 1860 respectively (Berry & Johnston, 1980).

Orkney, as already noted, has Microtus arvalis, as well as Apodemus sylvaticus and Sorex minutus; of the Outer Hebrides, Lewis and Barra have, like Ireland and Man, only Apodemus sylvaticus and Sorex minutus, while Uist also has Microtus agrestis. In the Inner Hebrides, Raasay has Clethrionomys glareolus but lacks Microtus agrestis; Eigg and Muck, Skye, Jura, Islay, Gigha and &ran, have Microtus agrestis but not Clethrionomys glareolus; while Mull and Bute have both. The Outer Hebrides, as well as Eigg and Muck, also lack Sorex araneus. There is little pattern here, except the ubiquitous distribution of Apodemus sylvaticus, and this haphazard series of Occurrences strongly argues for human introduction, accidental or deliberate, having played a major role. Bute, close to the mainland and with a ‘complete’ small mammal fauna (all five of the widespread species) may well have

: RUSSULA

- I . C SUAVEOLENS

Fig. 11. Distribution of some southern mammals (after Corbet, 1978). (A) Crocidura russula Greater white-toothed shrew, extends north only to about 53”N, though it does occur on some (Guernsey, Herm, Alderney) of the Channel Isles. (B) Crocidura suaveolens Lesser white-toothed shrew, has a more southern distribution, generally, than C. russula, but occurs on the Scilly Isles, some of the Channel Isles (Jersey, Sark) and also on the French isles of Sein, Yeu, and Oessant (Ushant), which are well north of the rest of its range. (C) Muscardinus avellanarius Dormouse, has a rather southern distribution, but occurs in southern Britain and southern Sweden. (D) Microrus arwalis Common Vole, widespread in much of Europe, is largely absent from Scandinavia and Britain. It does occur in Orkney and on Guernsey.

4USCARDlNUS

I ‘ IICROTUS ARVALIS

,?I ,

32 D. W. Yalden

received them naturally, but this is increasingly less likely with the more remote islands and incomplete faunas.

At the other end of Britain, Skomer, like Raasay, has a large form of Clethrionomys, as has Jersey, and Microtus is absent from all three. The Scilly Isles have a White-toothed shrew, as do the Channel Isles. However, in the Scilly Isles, Jersey and Sark this is Crocidura suaveolens, while in Guernsey, Herm and Alderney it is C. russula, the larger and more widely distributed species (Delany 81 Healy, 1966). (C. russula occurs in northern France, along the Channel coast (Fig. l l ) , while C. suaveolens has a much more southern distribution, mostly south of the Loire; St Girons, 1973). Guernsey, lacking Clethrionomys, has Microtus arvalis, Jersey has Sorex araneus (strictly, S. coronatus, see Meylan i? Hausser, 1978), and all the Channel Isles lack S. minutus. This is, again, a rather irregular pattern, which cannot be explained logically; Jersey, much the closest to the mainland of France and separated by a channel only 18 m deep, ought to have received a full complement of small mammals.

(iii) Genetic studies The supposed distinctiveness of the various island forms of small mammal played an important part in the formulation of early theories of their arrival. Many, at one time or another, were rated as full species, and this implied that they had been separated from their ancestors for many thousands of years. Thus, the large forms of Bank vole on Mull, Raasay, Skomer and Jersey were all regarded as full species, and considered to be more closely related to the form occurring in the Alps (Clethrionomys nugeri, now C. glareolus nagen] than to the rest of British Clethrionomys glareolus (Beirne, 1952); Ellerman 81 Morrison-Scott (1951) even went so far as to transfer the Raasay and Jersey forms (C.g. erica and C.g. caesarius) to the quite distinct, northern, species C. rufocanus. Similarly, the large island races of Apodemus (e.g. hirtensis on St Kilda,fridariensis in Shetland, hebridensis in the Outer Hebrides) have all been regarded at some time as full species, and Ellerman i? Morrison- Scott (1951) assigned them to the distinct, and larger, species A . jlavicollis. In Microtus agrestis the situation was slightly different in that a large form M.a. neglectus inhabits northern Scotland, and was presumed to be closely related to various subspecies named from the Hebrides, while a smaller form (M.u. hirtus), occurring southwards from the Great Glen in Scotland, England and Wales, was presumed to have been a later immigrant which displaced M.u. neglectus from these southern parts. The story was further complicated because Mkrotus arvalis, now present only in Orkney and Guernsey, was presumed to have been a still earlier immigrant, itself displaced by M. agrestis neglectus stock.

The first phase of revision of these views came from a traditional taxonomic approach, which demonstrated that the distinctiveness of these island forms had been grossly over- rated. For Clethrionomys, Corbet (1964) showed that there was a cline of size northwards through Britain, and that a comparison of the smallest forms (from S.E. England) with the island races exaggerated the size difference, especially if the difference in size between younger and older animals was also ignored. Corbet further showed that the difference in the form of the upper third molar (m3)- ‘complex’, typically, in voles from Skomer, Raasay and Jersey, but ‘simplex’ on Mull and the rest of Britain-was in fact also only a difference between the extremes of a fairly continuous range of variation (Fig. 12); ‘complex’ individuals occur in mainland Bank voles, and one population, from an isolated forestry plantation at Borland, Loch Tay had a complex pattern when first discovered (Corbet, 1963), though it has since reverted to a simplex pattern (Corbet, 1975). Further it has been shown by breeding experiments in the laboratory that these island races do indeed interbreed readily

10 RAASAY (R)

10 JERSEY(J)

101 MULL (MI

10- SKOMER (SKI

10-

10-

- 2

-

- L TAY -BALNEARN (T) 20-

10-

1C-

SUFFOLK (SF) 20

10

CAP GRIS NEZ (C)

7, , , - , - , ~

10-

(C ) I I

160 200 360,um

RAASAY m

JERSEY

1

MULL I

5- SKOMER

SUFFOLK I n I #

CAPGRISNEZ I I

16 17 18 19 20 HIND FOOT LENGTH (rnm)

(D) l5

Fig. 12. Variation in the Bank vole Clethrionomys glareolur. The third upper molar (m3) is variable; a fourth inner angle may be present, and varies in size from absent (A), the ‘simplex’ condition to well developed, (B), the ‘complex’ state.

Measurement of the size of this fourth loop shows (C) that most populations are predominantly simple, but the island populations of Raasay, Jersey and Skomer are typically complex. At Loch Tay, one plantation (Borland) also had a ‘complex’ population in 1955-57, but by 1975 this had reverted to ‘simplex’. The populations also vary somewhat in general body size, of which hind foot length (D) is a good indicator; the four island populations are in general larger than the others, but there is a spread in all populations, and a cline from south to north in Britain. The map (E) shows the collecting sites for voles shown in (C) and (D); localities are identified by their initials, in brackets, on (C). (Based on Corbet, 1964).

JERSEY(J)

GUERNSEY (G) l;lr ALDERNEY (A) u SARK (S)

1 HERM (HI u I -n

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ST. W R Y

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. -1; CF

A FLAVICOLLIS

CORNWA

E CAPGRlSNEZ( D NEW FOREST(NF) I

Fig. 13. Variation in the Wood mouse Apodemus syloclricus. Wood mouse populations differ markedly in size. Variation in a single metrical character, maximum skull length, is shown in (A) for some southern populations, including the Channel Isles. The larger relative, A.&oicdlis, the Yellow-necked mouse, is included in the comparison; note that Wood mice from Herm are as large. The mean for each population is shown by the arrows, and at the top of the diagram, further arrows indicate the position of the mean for other populations; note that some of them average larger than A.fiwtiollis.

