palaeogeographicalcontrolsonthecambriantrilobite ... · the axis of the late proterozoic to early...

Palaeogeographical controls on the Cambrian trilobiteimmigration and evolutionary patterns reported in

the western Gondwana margin

J. Javier AŁ lvaro a;�, Olaf Elicki b, Gerd Geyer c, Adrian W.A. Rushton d,John H. Shergold e

a UMA 8014 CNRS, Universite¤ de Sciences et Technologies de Lille, Sciences de la Terre, 59655 Villeneuve d’Ascq Cedex, Franceb Institut fu«r Geologie, TU Bergakademie Freiberg, Bernhard-von-Cotta-Str. 2, 09596 Freiberg/Sachsen, Germany

c Institut fu«r Pala«ontologie, Universita«t Wu«rzburg, Pleicherwall 1, 97070 Wu«rzburg, Germanyd Palaeontological Department, The Natural History Museum, London SW7 5BD, UK

e La Freunie, Benayes, 19510 Masseret, France

Received 12 April 2002; received in revised form 18 July 2002; accepted 25 January 2003

Abstract

Southward drifting of the western Gondwanan margin during the Cambrian has been demonstrated by means ofboth palaeomagnetic methods and lithological indicators of climate (such as carbonates and evaporites). Recentimprovements in biostratigraphical correlations permit an enhanced understanding of the climatic andpalaeobiogeographical constraints that controlled the distribution of Cambrian benthic communities. Palaeogeo-graphical and biogeographical reconstructions based on trilobites are reported in this paper in order to test interactionbetween migration, speciation and extinction rates. The variability of the documented biogeographical patterns isdirectly related to species diversity, in which wider distribution coincides with transgressive trends and subsequentconnection of neighbouring platforms. Early Cambrian trilobite faunas show a high degree of both substrate controland endemicity, although transgressions led to parallel shifts in faunal compositions. By contrast, Mid-Cambriantrilobite faunas are relatively uniform across western Gondwana, and latest Mid- and Late Cambrian associationsdocument influence of an increased similarity with Asian trilobite faunas.5 2003 Elsevier Science B.V. All rights reserved.

Keywords: biogeography; palaeogeography; extinction; trilobites; western Gondwana; Cambrian

1. Introduction

Four disciplines are the main sources for pa-laeogeographical reconstruction: palaeomagne-tism, sedimentology, palaeoclimatology, and bio-geography. The boundaries of the postulatedplates and terranes (commonly discussed fromthe viewpoints of geophysics, tectonophysics and

0031-0182 / 03 / $ ^ see front matter 5 2003 Elsevier Science B.V. All rights reserved.doi:10.1016/S0031-0182(03)00300-6

* Corresponding author. Tel. : +33-3-2033-6392;Fax: +33-3-2043-6900.

E-mail addresses: [email protected](J. Javier AŁ lvaro), [email protected] (O. Elicki),[email protected] (G. Geyer),[email protected] (A.W.A. Rushton),[email protected] (J.H. Shergold).

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structural geologists) do not always match thebiogeographical boundaries worked out by palae-ontologists and biostratigraphers. However, dothe biogeographical boundaries that prevented re-productive communication necessarily coincidewith those of the tectonic plates? In some casesthey do; many plate and terrane boundaries, how-ever, do not correspond with biogeographical bar-riers and, on the contrary, a single continentalmargin can display barriers, particularly duringregressive conditions, which separate di¡erentgroups of organisms that are signi¢cant biogeo-graphically. As a result, a biogeographical unitcontains similar biotas, but may or may notshow physical continuity.One of the main projects of the International

Subcommission on Cambrian Stratigraphy (ISCS)is the proposal of a global chronostratigraphicalchart useful for international correlation (seeGeyer and Shergold, 2000, for a recent statementof the problem). One of the major di⁄culties isthe resolution of the drastic provincialism exhib-ited by the benthic fauna, such as the trilobites(Palmer, 1998). From the earliest occurrences oftrilobite faunas there is clear evidence of biogeo-graphical di¡erentiation into two main provinces:the olenellid province, comprising much of Bal-tica, Laurentia, and Siberia, and the redlichiidprovince of Gondwana (Palmer, 1972). Subse-quently, an overlapping bigotinid province wasdistinguished (Pillola, 1991a). For the Mid- andLate Cambrian, Palmer (1972) reported a morecomplicated scheme involving four provinces forthe continental seas, and three other ones for ex-posed shorelines. In addition, conodont distribu-tion in the Late Cambrian indicates the presenceof two faunal realms (Bergstro«m, 1990), a tropicalMidcontinent region and a cool Atlantic region.One geographical area where the biostrati-

graphical correlations are still strongly debatedis the western Gondwana margin, which includesthe Mediterranean area and much of western andcentral Europe. Geophysical data that would in-dicate Cambrian plate boundaries are scarce with-in this area. However, biogeographical disparitiesbased on detailed and extensive trilobite studiesre£ect major palaeogeographical di¡erences andrelative sea-level £uctuations. Although theoreti-

cally the reproductive communication of trilobiteswas maintained by means of larvae, the Cambriantrilobite-based biogeographical patterns of thewestern Gondwanan margin, as it drifted towardsthe South Cambrian pole, are not homogeneous,and the changes in patterns of dispersal and evo-lution of some trilobite families remain uncertain.This paper o¡ers an examination of the biogeo-graphical patterns displayed by the Cambrian tri-lobites on the western margin of Gondwana, as amethod of testing the established palaeogeograph-ical models.

2. Basins and platforms: a nomenclatural approach

The Moroccan and European Hercynian (orVariscan) massifs, the latter located south of theRheno^Hercynian zone and north of the Alpinerealm (Fig. 1), contain a mosaic of Cambrian suc-cessions assigned to the western Gondwanan mar-gin, namely Morocco, the Iberian and Armoricanmassifs, Montagne Noire, Sardinia, Saxo^Thurin-gia and Bohemia. Associated with the same mar-gin are other small cratonic domains recognised inthe southern British Isles ; some of their bound-aries are less well-de¢ned (Cocks et al., 1997).

2.1. Morocco

The Neoproterozoic(?)^Cambrian successionsof the Moroccan Atlas are located in the Anti-Atlas and central High Atlas mountains, althoughsome disconnected outcrops occur in the Jbiletand Rehamna regions, and the Meseta plateau.The axis of the Late Proterozoic to Early Pa-laeozoic Souss Basin (Geyer, 1989) roughly coin-cides with the modern trend of the Anti-Atlas(SW^NE). Common west-to-east facies changesthroughout the Cambrian successions re£ect theeastern setting of proximal areas (Destombes etal., 1985; Geyer and Landing, 1995; Geyer etal., 1995).

2.2. The Iberian Massif

The Cambrian tectonosedimentary outcrops ofthe Iberian Massif were subdivided by Lotze

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J. Javier AŁ lvaro et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 195 (2003) 5^356

(1961) into zones, viz. the Cantabrian (CZ), West-ern Asturian^Leonese (WALZ), Central Iberian(CIZ) and Ossa^Morena (OMZ) zones. Their evo-lution has been interpreted in terms of two dis-tinct troughs: the CantabroÎberian and the An-dalusian basins, the latter including the OMZ.The CantabroÎberian Basin comprises the CZ,WALZ, northern CIZ, and their eastern prolon-gation into the Demanda Mountains (DM) andthe Iberian Chains (IC). It was limited to theNE by the CantabroÊbroan Land area, whichconstituted the main source of sediments forboth the CantabroÎberian and the Pyrenean ba-sins (Carls, 1983), and in the SW by some upliftedareas (or median highs; Lotze, 1961), which epi-sodically supplied sediments (Aramburu et al.,1992).The geodynamic a⁄nity of the OMZ is still

controversial. One hypothesis supposes its accre-tion to the Iberian Autochthon (which wouldinclude the rest of northern zones) during theCadomian orogeny (Quesada, 1991), whereas an-

other hypothesis considers the Iberian Massif as acomplex tectonic mosaic constructed from twodistinct plates that collided during the Early De-vonian (Acadian orogeny; Mart|¤nez-Garc|¤a andRolet, 1991). Because of this discussion, and thedisplacement of the OMZ from NW to SE alongthe Badajoz^Co¤rdoba shear zone during the Her-cynian orogeny, we will illustrate the OMZ as aneighbouring basin of the western Gondwananmargin.

2.3. The southern British Isles (‘eastern Avalonia’)

The main structural elements are the MidlandPlatform, bounded to the NW and NE by depo-sitional troughs, and truncated to the south by theHercynian front. The trough to the NE is theconcealed ‘Eastern England Caledonide Belt’, inwhich Cambrian rocks of the Tornquist Sea maybe present, but have not been proven. To the NWof the Midland Platform lies the Welsh Trough,an elongated ensialic basin that experienced sub-

? ?

RHENO-HERCYNIA

N

SAXO-THURINGIA

MOLDANU

-BIAN

TEPLA-BARRANDIAN

NORTH - GERMAN - POLISH

CALEDONIDES

TRANS - EUROPEAN

FAULT

ARMORICANMASSIF

CENTRALMASSIF

MONTAGNE NOIRE

SWSARDINIA

SOUTHPORTUGUESE ZONE

OSSA-MORENA ZONE

CENTRAL-IBERIANZONE

WESTASTURIAN-LEONESEZONE

CANTABRIANZONE

DEMANDAMOUNTAINS

IBERIANCHAINS

PYRENEES

Pre-Hercynian outcrops of

western and central Europe,

and Morocco

ALPINE REALM

Anti-Atlas

High Atlas

Western Plateau

Fig. 1. Main tectonostratigraphical units of the Variscan Belt (modi¢ed after Franke, 2000), and northwestern Africa.

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J. Javier AŁ lvaro et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 195 (2003) 5^35 7

sidence throughout much of the Cambrian Period.The Welsh Trough is limited to the NW by thetectonically active Monian Terranes that behavedas a positive area through the Cambrian, andwhich separate the Welsh Trough from the Lein-ster Basin of SE Ireland (Bluck et al., in Cope etal., 1992); the latter contains Cambrian rocks buthas not yielded any trilobites. To the north, theCambrian of Scotland is part of Laurentia, and isnot discussed further here.The Cambrian of England and Wales is devel-

oped in three main facies: (1) basinal clastic de-posits of the Welsh Trough, exposed in two mainareas in northern Wales (Harlech Dome and Ar-fon) and two in the south (St. David’s and Llan-gynog areas); (2) outer-shelf clastics on the Mid-land Platform (Nuneaton); and (3) shallow-waterclastics with very local limestones in the WelshBorderland (Comley and Rushton areas), on themargin of the Midland Platform. The geology ofthese areas was reviewed by Rushton (1974) andRushton et al. (1999), and the palaeogeographywas summarised by Brasier et al. (in Cope et al.,1992). McKerrow et al. (1992) showed easternAvalonia located close to Morocco at a latitudeof about 40‡S in the Early Cambrian, driftingsouth to about 60‡S in the Late Cambrian(L.R.M. Cocks, pers. commun., 2002).

2.4. The Armorican Massif

The Cambrian of the Armorican Massif con-sists dominantly of siliciclastic deposits. The faciesstrongly depends on the local position on the shelfand includes locally Lower Cambrian carbonatesdeposited in nearshore environments. Its apparentpolar wander path can be superimposed acrossthe Neoproterozoic^Early Palaeozoic interval onthe Gondwana path. A counter-clockwise rotationof the massif has been proposed for Early Palaeo-zoic times (Young, 1990), based on sedimentolog-ical and palaeogeographical similarities with somePortuguese outcrops.

2.5. The Montagne Noire and southwesternSardinia

To the north of the Cantabro^Ebroan Land

area, a mosaic of platforms is represented in theCambrian outcrops of the Pyrenees and Mon-tagne Noire. The Cambrian stratigraphical andbiogeographical patterns of southwestern Sardiniareveal a close similarity to the Montagne Noire sothat this platform can be considered neighbouringthe Montagne Noire segment at that time.