A better way of comparing populations, using simultaneously a number of measurements on each individual specimen, is multivariate analysis. The results of such an analysis may be expressed graphically in the type of diagram shown in (B), (C), and (D). Here, the centre of each circle is the mean for a population/ sample with respect to two canonical variates, and the circle expresses the range of individual variation round the mean-it should include 90% of the individuals of that population. In (B) the samples shown in (A) are compared; note that the mice on all the Channel Isles are very similar (their circles overlap extensively), the mice from S. England and France are very similar, and A. flaoicollis is quite distinct. In (C), a similar comparison of mice from the Scilly Isles (Tresco and St Mary's) with those from Mull (samples from 2 years) and various English and Scottish sites is made; there is considerable overlap between all of them, and only the St Mary's population is reasonably dstinct. In (D), various Scottish madand and island populations are compared; the mainland sites plus Lewis, Raasay and Mull form one cluster, the other Outer Hebrides (Barra, S. & N. Uist) form another, and Rhum and Colonsay populations are also distinct.

The collecting sites are shown on E, and most are identified by their initials (in brackets) on (AHD); the others are CF, Charnwood Forest; F, Foula; FI, Fair Isle; MO, Monikie; SK, St IGlda; SN, Sunart; W, Wytham Wood. (Based on Delany & Healy, 1964, 1967, 1967b; Delany & Whitaker, 1969; Delany, 1970).

w I&

P 3 2 F

h g t n of the British Mammals 35

with mainland bank voles, and their distinctive features are quite simple genetic differences; the pale colour of Skomer voles, for instance, seems due to a single gene which is dominant to the normal colour, and complex molars are also dominant to simplex (Steven, 1953, 1955).

For Apodemus sylvaticus, a similar taxonomic review was provided by Delany and his co-workers (Delany, 1964; Delany & Healy, 1964, 1967a, 1967b; Delany & Whitaker, 1969). They showed that, for instance, the mice of Lewis in the Outer Hebrides were scarcely distinguishable from those of nearby mainland Scotland, yet those of the other Outer Hebrides (North and South Uist, Barra) were more distinct. The rather continuous range of variation in size from the smallest mice (in mainland Scotland), to the largest (on Fair Isle and St Kilda) was well illustrated by Delany (1970), and the rather random nature of variation in colour between island populations was also indicated (Fig. 13). Laboratory breeding experiments have indicated that even the largest of these island forms will reproduce with the smallest of mainland forms (and also shown that they will not cross with Apodemus flavicollis), emphasizing their conspecificity (Jewel1 & Fullagar, 1965).

A full account of work done reanalysing the variation in Microtus ugrestis has not been published, though some results have been mentioned. (Corbet, 1961; Evans, in Corbet & Southern, 1977). The first upper molar (ml) often has an extra postero-internal loop (Fig. 14), the ‘complex’ condition; this condition is prevalent in voles from Scotland north of the Great Glen, from the Outer Hebrides, and from Skye, Scalpay, Eigg, Islay and Luing in the Inner Hebrides. The other islands of the Inner Hebrides which have voles, together with Britain south of the Glen, have populations predominantly simplex. The rather sharp discontinuity at the Great Glen suggests to Corbet that the complex condition is indeed characteristic of an earlier immigrant form, which got itself, naturally, to Skye and (via Jura) to Islay, and by human introduction to the other islands. It was then replaced by a form with simplex molars (M.a. hzrtus) in southern Britain which spread northwards, displacing the complex (M.a. neglectus) form south of the Great Glen, and also from Jura, and getting itself either naturally (to Bute, Lismore and Mull) or by human introduction (certainly to Muck, Gigha and Arran; Corbet argues, to Mull as well) to the other islands where it occurs. One could imagine other possible explanations, e.g. the ‘complex’ form could have been introduced to northern Scotland and the islands (along with Apodemus, see below) from Scandinavia. Direct evidence, in the form of fossils, is clearly needed. Corbet (1975) does, in fact, present evidence that M. agrestis on Jura had which were more evidently complex in Neolithic times, and have shown progressive simplification; this might be interpreted either as the immigration and increasing prevalence of a simplex gene, or as gradual selection acting on the natural variation in expression of the complex condition. The latter seems more likely.

A further advance from the traditional taxonomic approach has come from the study, especially by Berry and his co-authors, of epigenetic variation. There are a number of minor skull characterssmall foramina for nerves and blood vessels which may be single or double, for example-whose expression in different populations varies greatly; Hedges (1969) provides good illustrations of a number of these features (Fig. 15). The frequency of the different variants is very different in different populations; the particular combination of frequencies characterizing each population seems to act as a genetical marker for it. Comparison of the frequencies in different populations therefore indicates how closely they are related. Berry (1969) used such comparisons for the various northern island populations of Apodemus sylvaticus; he was able to demonstrate very clearly that all the Outer Hebridean populations, and those from Shetland, St Kilda, Ireland and Iceland, were more closely related to Norwegian Apodemus than Scottish mainland mice (Fig. 16). The clear implication is that they were transported to the islands by the Vikings. Extending his study to Apodemus from further south in Britain, he noted that Apodemus from St Mary’s, Scilly Isles, are

18rn (10 fathoms)

Fig. 14.

LEWIS @

Variation in the Field vole, Microtus agrestis. The first upper molar (m’) of M. agrestis may be simple (A) or may possess an extra postero-internal loop (B) (like that typically found on m2 in this species). Populations north of the Great Glen in Scotland typically have the extra loop (are ‘complex’), while those further south in Britain have ml simple. The populations of voles in the Hebrides present a confused pattern. Most of those north of the line of the Great Glen have complex ml (indicated by black silhouettes) but Muck, and also Mull and Lismore at the southern end of the Glen, have simple ml (white silhouettes). Conversely, most of the more southern islands have voles with ml simple, but Islay and Luing have them with ml complex.

Some of these islands (those lying close in shore, and separated by channels shallower than, say, 10 m) were probably colonized naturally by voles from the nearest mainland-this may apply to Skye, Mull, Lismore, Luing, Scarba, Jura, Islay and Bute. In that case, the Islay population, with its complex molars, may be a relict population of an early colonization, displaced later on Jura by ‘simple’ voles. For the other islands, accidental human introduction seems to have occurred. This is certainly true for the three islands of the Outer Hebrides where the vole occurs, and also for Muck, Eigg, Colonsay and Arran, which are separated by Channels 50 m deep. Other islands seem to lack Field voles, though they are no more (or less) isolated. (Based on Corbet, 1961).

origin of the British Mammals 37

1/2 "

112

( B )

( c 1

( D )

Fig. 15. Epigenetic variation in the skull of Apodemuc sylouficus. (A, dorsal; B, ventral; C, inner or lingual; D, outer or buccal).