2.6. Saxo^Thuringia and Bohemia

According to Havl|¤c›ek et al. (1994), both theGerman and Barrandian (an area situated in thecentral part of the Bohemian Massif) Cambriandeposits belong to the ‘Perunica terrane’. Thiscrustal segment was de¢ned as a separate micro-plate occupying ‘the major part of the BohemianMassif, and involving the Moldanubian, Barran-dian and Saxothuringian (Saxothuringian^Lu-gian) Zones’ (Havl|¤c›ek et al., 1994). This interpre-tation was based on biogeographical a⁄nities ofbenthic faunas with those of Baltica and a north-ward drift of Perunica from high (57.9‡S for theLower Cambrian Paseky Shale) to lower latitudes(31.1‡S for Middle Cambrian sediments and28.6‡S for Upper Cambrian andesites; Krs etal., 1987; Vra¤na and SYtefldra¤, 1997). By contrast,Kukal (1971) assumed a semi-arid, rather warmclimate of continental type for the Lower Cam-brian. The Barrandian palaeolatitude setting hasbeen widely debated. Recently, Linnemann et al.(2000) revised the concept of Perunica, eliminat-ing the Saxo^Thuringia area; they distinguishedthe Brittany^Normandy, Perunica and Saxo^Thuringia terranes, which would form the so-called ‘Armorican Terrane collage’. Discussionabout the existence of these microplates is beyondthe aims of this paper. Nonetheless, two tectono-stratigraphical areas are considered here: the Bar-randian and Saxo^Thuringian areas. The lattercomprises a fault-bounded crustal fragment thatcontains Cambrian outcrops in the Doberlug Syn-cline, Thuringia and the Franconian Forest area.The palaeogeographical a⁄nity of the Go«rlitzSyncline o¡ers some uncertainties but is here con-sidered as a relic of an overlapping succession onthe Cadomian basement of Saxo^Thuringia.In the Barrandian area, a distinct Cambrian

sedimentary trough, the Pr›|¤bram^Jince Basin, is

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commonly recognised. It contains a thick LowerCambrian continental succession covered by Mid-dle Cambrian, transgressive marine fossiliferousshales (the Jince Formation), and Upper Cambri-an volcaniclastic complexes. Laterally, the JinceFormation crops out in the Skryje^Tyr›ovice area.In the biogeographical discussions expressed be-low, the trilobites of the Skryje^Tyr›ovice areawill be reported with those of the Pr›|¤bram^JinceBasin.

3. Cambrian palaeogeographical evolution:an overview

Lack of uniformity in the successions of faunasand depositional environments in the westernGondwanan margin has led to the developmentof di¡erent regional biostratigraphies (Fig. 2). Inorder to avoid nomenclatural confusion in chro-nostratigraphical correlations, we here use a com-

bination of the Cambrian Moroccan and Iberianscales for the Lower Cambrian, and the Iberian^Montagne Noire scale for the Middle Cambrian.The sparse Upper Cambrian faunas are related tothe Australian succession, though in Britain thereare su⁄cient olenid trilobites to correlate with theScandinavian succession. For Britain, the strati-graphical classi¢cation used in Cowie et al.(1972) is employed because it supplies a singlestandard for England and Wales and facilitatesreference to Rushton’s (1974) stratigraphical sum-mary. It is recognised that the lower two of theBritish regional series divisions ^ Comley and St.David’s series ^ do not correspond precisely withthe usage of Lower and Middle Cambrian in useelsewhere, the inter-series boundary lying ratherhigher than the Lower^Middle Cambrian bound-ary in Morocco (Geyer, 1990a). The base of theMerioneth Series corresponds closely to that ofthe Upper Cambrian as generally understoodand its top lies very close to the internationally

Fig. 2. Chronostratigraphic correlation of regional charts (based on Thomas et al., 1984; Geyer, 1990a; AŁ lvaro and Vizca|«no,1998; Sdzuy et al., 1999; Geyer and Shergold, 2000).

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Fig. 3. Palaeogeographical evolution interpreted in the Cambrian basins and platforms of the western Gondwanan margin acrosseight intervals (not to scale).

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Fig. 3 (Continued).

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agreed base of the Ordovician (Landing et al.,2000). Fig. 3A,B provides graphic representationof the changing palaeoenvironmental relation-ships discussed on the following pages.

3.1. Morocco

The Late Proterozoic (‘Panafrican’) orogeny inMorocco and northwestern Africa was followedby deposition of a Neoproterozoic(?)^Cambriancover succession that illustrates a long-term tran-sition in lithofacies associations, and includes athick interval (locally more than 2000 m) of car-bonate-dominated rocks followed by siliciclastic-dominated rocks higher in the Cambrian in thewestern Anti-Atlas. The Neoproterozoic(?)^Cam-brian transition in this region, viz the Adoudou,Lie de vin, and Igoudine Formations, is dominat-ed by restricted marine platform limestones anddolostones, and is overlain by a long-term shale^carbonate cycle in the Lower Cambrian, theAmouslek Formation. Uppermost Lower Cam-brian units include up to 200 m of ¢ne-grainedsiliciclastics with relatively minor nodular andbedded limestones (Issafen Formation), overlainby the higher energy, shallow-marine, sandstone-dominated facies of the Asrir and Tazlaft Forma-tions, which characterises the Lower^MiddleCambrian transition. This period re£ects a signi¢-cant change in sedimentation, related to an im-portant phase of relative sea-level £uctuationwhich caused a major regression followed by animmediate transgressive event (Geyer, 1989;Geyer and Landing, 1995).Limited carbonates also characterise the lower

Middle Cambrian. Fossil-hash limestone beds areregularly encountered only in the ‘Bre'che a' Mic-macca’ Member (lower part of the Jbel WawrmastFormation, up to 300 m thick). Progressivelystronger, higher-latitude in£uences are shown bythe thick shallow-marine deposits that accumu-lated during most of the Mid- and Late Cambri-an; they are a monotonous succession, up to 700m thick, of siliciclastic sediments representingsandy shoals and tidal-£at environments as wellas £uvial to deltaic environments prograding ondistal muddy substrates (Jbel Afraou, Rich Khli-fa, Bailiella, Azlag, and Jbel Lmgaysmat Forma-

tions). The late Late Cambrian is unknown fromthe Moroccan Atlas but appears in part to becharacterised by some 115 m of shallow-marine¢ne-grained siliciclastics (Dar Bou Azza Forma-tion) in the Moroccan Meseta (Mergl et al., 1998).In this area this unit overlies a signi¢cant unit of100 to 200 m of sandstones representing anotherimportant regressive unit of apparently earliestLate Cambrian shoreline deposits, the El HankFormation.

3.2. The Iberian Massif

The Cambrian rocks rest unconformably onNeoproterozoic rocks in the whole massif exceptin the CIZ, where the Vendian^Cambrian transi-tion is tentatively located in the Pusa Shales(Brasier et al., 1979). The transition contains mega-breccias, conglomerates, quartzites, limestonesand anoxic sediments deposited in a mixed shelfand slope apron (Valladares, 1995). The Cambri-an commences with a dominantly terrigenous suc-cession (or lower lithosome): the Herrer|¤a Forma-tion, 900^1700 m thick, in the CZ, representingdeltaic deposits with £uvial and intertidal episodes(Rodr|¤guez Ferna¤ndez et al., 1991); the Ca¤ndanaGroup, up to 2500 m, in the WALZ with shallow-water marine and continental facies (Crimes et al.,1977); the Tamames^Azorejo Formations in theCIZ, 500^600 m thick; the Torrea¤rboles Forma-tion, 0^350 m thick, in the OMZ with shallowsubtidal and intertidal deposits ; and the Ba¤mbolaand Embid Formations, 500^900 m thick, in theIC, interpreted as progradational deltaic systemsand storm- and wave-dominated nearshore set-tings passing upward into tidally in£uenced sub-tidal and intertidal environments. During EarlyCambrian times, the OMZ displayed the settingof rifting processes (Vegas, 1978; Mata andMunha¤, 1990).The second lithosome re£ects the diachronous

establishment of mixed carbonate^siliciclasticplatforms, ranging from Ovetian to early Mid-Cambrian times: the lower member of the La¤n-cara Formation (150^225 m thick) in the CZ,and the Vegadeo Formation (50^500 m) in theWALZ, displaying peritidal to shallow-waterdeposits with isolated microbial^archaeocyathan

PALAEO 3071 9-5-03


buildups (Zamarren‹o, 1972; Russo and Bech-sta«dt, 1994; AŁ lvaro et al., 2000b). Mixed plat-forms were established across the late Ovetian^Marianian interval in the IC (the Jalo¤n, Ribotaand Hue¤rmeda Formations; 300^500 m thick),Los Navalucillos and Tamames Limestones inthe CIZ (120^600 m thick), and the Pedrocheand Alconera Formations in the OMZ (350^900m thick). These contain stromatolitic and archae-ocyathan-microbial buildups, sandstones and var-iegated shales deposited in shallow subtidal to in-tertidal and supratidal environments; locally theybear anhydrite or gypsum pseudomorphs (see AŁ l-varo et al., 2000a, and references therein). By con-trast, the mixed sedimentation of the second lith-osome was interrupted in the IC, CIZ and OMZby early Bilbilian regressive siliciclastic sedimentsrelated to the Daroca regression (AŁ lvaro and Ven-nin, 1998), lithostratigraphically recognised as theDaroca (IC), Los Cortijos^Endrinal (CIZ) andCastellar (OMZ) Formations (50^150 m thick).An early Bilbilian prograding shoreline is postu-lated in these areas, in which coastal and shoalcomplexes grew and migrated seawards duringtimes of rapid sediment in£ux and regressive con-ditions.The Lower^Middle Cambrian transition is

characterised by a signi¢cant change in the styleof sedimentation, related primarily to an impor-tant phase of tectonic activity. As a result, anirregular topography was widely developed onthe platforms, which were di¡erentiated into amosaic of horsts and grabens. In situ carbonateproduction on relative topographic highs gave riseto the ‘griotte’ facies of the upper member of theLa¤ncara (CZ) and Mansilla (IC) Formations.The third lithosome re£ects the disappearance

of carbonate deposition in the whole Iberian Mas-sif ; it is interpreted as shallow platform depositsof intertidal and braid-plain deltaic environmentsin the Oville Formation (80^800 m) and the lowerpart of the Barrios Formation (300^400 m thick)in the CZ (Aramburu et al., 1992; Aramburu andGarc|¤a-Ramos, 1993), the shallow-marine depos-its of the Cabos Series (up to 4000 m thick; Bald-win, 1977; Marcos and Pe¤rez Estau¤n, 1981), andthe Murero Formation, Aco¤n Group and Valcon-cha¤n Formation in the IC (600^1400 m thick)

representing storm-dominated o¡shore, prograd-ing sandy shoals, and tidal-£at environments (AŁ l-varo and Vennin, 2001).

3.3. The southern British Isles

In the southern British Isles, Cambrian rocksoccur in England and Wales in scattered inliers,the largest of which are: (1) the Harlech Dome(and the St. Tudwal’s Peninsula), and the Arfonarea of northern Wales; (2) Pembrokeshire insouth Wales; (3) a number of small outcrops inShropshire; (4) the Malvern Hills ; and (5) theNuneaton area in Warwickshire, British Mid-lands.The lithostratigraphical successions of these

areas di¡er greatly. The relatively thick and com-paratively complete succession of northern Walesincludes Lower Cambrian deposits consisting ofs 450 m of deltaic sandstones (Dolwen Grits)overlain by up to 200 m of prodeltaic shales(Llanbedr Slates), and a formation of up to 800m of proximal turbiditic sandstones with shaleintercalations (Rhinog Grits). Middle Cambriandeposits are generally basinal turbiditic sand-stones and shales, together about 700 m thick,capped by 100 m of dark fossiliferous shales.Manganese-bearing rocks occur close to the baseof the Middle Cambrian. The Upper Cambrianconsists of 2000 m of ¢ne-grained shales (orslates) and feldspathic £ags, capped by a con-densed unit of black mudstone, the DolgellauFormation, about 100 m thick. The lower UpperCambrian (Maentwrog Formation) is basinal, butthe Festiniog and Dolgellau are open-shelf depos-its.The successions in Pembrokeshire show a com-