There are a number of small foramina which may be single or double (1/2); other foramina, and small bones and bony processes, may be present or absent (k), while one foramen may be absent, single or double (0/1/2). These variants occur in different proportions in different populations, and the extent of similarity seems to reflect how closely related the populations are. (Based on Hedges, 1969).

similar to those of Cornwall (but those on Tresco are rather different, presumably by divergence in situ) and those from the Channel Isles were also closest to those from Cornwall (Berry, 1973). Partly as a result of this last resemblance, he argued for the survival of Apodemus sylvaticus in the Channel Isles through the Last Glaciation, but this is hardly compatible with the geological evidence cited earlier. Berry's argument is rather weakened by the fact that he had no sample of Apodemus from the Cotentin Peninsula, nearest to Jersey, but used a sample from Cap Gris Nez. Since he found evidence of some dissimilarity between Apodemus populations of eastern and western England, the apparent distinctiveness of Channel Isles mice from those of western France remains to be investigated.

Another very useful application of epigenetic variation concerned the populations of Microtus arvalis in Orkney and Guernsey; these were compared with various Continental samples by Berry & Rose (1975). Not surprisingly, the Guernsey population was indistinguishable from one from northern Germany, and (in the absence of a sample from northern France for comparison) it seems clear that it has been introduced to Guernsey from the nearby continent in relatively recent times. On the other hand, the Orkney population was closest, not to north European voles, but to a sample from Yugoslavia. This implies that M. arvalis was carried to the Orkney by the earliest Neolithic settlers, but this is compatible with their presence in Neolithic archaeological sites. It would be interesting to examine

38 D. W. Yulden

Fig. 16. Apodemus sylwaicus, routes to the Isles. From an examination of the epigenetic variation in sixteen island populations of Wood mice, Berry (1969) concluded that most of them were more closely related to mice from Norway than mice from Scotland. He suggested seven separate introductions of Norwegian mice (1) to Iceland; (2) to St Kilda; (3) to Yell, thence, separately to Foula and Fair Isle; (4) to Barra (B), thence to South Uist (SU) and Lewis (LE); (5) to Eigg (E); thence to Rhum (RH) and Canna (CA), and from Canna to North Uist (NU); (6) to Rathlin Island (RI); (7) to Ireland. The mice of Mull (M) and Colonsay (C) came from the Scottish mainland. (After Berry, 1969).

Apodemus from Orkney for epigenetic variation, since they too were present in the Neolithic levels (Corbet, 1979). It seems clear that Danish settlers, who would a priori seem the most likely culprits for carrying rodents to the Orkneys, cannot be blamed (and M. arvdis does not occur in Norway, so the Vikings are unlikely to have been involved).

It seems remarkable that the shrews have scarcely figured in any of this work; neither classical taxonomists nor those examining epigenetic variation have considered variation in Sorex araneus or S. minutus. The latter, in particular, is widely distributed on the islands, and an examination of its variability might help to establish whether it reached any or all islands unaided.


A WORKING HYPOTHESIS

It seems worthwhile at this point to assemble an overall view of the history of the British mammal fauna in Post-Glacial times, combining the various lines of argument already given. It will then be rather easier to see the outstanding difficulties in perspective, and perhaps suggest ways in which, by further research, they might be resolved.

From around 20,000 to 15,000 b.p., the north of Britain was covered by an ice sheet, which also spread across the Irish Sea, and covered most of Ireland. South of this ice sheet, severe periglacial conditions are indicated, on geological, beetle and pollen evidence. There is no direct evidence of any mammal fauna at that time; if there was one, it would have been a high Arctic fauna, and unlikely to have included any of our present day mammal fauna (cf. Watson, 1977; Coope, 1977).

From 14,000 or 13,000 b.p. to 11,000 b.p., the period of the Windermere Interstadial, there was a temperate interlude when good plant cover, albeit of an open, herbaceous, type, developed in Britain and Ireland; towards the end of this time, open birch scrub developed (recognized as the Allered Interstadial, Pollen Zone 11, 12,000-1 1,000 b.p.). During this period, reindeer and ‘Irish elk’ (Megaceros giganreus) were numerous in Ireland, and present too in Britain and the Isle of Man; presumably terminal moraines of the last ice sheet left a ridge across what is now the Irish Sea, which they could use as a land bridge. Lemmings also occurred in Ireland at some time during the Last Glaciation; whether this was in an early phase, after which expansion of the ice sheet exterminated them, or contemporaneously with the ‘Irish elk’ in Allered times, is not certain.

From 11,000 to 10,000 b.p., the climate was again much colder. Deteriorating conditions seem to have exterminated the Irish elk and reindeer in Ireland; in Britain, further south from the ice of the Loch Lomond Readvance, a mammal fauna including reindeer, lemmings and the northern voles Microtus oeconomus and M . gregalis seems to have been present; certainly the beetle fauna of this time indicates a high arctic climate, even at a southern site such as Hawks Tor in Cornwall (Coope, 1977). It seems probable that a few of the present- day British mammals were present here then, since Microtus agrestis and Lepus timidus were present at Robin Hood’s Cave, and Arvicola also at Nazeing.

At about 10,300 b.p., there was a sudden warming of the climate; within about 50 years conditions changed from arctic (with mean annual temperatures of -6°C) to warm temperate (with mean annual temperatures of 10°C, and July temperatures of 16°C). With this rapid improvement, plants, beetles, mammals and molluscs typical of warm temperate regions spread back into Britain very rapidly. The animals did so very quickly (Osborne, 1980), but it took deciduous trees over 1000 years to establish full forest conditions. Of the mammals, we know from Thatcham and Star Carr that several species which are typical today of deciduous woodland (Roe deer and hedgehog, for example) had arrived in Britain by 9500 b.p. Probably all of the native British mammal fauna was present by that time, since the two most characteristically southern species, the dormouse Muscardinus avellanarius and Yellow-necked mouse Apodemus Javicollis, would have found sufficient food in the deciduous woodlands of Pollen Zone VI, which were already forming at Thatcham by about 9500 b.p.

This general warming of the climate caused a further melting of the ice sheets, resulting in a rapid rise in sea level. If the English Channel was already a valley as deep as it now is, it would have flooded by 9000 b.p., at about the beginning of Pollen Zone VI. Whenever this event occurred, it marked the end of natural immigration of land mammals. (Interesting confirmation of this suggested timing might be obtained from events in the Baltic area. There was a land bridge from Denmark to Sweden in the period 8500 to 8000 b.p. and the ‘Baltic

40 D. W. Yalden

Sea’ was then a freshwater lake-‘Ancylus Lake’; subsequently, the land bridge was flooded, and marine conditions returned to the Baltic (West, 1977). Now southern Sweden has practically all of the land mammals, and reptiles, that reached southern Britain, with the exception of the Brown hare, introduced into Sweden in the 19th century. Conversely, those more southern mammals-Eliomys, pitymys, Crociduru, Microtus urvalis-which are absent from Britain are also absent from Sweden. This similarity in faunas strongly suggests a similar timing of invasion and isolation).