parable depositional sequence, commencing withthe Lower Cambrian Caerfai Group, about 300 mthick, consisting of sandstones and shales overly-ing a transgressive basal conglomerate depositedin a shallow-marine, continuously subsiding ba-sin. A depositional break testi¢es a regressivetrend near the top of the group. The lower MiddleCambrian Solva Group consists of roughly 500 mof sandstones and shales deposited in relativelyhigh-energy waters, whereas the upper MiddleCambrian, about 250 m of pyritous mudstones,

PALAEO 3071 9-5-03


termed the Menevian Group, suggests quiet, oxy-gen-depleted environments. The Upper Cambrian‘Lingula Flags’ (more than 600 m thick) are shal-low-water storm-in£uenced deposits of a rapidlysubsiding marine basin.The shelf deposits in the English Midlands

commence with Early Cambrian sandstones, theHartshill Formation (260 m thick), which showsprogressive deepening from a basal shorefacesand. There follow 600 m of outer-shelf mud-stone, the Stockingford Shales, which extendfrom the higher Lower Cambrian to the top ofthe Upper Cambrian. The upper Middle Cambri-an and the uppermost Upper Cambrian showstrong lithological and palaeontological similar-ities to the correlative rocks in Wales, thoughthe formations are thinner. The upper MiddleCambrian to Tremadoc succession is relativelycomplete. The successions in Shropshire, on themargin of the Midlands Platform, partly depos-ited in fault-bounded depocentres (Smith andRushton, 1993), suggest the in£uence of shallowwater almost throughout. At the top of the Com-ley Series are the condensed Comley Limestonesthat represent shallow-marine carbonates contain-ing the best preserved and most important lateEarly Cambrian (in the traditional British sense)trilobite faunas of the British Isles.In summary, the lowest Cambrian deposits

were deposited unconformably on a variegatedvolcanic basement in both Wales and England,whereas younger Cambrian sediments in Walesformed in a subsiding ensialic basin, while theirEnglish equivalents formed on the subsiding Mid-land Platform.

3.4. The Armorican Massif

The Cambrian rocks of the Armorican Massifunconformably overlie the Cadomian basement.The most representative and complete LowerCambrian succession is that of the North Conten-tin Syncline. A SW^NE-trending trough was ¢lledby a succession ranging from 590 to 2100 m inthickness (Dore¤, 1994) with, from bottom to top:(1) the Couville Formation, up to 400 m thick,composed of conglomerates and sandstones inter-bedded with a volcanoclastic complex; (2) the

Carteret Formation, ca. 1000 m, consisting of lit-toral alternations of shales and silty sandstonesrich in microfossils and ichnofossils of Cordubianage; and (3) the Saint-Jean-de-la-Rivie're Forma-tion, ca. 200 m thick, made up of peritidal alter-nations of limestones and siltstones, containingsalt pseudomorphs, microbial^archaeocyathanbuildups and oolitic shoals. Other Lower Cambri-an successions are known in central and southernNormandy and Maine, and comprise small shellyfossils, brachiopods and ichnofossils. A post-oro-genic volcanism took place along two volcano^tectonic structures : ignimbrite sequences underly-ing Lower Cambrian sediments, and the Mainegraben interbedded with Lower Cambrian toArenig? sediments. Other Cambrian fossiliferousrocks are incompletely known, such as a MiddleCambrian trilobite assemblage from the north-eastern Vende¤an Massif and a suspected LateCambrian brachiopod assemblage from the Chan-tonnay synclinorium.

3.5. The Montagne Noire and southwesternSardinia

Both areas display the same three-fold litho-some subdivision as that reported in the CZand WALZ for Early to Mid-Cambrian times.The Neoproterozoic^Cambrian transition is notclearly identi¢ed in the former, where the lowerterrigenously dominated lithosome begins withthe Marcory (more than 1000 m thick) and Ma-toppa (300^600 m) Formations. The second, Ove-tian^Bilbilian, lithosome represents the establish-ment of carbonate platforms; in the southernMontagne Noire these are subdivided into thePardailhan, Lastours and Pont de Poussarou For-mations (100^500 m thick), and in Sardinia thePunta Manna, Santa Barbara, Planu^Sartu andSan Giovanni Formations (600^1200 m thick).These platforms were widely colonised by build-ups of microbial-archaeoyathan communities, as-sociated in Sardinia with a tensional tectonic ac-tivity re£ecting the establishment of an isolatedcarbonate platform (see a summary in Perejo¤n etal., 2000). In both areas the Lower^Middle Cam-brian transition displays the tectonic instabilityalready reported in the CZ and the IC, with de-

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velopment of palaeotopographies and depositionof the ‘griotte’ facies (La Tanque and Campo Pi-sano Formations; up to 80 m thick). After drown-ing of the platforms and burial of the palaeoto-pography by o¡shore shaly sediments (Couloumaand lower Cabitza Formations; 30^80 m thick),the evolution in both areas di¡ered drastically.The southern Montagne Noire exhibits the pro-gradation of sandy shoals (Ferrals Formation;130^200 m thick) and tidally induced systems(La Gardie Formation; 0^500 m thick), followedby important carbonate developments leading todeposition of new mixed carbonate^siliciclasticsediments on intra-shelf ramps (Val d’Homs For-mation; 60^200 m thick). This episodic carbonatesedimentation is unique across the whole westernGondwanan margin for Late Cambrian times. Bycontrast, southwestern Sardinia shows a Middle^Upper Cambrian shale-dominated deposition(Cabitza Formation), distinctly condensed and re-markably uniform (ca. 190 m thick; Loi et al.,1995), which gradually passes into the Tremado-cian.