Immigration to Ireland, and the other islands, remains much more difficult to explain. Since isostatic depression of Scotland and north-east Ireland actually produced a raised sea level of +22 m at 12,500 b.p. and +5 m at 10,000 b.p., the low sea levels around southern Britain at these times are irrelevant. Moreover, the present channel between Scotland and Ireland is at -53 my and could not possibly have provided a land bridge in its present topography. The period of minimum sea level, around 8000 to 7800 b.p. (Donner, 1970; Cullingford, Caseldine & Gotts, 1980) seems to offer the most likely time for the native mammals to have reached Ireland, and probably also the Isle of Man. However, even that would not have been possible given the present topography of the relevant sea floors; there must have been morainic ridges which, since the sea broke through, have been scoured away. However, a land bridge present at 8000 b.p. would have been available to, essentially, the whole of the present land fauna and flora; the absence of so many forms from Ireland needs to be explained as much as the presence of the forms that did get there. It seems likely that any land bridge was, at best, low-lying, waterlogged, and perhaps dissected by streams or rivers; the deep (over 100m) but narrow channel down the centre of the Irish Sea was presumably formed as a river valley draining the melting ice cap, and probably already extended north as far as it does now (Figs 8,20). Corbet (1961) suggests that a south-flowing river may have acted as a filter, allowing some plants and animals to cross but not others; considering the mammals, the larger mammals would have had least difficulty in crossing such a barrier, and those small mammals which occur in saturated or coastal environments would also have been favoured. It seems significant that Pigmy shrews, which outnumber Common shrews on wet peaty moorland (Yalden, 1981; Butterfield, Coulson & Wanless, 1981) reached Ireland and Man; Common shrews and moles, both of which burrow extensively and feed heavily on earthworms, did not. Even so, it is surprising that neither Field voles Microtus ugrestis nor Water voles Amicolu terrestris managed to cross, since both are also found in damp habitats; on the other hand, coastal conditions may be unattractive to them, since they obviously rely on particular types of grassland for their food. It is notable that Arvicolu has apparently failed to reach any of the Hebrides, not even those islands (e.g. Skye, Mull) which are close to the mainland (map in Arnold, 1978).

Alternatives to a land bridge to Ireland in the period around 8000 b.p. might be extensive human introduction, or use of the land bridge in Windermere Interstadial times followed by survival through the Younger Dryas period. Such species as Lepus timidus, Sorex minutus and Meles meles are very unlikely to have been introduced by man; hares are notoriously temperamental, difficult to catch and handle, shrews starve too easily, and badgers, conversely, are rather aggressive. The possibility that the native mammals used the earlier land bridge, which Meguceros and reindeer certainly used, to get to Ireland for Allered times, and then survived through the deterioration of Younger Dryas times, is a more tempting explanation. The presence of some, at least, of the native species at 8700 b.p. (Woodman, 1978) also argues for their use of an earlier land bridge. The arguments about the land bridge being a filter would, of course, still apply; moose, for example, did not get into Ireland, so far as we know, in either Allered or Flandrian times. The main difficulty with this explanation is that, of the Irish fauna of Allered times, only the reindeer is known to have survived through


Younger Dryas times. However, the fauna is very poorly documented, and more, reliable, records are needed to resolve the problem.

The subsequent history of the British mammal fauna has been largely governed by the activities of man. The extinction of Megaceros in Ireland was presumably caused by climatic changes (since man did not reach Ireland until after 9000 b.p., Mitchell & Parkes, 1949; Smith, 1974; Woodman, 1978) as were the extinction of lemmings and northern voles in Britain. Lemmings rely largely on sedges (Lemmus) or willows (Dicrostonyx) for their diet and cannot survive on heathland plants (Jung & Batzli, 1981); the loss of the tundra sedge- willow communities could therefore account for their extinction. Subsequently, reindeer, horse, moose, aurochs, beaver, bear, boar and wolf were exterminated, probably in that order, and the later the extinction, the more certain it is that man was responsible; habitat change, though was certainly a factor that militated against all of these species.

These extinctions were reviewed briefly by Corbet (1974), and more extensively by Perry (1978). These extinctions were probably the result of deliberate hunting, for fur or for protection of crops, stock or people. Destruction of the forest cover, responsible for the extinction of several beetle species, and for the reappearance of open ground molluscs in Mollusc Zone e, has also affected the mammals. The Roe deer, still common in Medieval times, was probably extinct in England and Wales by about 1730; it was reintroduced to Dorset in 1800 and to the Breckland of East Anglia in about 1884 (Page, 1962). The Red squirrel Sciurus vulgaris similarly became extinct in Scotland about 1840 and in Ireland by 1800. While it is possible that a few Red squirrels survived in Speyside or further north (and similar claims have been made for Roe deer in northern England), it is clear that their present status derives from a series of reintroductions, starting at Dalkeith in 1772 and Dunkeld in 1793 (Ritchie, 1920); similarly, they were introduced to Ireland from about 1815 (Shorten, 1954).

Conversely, a number of mammals have been added to the British fauna, wittingly or unwittingly, though in many cases we are lacking in precise information. The rabbit and fallow deer, for example, were popular food animals in Medieval times, but absent from Neolithic to Iron Age times; who, from Roman to Norman times, was responsible for importing them remains uncertain though there are good reasons for suspecting the Normans in both cases (Chapman & Chapman, 1975; Sheail, 1971). The House mouse may well have come with Neolithic farmers, since it was certainly here before Roman times; it is possible that they also brought the Harvest mouse Micromys minutus (Harris, 1979), though that could be a native. More recent introductions have been well documented by Fitter (1959) and Lever (1977).

The question of when various forms were introduced to small islands remains to be considered. Archaeological remains tell us that Apodemus sylvaticus and Microtus arvalis were present in Orkney in Neolithic times (Corbet, 1979), and epigenetic character analysis confirms that the latter is likely to have been brought not from the nearest European populations but from the Mediterranean area (Berry & Rose, 1975). For some other island races of Apodemus sylvaticus (Berry, 1969), and also for Mus (Berry, 1970), there is evidence from epigenetic studies that they originated with Norwegian stock, and were carried by the Vikings. Other islands seem to have received their founding mice from the nearest mainland (e.g. Apodemus in the Scilly Isles, Berry, 1973; Mus on Skokholm, Berry, 1964). Still others, closest to the mainland, were probably colonized naturally; Skye and Mull, which both the mole and Common shrew have reached, are @ace Corbet, 1961) likely to be in this category. However, in all these cases, archaeological evidence to confirm the presence of species on particular islands at particular dates is urgently needed; this is especially true for Ireland, Man, and the Outer Hebrides.

42 D. W. Yalden

Crucial to the notion that island races have resulted from introduction by man and subsequent evolution in situ is the genetic principle of ‘founder effect’; if only one or a few individuals colonized an island (whether they did so naturally or by assisted passage), they could not have possessed all the genetic variants which occurred in the mainland populations (Berry, 1970). This of itself would ensure that the island forms differ from the mainland population, but subsequent evolution, acting by natural selection on the unrepresentative range of variation, would exaggerate the differences. Island environments in any case differ from mainland ones; small ground predators (weasels, Mustela nivalis and adders Vipera berus, in particular) are often missing, competitors also may be missing, some habitats and sources of food may be lacking, and exposure to winds may be severe. Larger size probably offers considerable advantages in resisting exposure, while in the absence of ground predators, the ability to escape down a small burrow is less important; larger size is, of course, a frequent characteristic of island races (cf. Corbet, 1961). Absence of the competing Microtus may well be a major factor in allowing the ‘complex’ forms of Clethrionomys to persist on Skomer, Raasay and Jersey. The absence of such competitors may have allowed a species to colonize one island, while their presence prevented it colonizing another, nearby, island.

OUTSTANDING PROBLEMS Awkward fossil occurrences The hypothesis outlined above assumes that most of the island races of small mammals owe their origin to accidental introduction by man; in contrast to the ‘glacial relict’ hypothesis, this implies that their ancestors were not widespread members of the British mammal fauna in some previous phase of colonization. Transferring the glacial relict hypothesis to a more modern geological framework, one could imagine that in the Windermere (or Allered) Interstadial, the essentially southern Microtus arvalis and Crocidura russula did colonize Britain (along with, for example, the now southern beetle Bembidion callosum); if they then survived the climatic deterioration of Younger Dryas times, they might have been well- placed to colonize such sites as the Channel Isles.

The problems associated with this line of reasoning have already in part been outlined. It is inherently unlikely that such southern forms could survive through the Younger Dryas when their northern counterparts, such as Microtus agrestis and Sorex araneus, did not, and the problems of the depths of various land bridges have also been indicated. However, the presence of fossils of, apparently, both Microtus arvalis and Crocidura sp. have been reported in southern Britain, and this has been taken to indicate that since both forms were once present here, they could be glacial relicts in the islands where they now occur.

Indeed fossils of ‘Microtus cornen’, regarded as ancestral to, or a form of, M . arvalis, were important for formulating the glacial relict hypothesis. M . corneri was described from material collected at Ightham Fissures in Kent, and subsequently reported from at least seven other sites (Sutcliffe & Kowalski, 1976). However, there are two grounds on which the validity of these records is questionable, timing and identification. At most of the sites, little attention was paid by the excavators to stratigraphy, and the resultant faunas are a hopeless muddle of glacial and post-glacial forms. This is especially true of Ightham Fissures, where Apodemus and Clethrionomys are mixed up with both Lemmus and Dicrostonyx; M . agrestis, M. gregalis and M. oeconomus are certainly present (the latter including the type of M . cornen), but the presence of M. arvalis remains to be clearly demonstrated (Sutcliffe & Kowalski, 1976). Recently, M. arvalis has been claimed to be present at Ogof-yr-Ychen, Caldey Island (Bateman, 1973), but the fauna is, again, a mixture of tundra and temperate

nt

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9 2. 3

GR OE. AR.AG s (8) (C 1 3

F

s rb

OE.AR,GR AG

Fig. 17. Identification of Mzcrotus species. Much of the confusion in the literature stems from a failure to appreciate the full range of variation in the different species of Microfus, and the difficulties of distinguishing them on incomplete material. The first lower molar (MI) (A) varies considerably in the shape of the anterior loop (stippled region); M . oeconomus (OE) always has a rather simple M , but the variants overlap with those seen in M. gregdzs (GR) and that again shares all the most complex variants seen in M . urvdis (AR) and M . agrestis (AG). In the upper molars, (B) M. ugretis typically has an extra postero-internal loop on the second upper molar (m2); a similar loop on ml characterizes the complex form of M . ugreszis-Fig. 14. The skull of M . gregalis is also distinctive, being much narrower and having a distinctive interorbital crest (C). A multivariate analysis of the lower jaws (D) showed that M. oeconomus could be separated by their larger size and the simpler form

$

rp of m,, but lower jaws of the other three species are quite indistinguishable (A, from an original by Mrs J. Hall; D, from Hall & Yalden, 1978). w

44 D. W. Yalden

mammals. Even if M. arvalis was present in Britain, this would only be important if it were at a Late-Glacial or Flandrian date; its presence here in the Ipswichian Interglacial (before the start of the Weichsel Glaciation) would be no more relevant to the Post-Glacial fauna than for example, the presence of Hippopotamus amphibius, which certainly was here in Ipswichian times, and equally certainly did not return in Flandrian times.

The problem of identification of Microtus is at least as critical as the problem of timing. Four species occurred in Europe in Late-Glacial or Flandrian times: M. oeconomus, M . gregalis, M. agrestis and M. arvalis, and at least the first three of these also occurred in Britain. M. oeconomus (= M . rm’ceps) is slightly larger than the other three, and has a simpler m, (Fig. 17), so that even lower jaws, lacking skulls, can be identified. M . gregalis (= M. anglicus) has a very narrow skull, but its dentition is virtually indistinguishable from that of M. arvalis. M. agrestis and M. arvalis are very difficult to distinguish on their skulls, except that M. agrestis regularly has an extra loop on m2; it follows that lower jaws alone are indistinguishable (as are those of M. gregalis) (Hall & Yalden, 1978). At present, M. gregalis does not occur in Europe, being confined to the tundra zone of Siberia and N.W. America and the high steppes of Central Asia, and this may have led to it being overlooked. Of the reputed occurrences of M. arvalis, that at Dog Holes, Lancashire, was evidently in fact M. agrestis, and that from Brixham Cave, Devon, was probably M. oeconomus (Sutcliffe & Kowalski, 1976). Two localities, Marlow, Buckinghamshire and Beckford, Worcestershire (Sutcliffe & Kowalski, 1976), have yielded collections of isolated molars which, lacking the characteristic m, of M. oeconomus or the mz of M. agrestis, could be M . arvalis but could equally be M . gregalis (Hall & Yalden, 1978). Those from Levaton (M. cf. arvalis) were based on lower jaws only (Carreck, 1958). The skulls from Ogof-yr-Ychen were apparently not those of M. gregalis (Bateman, pers. comm.) but details have not been published. The conclusion seems to be that, at present, the occurrence of M. arvalis in mainland Britain in Late-Glacial or Flandrian times is ‘not proven’.

The situation with Crocidura is clearer. Rzebik (1968) described a number of shrew fossils from Tornewton Cave, Devon, including one complete and eleven fragmentary lower jaws of Crocidura. Three species of Crocidura occur in Europe at present, C. sauveolens, C. russula and C. leucodon; the first is smaller than the other two, and is not the form present at Tornewton. The other two cannot be distinguished on lower jaws alone; the fossils therefore belong to one or other of them, but which one is uncertain. Tornewton Cave contains an extensive series of mammal faunas, extending from the Riss (Wolstonian) Glaciation through the Ipswichian Interglacial (with Hippopotamus), through the Devensian Glaciation, and into the Flandrian (Sutcliffe & Zeuner, 1962; Kowalski, 1967). The part of the cave containing the Crocidura specimens was, however, not one that contained this stratified sequence, and it is impossible to say when Crocidura was present in England. Rzebik (1968) notes that Crocidura was widespread in Europe in Interglacial times (‘warm periods’) and the likelihood must be that these remains date from the Ipswichian Interglacial.

Numerous records of Apodemus sylvaticus in cave faunas along with, for example, Dicrostonyx and Lemmus at Castlepook Cave, Co. Cork, and with Dicrostonyx at seven other Irish caves (Sutcliffe & Kowalski, 1976, table 9), or with these and the northern Microtus sp. in the English caves (ibid. table 10) are also embarrassing for the arguments advanced here. At present, Apodemus sylvaticus does not overlap geographically with Lemmus or Lhcrostonyx, and meets M . gregalis only on the Central Steppes of Asia. There are no other, more northern, species of Apodemus which could cause confusion either, and the conclusion, at present, seems to be that tundra and temperate faunas have become muddled during excavation. The same is likely to be true for faunas (e.g. Ightham Fissures, Chudleigh Fissure, Langwith Cave) where Clethrionomys glareolus apparently occurs with lemmings.