3.6. Saxo^Thuringia

Lower and Middle Cambrian successions occurin the Doberlug^Delitzsch and Go«rlitz synclines,the Franconian Forest and Thuringia (Berga An-ticline). Early Cambrian relationships betweenthem are controversial because they are of di¡er-ent ages and the limited occurrence of fossils.Upper Cambrian successions are generally notpreserved, as a result of uplift combined with de-nudation and weathering (Linnemann and Busch-mann, 1995). The incomplete Early Cambriansuccession of the Doberlug Syncline represents acarbonate ramp with archaeocyathan-microbialbuildups (Zwethau Formation, more than 700 mthick) developed during early Ovetian times onthe Cadomian basement of Saxo^Thuringia (Eli-cki, 1999). The Lower^Middle Cambrian transi-tion is unknown because of the scarcity of out-crops and structural di⁄culties. The MiddleCambrian of the Doberlug area (Tro«bitz andDelitzsch Formations, ca. 600 m thick), earlyLeonian^Caesaraugustian in age, is relativelyfossiliferous (Sdzuy, 1972). The sediments were

deposited on an open siliciclastic platform. TheMiddle Cambrian of the Franconian Forest con-sists of isolated outcrops that represent at least¢ve di¡erent formations, in ascending order theGalgenberg, Wildenstein, Triebenreuth, Lipperts-gru«n and Bergleshof Formations, of unknownthickness, all characterised by signi¢cant trilobitefaunas.The Early Cambrian of the Go«rlitz Syncline is

very incomplete, but has yielded important trilo-bites. The Charlottenhof Formation is a fragmentof a carbonate platform and represents the tran-sition from rather shallow to deep subtidal con-ditions (Ludwigsdorf Member) and subsequentlyto deeper platform environments (Lusatiops Mem-ber). The age of the formation is late Marianianor late Banian (Elicki, 1994; Geyer and Elicki,1995). Finally, the Cambrian (s.l.) of Thuringia(Berga Anticline) is known only from two bore-holes: it is a slightly metamorphosed carbonatesuccession that has yielded a poorly preserved mi-crofauna, with siliciclastic deposits at the base(Elicki, 1997).

3.7. Bohemia

The Pr›|¤bram^Jince Basin of the Barrandianarea contains about 2000 m of Lower Cambriancontinental conglomerates and sandstones, punc-tuated by the deposition of sediments in a brack-ish environment (the Paseky Shale) containing en-demic merostome(?) arthropods and possiblyphyllocarids (Chlupa¤c›, 1995). Deposition of Mid-dle Cambrian transgressive marine fossiliferousshales was accompanied by two major palaeogeo-graphical changes: (1) an eastward shift of thearea of maximum subsidence, and (2) a changein orientation of the main structural directionsand longitudinal basin axis from a NE^SW to aNW^SE trend (Havl|¤c›ek, in Chlupa¤c› et al., 1998).The Jince Formation (up to 400 m thick) consistsof shales and ¢ne-grained sandstones containing arich and diverse fossil fauna of trilobites, brachio-pods and echinoderms. The unfossiliferous UpperCambrian deposits consist of volcanic complexeswith rhyolitic e¡usions and pyroclastic layers. Inthe Skryje^Tyr›ovice area, the Cambrian succes-sion consists only of the lower part of the Jince

PALAEO 3071 9-5-03


Formation, which rests unconformably on UpperProterozoic rocks. These outcrops re£ect a Mid-dle Cambrian deposition in a ‘channel-like’trough a¡ected by synsedimentary deformationand partly eroded during Upper Cambrian regres-sive conditions. The thickness of the Middle Cam-brian decreases from about 200 m in Skryje toseveral tens of metres towards the northeast.

4. Palaeooceanographic and palaeoclimaticconstraints

The western Gondwanan margin during theCambrian is commonly portrayed as oriented ap-proximately SW^NE in a position on the South-ern Hemisphere (McKerrow et al., 1992). As aconsequence of this orientation, the shape of thetropical zones was a¡ected by the course of majorocean currents transporting cool waters towardsthe tropics (Fig. 4). This would have caused thetropics to occupy a narrower latitudinal area onthis western margin compared with the easternmargins (Briggs, 1995). This inferred palaeogeo-graphical situation may be compared with that ofthe western Australian margin during the Eocene(McGowran et al., 1997), when Australia andAntarctica were close enough to prevent the de-velopment of a circum-Antarctica circulation. Asa result, two main surface circulation patternstook place: a South Equatorial Current £owing(up to the Recent) as a low-salinity surface com-ponent of the global thermocline system, and a

western Australian Current (parallel to the west-ern face of Australia) that disappeared with theEocene opening of a gap between both continents.Apparently a similar western Gondwanan SW^NE current rising in latitude from polar areasoperated during Cambrian times, which obviouslya¡ected faunal migrations and the palaeoclimates.The lack of extensive carbonate deposition

across the Precambrian^Cambrian transition inthe western Gondwanan margin, except theAdoudou and Lie-de-vin Formations in Morocco,can be explained by a combination of active Ca-domian orogenic phases, and active source re-gions, which supplied coarse-grained siliciclasticsand inhibited carbonate production. Much of ourunderstanding of the Lower Cambrian palaeocli-mate evolution on the western Gondwanan mar-gin comes exclusively from lithological indicatorsof climate, such as limestone (mainly reefal) andevaporite precipitation (AŁ lvaro et al., 2000a).Lower Cambrian salinities seem to have been epi-sodically and locally high enough to generatewidespread deposition of evaporates, suggestinga generally subtropical arid climate. Althoughan Early Cambrian warm period has beenclaimed, global warming does not necessarily im-ply a poleward shifting of isotherms but mightre£ect their spacing when thermal gradients be-came £atter.During Early and earliest Mid-Cambrian times,

the western Gondwanan margin was rimmed byenormously extensive shallow-water subtropicalplatforms, where carbonate deposition was some-

Fig. 4. Global Cambrian palaeogeography (modi¢ed after McKerrow et al., 1992) and proposal of palaeocurrents discussed inthe text.