However, there is the additional possibility that C. rufocanus and C. rutilus, northern voles which do co-exist with lemmings, are present rather than C. glareolus. They have not so far been recognized in British Pleistocene deposits, but they might be expected; they are, however, difficult to separate from C. glareolus on skeletal material. However, the careful stratigraphy unravelled at Robin Hood’s Cave and at Cathole (Campbell, 1977) seems to demonstrate more clearly that Apodemus did occur with lemmings and northern Microtus, at least in the warmer, birch scrub, of the Allered Interstadial. At both sites, only a few specimens of Apodemus were obtained and it is possible that the mice were actually much later, intrusive elements. Cave sites with scree-type sediments could easily allow Apodemus to penetrate deeply into earlier layers, and it is of course a burrow-living rodent.

The Lusitanian element There occur in western and south-western Ireland a number of plants and animals which are otherwise scarce or absent in Britain, but which are common in Spain and Portugal (Corbet, 1962). The most characteristic of these are various heaths, including Erica mackaiana, E. vagans, E. ciliaris, E. hibernica, and Daboeciu cantabrica, and the strawberry tree Arbutus unedo. Also included are two saxifrages (Saxifiaga spathularis, S. hirsuta), two butterworts (Ptnguicula grandflora, P. lusiranica) and six or so other species (Godwin, 1975, p. 487). There are fewer animals with similar distributions, but the slug Geomalacus maculosus, moth Calamiu tridens and woodlouse Oritoniscuspavus are certainly examples (Table 6, Fig. 18). The Natterjack toad Bufo calamuu might be regarded as a member of this group: it occurs in S.W. Ireland, in coastal areas of N.W. England and S.W. Scotland, and more widely in S.E. England. There are no mammals with this ‘Lusitanian’ distribution, and this element might be irrelevant to this review but for the repercussions which discussions of it have for the mammals. Beirne (1952) argued that the Lusitanian species had survived through a glacial

Table 6 Lusitanian elements of the j o r a and fauna. Those marked* occur, in the British Isles, only in Ireland, the others occur also in various other parts of

western or south western Britain (see also Corbet, 1962)

Saxifraga spathularis‘ Saxifraga hirsuta * Arbutus unedo* Erica mediterranea * Erica mackaiana* Erica ciliaris Erica vagans Daboecia cantabrica‘ Sibthorpia europaea Pinguicula lusitanica Pinguicula grandifora * Euphorbia hyberna Euphorbia peplis Neotinea intacta* Simethis planifoia* Puccinellia foucadii

Geomalacus maculosus* Calamia widensf Metoponorthus cinguendus Oritoniscus j a w s *

St Patrick’s cabbage Kidney saxifrage Strawberry tree

Dorset heath Cornish heath St Dabeoc’s heath

Irish spurge Purple spurge

Kerry slug Burren green moth (a woodlouse) (a woodlouse)

46 D. W. Yalden

4RBUTUS

‘\

- E MACKAIANA

/

Fig. 18. Distribution maps for some ‘Lusitanian’ elements. (A) Daboecia canzabrica St. Dabeoc’s Heath (after Woodell, 1958). (B) Arbutus unedo Strawberry Tree (after Sealy, 1949). (C) Erica mackaiana (after Webb, 1955). (D) G e o d a c u s maculosus (after Kerney & Cameron, 1979, in part). All are common in Iberia, and occur in western or south-western Ireland. Arburus and Daboecia also extend up the French Atlantic coast, while there is an old record of Geomalacus from Brittany, and a single site for Arburus on the north coast of Brittany. These scattered records may suggest a more extended range in the early Flandrian up the western coast of Europe.

phase on a ‘Celtic Land’, part of the continental shelf off S.W. Britain which was exposed by the lowered sea level associated with the glaciation but is now submerged. A more rigorous analysis of this problem by Watts & Mitchell (1970) considered glacial refuges off the west coast of Ireland as sites where the Erica sp. and Duboecia could have survived through the last glaciation; some of them are known, from fossil evidence, to have been present in at least one interglacial, the Hoxnian (or Great) Interglacial.

Implicit in these arguments is the assumption that western Ireland was, as now, bathed by the Gulf Stream Drift; if that source of warmer water had modified the coastal climate, the warmth-loving (or, more correctly, frost-intolerant) species might have survived. This would, even so, have required a very steep gradient of climate from the coast inland to the ice sheet. Recent evidence from deep sea cores proves quite conclusively that even this version of the ‘glacial refuge’ hypothesis is untenable. At the glacial maximum, the front of polar water in the North Atlantic lay at about the latitude of Lisbon (Ruddiman er al., 1977), and, though it swung back in Allerad times to something like its present disposition, in Younger Dryas times it lay across to S.W. Ireland (Fig. 19). Even Watts, formerly an advocate of


glacial refuges for the Lusitanian forms, accepts that they cannot be advocated in the face of this evidence (Watts, 1977, p. 283).

A more probable sequence of events can be hypothesized from the elements of evidence already given. It seems clear that the improvement in temperature at around 10,OOO b.p. was very rapid, and at that time the world’s sea level was still about - 50 m from its present level. However greatly Scotland and N.E. Ireland were suffering from isostatic depression at that time, S.W. Britain would have extended as dry ground some distance from what is now the shore line, and the migration route from Iberia to S.W. Ireland would have been more direct than it now is (Fig. 20). Most of the Lusitanian elements have a coastal or maritime distribution from Iberia into France, as also in England and Ireland, and such an exposed coastline, recently relieved of periglacial disturbance, would have been ideal ground for them to colonize. Many of them have small, wind-dispersed seeds, and would have been rapid colonizers. We know that many beetles, currently regarded as characteristic of southern Europe, were able to exploit the short-lived climatic improvement of the Windermere Interstadial to reach as far north as Cumbria (Coope & Joachim, 1980) and southern Scotland (Bishop & Coope, 1977); they did so again, and it is reasonable to presume that some at least of their accompanying flora did so too, in Pre-Boreal times. The configuration of the floor of the Irish Sea does, however, suggest that a deep fresh-water channel, flowing south, would have provided a barrier to strictly terrestrial forms, including the small mammals whose absence from Ireland would otherwise be inexplicable.

One or two organisms do not fit at all well in this hypothesis. The strawberry tree has large, heavy, fruits, which are dispersed by birds (Sealy, 1949), and that may have happened. The Kerry slug, Geomalacus, can hardly have been dispersed by birds, nor is it a likely

‘1 2 0 o w I I ’ 0’

I 4 0 ’ W

60.& i Fig. 19 The position of the polar front at various dates in the late and post-glacial (after Ruddiman et d., 1977).

The position of the edge of arctic water is indicated in deep sea cores by the Occurrence of typical cold- water foraminiferans, and dated by radio-carbon analysis. At the glacial maximum, the front lay across to Portugal; in the Windermere interstadial, 11,OOO b.p., the front had retreated to near Iceland, but swung back to near S.W. Ireland in Younger Dryas times.

48 D. W. Yalden

Fig. 20. Reduced sea level off western Europe, at the glacial maximum. If the sea level was at - 100 m, there would have been a much more direct route from the Iberian peninsula across to S.W. Britain, but even then the channel, more than 100 m deep, up the Irish Sea would have prevented direct entry to Ireland of terrestrial animals.

candidate for fast migration up the European coast. Corbet (1961, 1962) argued that it was probably transported by early man, and that explanation could equally well apply to the strawberry tree.