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times outpaced by local, craton-derived terrige-nous clastic sediments. As a result, the proportionof carbonate and siliciclastic rocks varies bothregionally and stratigraphically. Incursions ofshale onto the platforms is commonly related tosea-level rises, when the remobilisation and trans-port seaward of ¢ne-grained siliciclastic sedimentsoutpaced carbonate production. By contrast, re-gressive trends are represented by basinward shiftsof peritidal deposits in carbonate and mixed plat-forms, and deltaic and coastal plain dilution re-lated to terrigenous in£ux on platforms close tocontinental sources. For instance, the siliciclasticdeposition related to the Daroca regression duringthe earliest Tissa¢nian or early Bilbilian age, af-fected the Souss Basin, the OMZ, CIZ and ICplatforms, but was absent in the CIZ, WALZ,Montagne Noire and southwestern Sardinia plat-forms.Despite the lack of lithological indicators for

the climate during the Mid- to Late Cambrian,a transition from carbonate-dominated rimmedplatforms to siliciclastic temperate- to cool-waterplatforms is attributed to changing oceanographicconditions across the Lower^Middle Cambriantransition. The southward-moving warm tropicalwaters of western Gondwana progressively in-vaded cold seas; as a result, Middle Cambriansediments re£ect a transition to temperate-waterplatform sediments. In a study of the CambrianIberian platform (IC), AŁ lvaro and Vennin (2001)described a succession of benthic communities in-terpreted to have developed during a cooling pe-riod. This cooling trend, from subtropical to tem-perate waters, was marked by the successivedecline and disappearance of: (1) ooids, evaporiterelics and microbial carbonates; (2) eocrinoid-sponge meadows; (3) hydrodynamic bioclasticcarbonates related to the contraction of carbonatefactories into increasingly narrow belts (‘griotte’facies) ; and there are trends to (4) an increaseof trilobite-dominant communities on muddy bot-toms; and (5) to decreasingly fossiliferous com-munities, then dominated by linguliform brachio-pods in sandy shoals. The Languedocian^LateCambrian interval seems to mark the culminationof the cooling trend as shown by the pervasivedecrease in diversity, associated with the drastic

disappearance of bioclastic carbonates. On theother hand, the development of Late Cambrianisolated carbonate platforms within temperate-water settings of the southern Montagne Noire(Val d’Homs Formation) signals short-lived warm-ing episodes in meridional parts of the westernGondwana margin extending into Tremadociantimes.

5. Biogeographical patterns

5.1. Lower Cambrian

The Lower Cambrian has for a long time beenbased on the concept of being characterised bythe earliest Phanerozoic fossils. Bro«gger (1886)and subsequently Walcott (1891) developed theidea of a Lower^Middle Cambrian boundarybased on the ¢rst appearance of the trilobite ge-nus Paradoxides (s.l.) in Baltica and the disap-pearance of Olenellus (s.l.) in Laurentia. The low-er boundary of the Cambrian was not ¢xed withany precision but in early days was assumed tocoincide roughly with the ¢rst appearance oftrilobites. Since 1974 work by the InternationalSubcommission on Cambrian Stratigraphy haschanged this concept. Incorporation of stratawith ‘primitive’ Metazoan sclerites of uncertainsystematic a⁄nities (the so-called small shelly fos-sils) pushed down the level of the Cambrian lowerboundary considerably. In the present concept,the earliest Cambrian strata are characterised byrocks with the ¢rst diversi¢ed trace fossil assem-blages that underly strata with small shelly fossils.As a result, the Lower Cambrian was extendeddramatically and the trilobite-bearing strata rep-resent less than half of Early Cambrian time aspresently understood (Landing, 1994; Landing etal., 1998).In the Lower Cambrian, the ideal of a global

biostratigraphy and palaeobiogeography su¡ersfrom both a relatively limited diversity of trilo-bites and their pronounced endemism. Further-more, the distribution of trilobites is, from theearliest times, strongly controlled by facies, sothat even a precise interregional correlation is dif-¢cult. The most complete Early Cambrian trilo-

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PALAEO 3071 9-5-03


bite succession of western Gondwana, with rela-tively diverse trilobite faunas from various biofa-cies, comes from the Moroccan Atlas ranges (Fig.5A), with more than 200 trilobite species de-scribed from Lower and lowermost Middle Cam-brian strata. This area clearly is the key for ourunderstanding of the Lower Cambrian palaeobio-geography of western Gondwana and the neigh-bouring regions. Historically, this is the regionwhere the ¢rst elaborate Lower Cambrian biozo-nation was developed (Hupe¤, 1952). The LowerCambrian to lowermost Middle Cambrian bio-stratigraphy of Morocco now comprises ten bio-zones assigned to three stages (Fig. 2), which areall characterised by signi¢cant trilobite faunas andseparated by faunal turnovers (Geyer, 1990a;Geyer and Landing, 1995).Recent correlations suggest that the earliest tri-

lobites from the lowermost Issendalenian Stagepre-date other trilobite occurrences in westernGondwana and are among the ¢rst trilobitesknown so far on a global scale (Sdzuy and Geyer,1988; Geyer and Landing, 1995). These faunas ofthe Eofallotaspis Zone include several species ofthe fallotaspidid genus Eofallotaspis and the big-otinid genera Hupetina and Bigotinops, all ofwhich are endemic, as are most trilobites of theoverlying Fallotaspis tazemmourtensis, Choubertel-la and Daguinaspis Zones. The entire Issendale-nian is characterised by fallotaspidid, redlichiidand bigotinid trilobites. Fallotaspidids (the generaChoubertella, Daguinaspis, Eofallotaspis, and Fal-lotaspis) appear to have preferred ¢ne-grained sil-iciclastic substrates and are now found almost ex-clusively in shales ; deposition took place on aslightly restricted carbonate platform with archae-ocyathan-microbial buildups, apparently with lim-ited connections to the open sea. Elsewhere fallo-taspidids (in a broad sense) occur with certaintyin the Lower Cambrian of the White^Inyo Moun-tains, California, adjacent Nevada, and Siberia.Redlichiid^saukiandid trilobites (such as Marsai-

sia, Pararedlichia, and Resserops) are also typicalin ¢ne-grained siliciclastics. They share characterswith genera from other Cambrian regions such asthe South China (or Yangtze) Platform (Nan-gaoian Stage) and are sometimes erroneously syn-onynised with Eoredlichia because of the limitedset of characters.