The problem of how and when some other species of tree got to Ireland is worth touching at this point. Neither hazel nor oak were present, apparently, in Ireland in Late Glacial (Pollen Zones 1-111) times, but they appear promptly in Pre-Boreal times; interestingly, and perhaps significantly, the earliest records for both species are in N.E. Ireland (Godwin, 1975, figs 91,93). Both have large fruits that are dispersed by animals; squirrels and jays (Gumulus glandarius), both of which hoard nuts and acorns in autumn, are generally implicated in discussion, though their role as long-distance transport agents has not been verified. In Pollen Zone IVY hazel appears very quickly in N.W. Britain, and Deacon (1974) argued for its


survival in a glacial refuge in that area. Since it failed to show itself during the Windermere Interstadial, human introduction seems highly likely, in which case it might also have been introduced to Ireland.

FURTHER PROGRESS This review has drawn on a number of lines of evidence in attempting to build a coherent thesis which takes account of recent advances in those several lines of knowledge. The thesis that results is not a tidy one; while natural migration may have produced a steady recovery of a temperate woodland fauna and flora over much of Britain, the effects of rising sea level in isolating some islands before others, the extinction of open-ground animals by forest development and the influence of man introducing animals, and plants, to some places but not others, will have been (in some cases clearly have been) random events. It follows that a general hypothesis to explain present distribution patterns, such as Beirne (1952) promoted, is a hopelessly unrealistic proposition, even if the factual basis for the hypothesis is corrected. In that case, only detailed consideration of each island and each species can be expected to produce a reasonable interpretation of the distributions. Godwin (1975) has tried to assemble such detail for the flora of the British Isles, and a glance at his monograph serves to emphasize how little we really know of the history of our mammal fauna. Some areas for further study are very obvious.

Archaeological evidence The absence of critical archaeological evidence has already been mentioned several times. It is, for example, remarkable that we have no specimens of Sorex minutus from Ireland in sub-fossil or archaeological deposits, and therefore no idea whether, as argued above, it was a Post-Glacial immigrant. It is important to emphasize that in this and all other cases, the stratigraphic control for sub-fossil remains of small mammals is vital. The hopeless confusion of Apodemus with Dicrostonyx in the same fauna at several cave sites has already received comment. It seems most unlikely, on their present distribution and habitat, that these two could co-exist, yet their frequent association could be used to argue that one of them (presumably Apodernus) has changed its habitat tolerance considerably. Apodemus does sometimes occur well away from woodland, for example in scree slopes, bracken patches and, of course, on such islands as St Kilda. The careful excavations documented by Campbell (1977) suggest that indeed this ‘woodland’ species did occur, with lemmings, in the scrubby birchhundra conditions of Pollen Zone 11, but further confirmation would be desirable.

There is, in principle, no reason why mammal diagrams, comparable to pollen diagrams and mollusc diagrams, should not be obtained; one has been published for a small cave in the Peak District (Yalden, 1977) (Fig. 21) and Campbell (1977) has produced two others. Generally speaking, archaeologists have tended to ignore the small mammals, and concen- trate on the large mammals which provided a more obvious source of food. This is doubly unfortunate, since not only is information on the history of the mammals themselves lost, but the small mammals would provide a good indication of habitat change. Muscardinus and Clethrionomys would signal scrub as surely as Microtus and Micromys indicate grassland, while there is evidence that several of them are sensitive indicators of changes in grazing pressure.

There are a number of cases where what we perceive as an absence of a species from an island, and try to explain on the basis of sea levels, isolation and accidental introduction, may actually be colonization followed by extinction, as a result of habitat loss, persecution, or

50 D. W. Yulden

n

1 1 0 2 0 3 0 4 0 1 2 1 2 3 4 1 2 3 F 10 7

2 0 1

OSSOMS EYRIE CAVE, STAFFS. PERCENTAGE OF THE SMALL M A M M A L FAUNA AT EACH DEPTH NOTE DIFFERENT SCALE O F SHADED COLUMNS

7 60 1

D z

Fig. 21. Small mammal diagram, for Ossom’s Eyrie Cave. This cave was excavated in 3 inch layers, and Roman artifacts at 15-11” provided a valuable dating horizon; the nestling eagle at 12-15” also represents a useful datum. The presence of Mus in the pre-Roman layer is noteworthy, as is the greater abundance of Arwicola in the earlier layers. This analysis is based on a total of 2905 small mammals, with a minimum count in any one layer of 75 (after Yalden, 1977). More analyses of this sort, particularly for the beginning of the Flandrian, are needed before we can discuss the origin of the present mammal fauna with any confidence. .

competition with later introduced mammals. The absence at present of deer, fox and badger from the Isle of Man seems to be a case in point; confirmation, by finding sub-fossil remains of badger on the Isle, would be welcome. Le Sueur (1975) puzzles over the possibility that Arvicolu was once present in Jersey; this is a common species in many archaeological contexts, and it should be possible to resolve the problem.

It is possible that useful information, particularly on the history of deer and other larger mammals, already exists in the archaeological literature (e.g. Jarman, 1972). This is vast, but the reports on mammals are frequently relegated to an appendix, and it is impossible to know what facts might be thus hidden. Information on rates and times of extinction (for example for Alces, Ursus and Castor, very poorly documented at present) as well as the times of introduction of Cerwus duma and Oryctolugus, ought to be forthcoming. Even the introduction of sheep and cattle are but poorly documented; for instance, the role of local domestication of aurochs or its use for crossing with Neolithic cattle remains uncertain.

Genetic studies Some of the outstanding problems would be amenable to fairly easy solutions. For instance, Apodemus sylvaticus seems to have been carried to most of the Scottish islands from Norway, presumably by Vikings (Berry, 1969). However, Apodemus seems to have been present in Orkney in Neolithic times. Epigenetic studies of Orkney Apodemus should confirm their separate origin.

The failure of taxonomists to examine shrews has already been mentioned. Epigenetic studies of Sorex minutus, in particular, might be revealing, since it has been argued above


that its widespread occurrence on the islands is partly the result of natural colonization. Epigenetic disparities between shrews on different islands might disprove this. I am not sure that shrews show appropriate epigenetic variation, as most studies have concerned rodents, but primates certainly do (initial studies concerned human variants), and insectivores are likely also to do so.

Geological problems Some of the major uncertainties concern areas of geological rather than biological research. Most notably, the relative levels of sea and land, particularly as they relate to a land bridge to Ireland, but also those to the Isle of Man and the Channel Isles, need clarifying. It seems likely that the floor of the Irish Sea, perhaps in the Liverpool Bay-Morecambe Bay area, contains evidence on whether this was a largely fresh water or brackish lake (like the present Baltic Sea) and, if so, when the sea broke through. If there were morainic land bridges, then evidence of them ought also to be present somewhere-it is unlikely that they were totally destroyed by subsequent marine erosion, even if they were breached. Possibly geological, archaeological or even palynological work on Islay would reveal some evidence of a functional land bridge to Ireland. Even the date at which the connection from the English Channel to the North Sea formed is, at present, somewhat circumstantial, being based on extrapolation of the curve of eustatic rise of sea level, on dating and pollen analysis of North Sea ‘moorlog’ and, a somewhat circular argument for present purposes, on distribution patterns. Independent geological evidence, for example from cores in the southern North Sea, would be valuable.

SUMMARY A mammal fauna was certainly present in Britain during Windermere Interstadial times, 13,00&11,000 years ago, which included such animals as giant deer Megaceros giganteus, reindeer Rangifer tarandus and probably the lemmings Lemmus lemmus and Dicrostonyx torquatus. Various lines of evidence concur in suggesting an equable, warm temperate, climate at that time, but an open type of vegetation with, at best, birch scrub. The subsequent deterioration in climate in Younger Dryas times, from about 11,000 to 10,000 b.p. extinguished most if not all of this fauna, but reindeer and lemmings, at least, survived in southern England and spread north as the ice melted with a further improvement in climate. This improvement was very rapid-beetle faunas dated by 14C suggest that it may have been largely accomplished in 50 years, and deep sea cores confirm the rapidity of the change. The earliest well-dated mammal faunas from Flandrian (post-glacial) times in England, from the Mesolithic sites of Starr Carr and Thatcham, are essentially modern, temperate woodland, faunas (though the small mammals were apparently not properly sampled at those sites). Subsequent events in mainland Britain, which was apparently cut off from Europe about 9000 b.p. by rising sea levels, have been strongly influenced by man; some species were introduced, accidentally or deliberately, while others were exterminated.

Islands around the main island have had a more elusive history, and most of it remains conjectural. Ireland probably received its present native mammals over a low-lying land bridge from Scotland via Islay in a short period around 8000 b.p. when sea level was, relative to the still-rising land, at its lowest. If so, this land bridge acted as a filter which allowed most of the larger species to cross, but stopped all of the small mammals except the Pigmy shrew. The Isle of Man was probably colonized by essentially the same fauna at about the same time. Other islands, Shetland, Orkney, St Kilda, Outer Hebrides, have never been connected to

52 D. W. Yalden

any mainland, and their terrestrial mammal faunas probably result entirely from human introduction. The same may well explain the presence of some species on other islands (Apodernus in Ireland, Crociduru in Scilly). Of the other islands, some (e.g. Rhum, Guernsey) probably owe all their fauna to accidental introduction, others probably got most of theirs by natural immigration (e.g. Skye), and some may actually have a fauna of ‘mixed’ origin. Overall, the mammal fauna of these islands certainly cannot be explained by a simple process of natural recolonization following the retreat of the ice; the process was much more complicated, and accidental human introduction has certainly played a part. In these circum- stances, the case for each species of mammal, and each island, has to be considered seperately. On the other hand, such consideration must also take account of the general pattern outlined here, and cannot succeed if it fails to produce explanations which are acceptable for other species. That has, in the past, happened all too frequently.

ACKNOWLEDGMENTS I am grateful to a number of people who have helped with this review. Dr J. C. Brown, Dr G. B. Corbet, and Dr Juliet Clutton-Brock kindly read through the first draft and made a number of comments and corrections; they and Dr D. Bramwell also drew my attention to several important sources which I should otherwise have overlooked. Dr M. P. Kerney amended the mollusc distribution maps (Fig. 4) for me, and Mrs Jenny Hall very kindly allowed me to adapt her original drawing as Fig. 17A.

A review such as this depends very much on the ready availability of the appropriate literature; I am therefore especially grateful to many authors who have kindly supplied reprints over the years, even if they wondered whether someone who was clearly not working in their field was an appropriate recipient.

Lastly, I must also thank the photographer, Mr L. Lockey, for rapidly producing the requisite copies of my diagrams, and the secretaries who also quickly converted my manuscript into a legible form.

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Devensian (Last) cold stage. Philosophical Transactions ofthe Royal Society of London, B 280,313-340. Coope, G. R. & Joachim, M. J. (1980) Lateglacial environmental changes interpreted from fossil Coleoptera from

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Postscript Since completing the above, my attention has been drawn to several further sources of information which extend and correct the discussion of, particularly, events concerning the history of the Irish Sea.

I suggest above (page 17) that there is a continuous channel, deeper than 100 my up the centre of the Irish Sea to the vicinity of Rathlin Island. More accurate charts than the atlases which I consulted show that this is not quite true. There is a ridge running across the sea floor WSW from the Lleyn Peninsula toward Wicklow (Dobson, Evans & James, 1971) which interrupts the 100 m deep channel. However, even this ridge is between 90 m and 80 m deep at its shallowest. Such a depth is too great for the sea floor to have been exposed as a land bridge in Flandrian (Pre-Boreal) times, if the level of 70 m at 10,000 b.p. (Fig. 7) is correct. The ridge is morainic in origin, however (Dobson et ul., 1971) and might have been eroded.

The geological history of the Irish Sea is currently a source of much study and discussion, but there is as yet no consensus on events during the last 15,000 years. The volume edited by Kidson L? Tooley (1977) presents a recent account of the activities and views of research workers concerned with the basin. Another useful contribution is the paper by Haynes et al. (1977) analysing the sea bottom in Cardigan Bay. They suggest that sea level had risen to -60 m at 12,000 b.p. and to -40 m at 10,000 b.p. These figures likewise emphasize that the ridge across to Ireland cannot possibly have been available as a land bridge in Flandrian times; moreover, if it constituted a land bridge in Late Glacial (Windermere Interstadial) times, then it was probably only for a short time at the beginning of that period.

The possibility of a northern land bridge, in the region of Islay, has not been discussed by geologists, but Dr R. J. Whinington (pers. comm.) informs me that the relatively shallow sea floor there is solid rock, and is not morainic in origin.

Finally, the problem of the fauna reaching Ireland has been thoroughly discussed from an archaeological viewpoint by Woodman (1978). He argues very firmly that there has not been a land bridge to Ireland in Flandrian (Post-Glacial, Holocene) times, and that man must have arrived in boats. He considers that the native mammals either represent earlier immigrants which survived through the climatic deterioration of Younger Dryas times, or later arrivals which swam. Given that there are no well-dated mammal faunas of Windermere Interstadial times which contain native mammals, the former must await confirmation. Several of the native species (Red deer and Wild boar, especially) are rather unlikely to have occurred in Windermere Interstadial times, and then survived through the Younger Dryas, but could easily have swam across from Scotland or elsewhere. On the other hand, the Pigmy shrew and Mountain (Irish) hare could easily have got to Ireland early on in the Interstadial and survived through the Younger Dryas. The problem of why the other small mammals did not also do so then remains; the selective barrier of a low-lying bridge still seems the most likely explanation.


I thank Dr Amanda Williams (Marine Information and Advisory Service, Institute of Oceanographic Sciences) and Dr R. J. Whittington (Dept. of Geology, University College, Aberystwyth) for drawing to my attention to several of these sources and answering my enquiries so helpfully.

Additional References Dobson, M. R., Evans, W. E. & James, K. H. (1971) The sediment on the floor of the southern Irish Sea. Marine

Haynes, J. R., Kiteley, R. J., Whatley, R. C. & Wilks, P. J. (1977) Microfaunas, microfloras and the environmental

Kidson, C. & Tooley, M. J. (eds) (1977) The Quaternary Hisrory of the Irish Sea. (Geological Journal Special Issue

Woodman, P. C. (1978) The Mesolithic in Ireland: hunter-gatherers in an insular environment. (British

Geologv, 11,2749.

stratigraphy of the Late Glacial and Holocene in Cardigan Bay. Geologicdyournal, 12,129-158.

7).

Archaeological Reports, British Series, 58, Oxford).

when did the mammal fauna of the british isles arrive?

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