Lemdadella and bigotinid trilobites are typicalof well-bedded platform carbonates. They may behelpful for future correlations but are still imper-fectly known because the past research concen-trated largely on shale faunas. Similar bigotinidsare known from the Lower Cambrian of the Ar-morican Massif and the Co¤rdoba platform of theOMZ (Bigotina) but also from the Lower Cam-brian Pagetiellus anabarus Zone (middle Atdaba-nian) of the Siberian Platform, in both areas alsoin platform carbonates. Finally, Lemdadella hasbeen reported from the Lower Cambrian of theOMZ (Lin‹a¤n and Sdzuy, 1978) and Antarctica(Palmer and Rowell, 1995).In summary, the Issendalenian shows a moder-

ate diversi¢cation of trilobite faunas with highpreference for ¢ne-grained siliciclastics and periti-dal carbonate muds. Most species and the major-ity of the genera are endemic, which is probablydue to a lack of knowledge of coeval faunas fromother western Gondwanan areas.The Issendalenian^Banian transition in Moroc-

co is marked by a dramatic decrease of the fallo-taspidids, whereas primordial ellipsocephaloids(the antatlasiids) arise and increase impressivelyin number and diversity to form the most charac-teristic faunal element in the Banian. Fallotaspi-dids are replaced by neltneriids. The ¢rst eodis-coids (Delgadella, Hebediscus) occur in the lowerBanian. All trilobites apparently have noticeablepreferences for distinct lithofacies, but the frame-work is more complicated than the simple distinc-tion between shale and carbonate facies in theIssendalenian. Banian rocks are dominated by¢ne-grained siliciclastics with relatively minor

Fig. 5. Global distribution chart illustrating presence (black) and absence (white) of Lower Cambrian trilobite genera reported inthe western Gondwanan margin (A), biodiversity patterns (B), and analysis of hierarchical Phi^Pearson similarity (C). Some ofthe reported genera could occur in Fig. 6 as Middle Cambrian trilobites according to the considered horizon of the Lower^Mid-dle Cambrian boundary.

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nodular and bedded limestones deposited in alow-energy middle to upper platform environ-ment.During the Banian, for the ¢rst time, strong

similarities of trilobite faunas can be detected be-tween the Moroccan Atlas region and the OMZ.Delgadella is found in the lower, middle and evenin the upper part of the Banian (A. hollardi, A.guttapluviae and Sectigena Zones) and is also afrequent faunal element in the earliest assemblag-es of the OMZ. The Sectigena Zone of Moroccoshares a number of genera and even species withthe lower two assemblages of the OMZ, such asAndalusiana cornuta, Gigantopygus, Rinconia, Sau-kianda andalusiae, Termierella and Triangulaspis.This distinctive zone is represented in Morocco byseveral quite di¡erent local assemblages that por-tray a variety of coeval biofacies. Stratigraphicalsuccessions dominated by low-energy shales in-clude the Ossa^Morena-type Saukianda^Termier-ella assemblage. Shales to ¢ne-grained sandstonesdeposited in more agitated shallow-marine envi-ronments tend to yield a Berabichia^Delgadella^Pentagonalia fauna. Limestones and marls yieldan assemblage with Hebediscus and Sectigena.Other trilobites are facies-independent and occurin di¡erent types of rocks. Issafeniella has beenidenti¢ed from the Marianian of the OMZ, whichalso shares a ¢rst and simple species Acanthomic-macca) with the Moroccan Sectigena Zone. Secti-gena itself, Berabichia and Issafeniella, have theircounterparts, and obviously close relatives, in theSiberian genera Charaulaspis and Chorbusulina.Both occur in the latest Atdabanian and earlyBotoman of the northern Siberian Platform,where they are accompanied by Hebediscus spp.and Triangulaspis. Two additional species ofChorbusulina were described from the LowerCambrian of Antarctica, but in fact could belongto Berabichia.An important genus that has its ¢rst appear-

ance in the Sectigena Zone is Serrodiscus. Thisgenus has a long stratigraphical range, but it isremarkable that the same type of red shales of theIssafen Formation, in which Serrodiscus ¢rst oc-curs in the Anti-Atlas, also contains the ¢rst Ser-rodiscus species (S. silesius and other species) inthe Marianian of the CIZ and OMZ, and in the

Charlottenhof Formation of the Go«rlitz area,Saxo^Thuringia (S. silesius). The German andSpanish Marianian assemblages also share thegenera Ferralsia and Lusatiops. Ferralsia is knownfrom the OMZ, but was originally described fromthe uppermost(?) Lower Cambrian of the Mon-tagne Noire. An Acanthomicmacca, although dif-¢cult to judge in terms of precise ontogeny, is alsoknown from the Charlottenhof Formation. Aprobable species of Holmia is also identi¢edfrom the Charlottenhof Formation, and speciestentatively assigned to Holmia and Kjerul¢a weredescribed from the lower? Sectigena Zone of theAnti-Atlas. Both genera are distinctive of the Hol-mia kjerul¢ Zone of Baltica.The faunas of the Moroccan Sectigena Zone

and the coeval assemblages from Iberia, Saxo^Thuringia, the southern British Isles, southeasternNewfoundland and other areas are characterisedby an assemblage that include relatively wide-spread eodiscoids. Typical species of this assem-blage include Calodiscus helena, C. lobatus (instrict sense), C. schucherti, Hebediscus (or Dipha-rus) attleborensis, Serrodiscus bellimarginatus, S.silesius, S. speciosus, Triangulaspis annio, T. vigi-lans, and other species of Triangulaspis (Fletcher,1972; Robison et al., 1977; Geyer, 1990b; Geyerand Palmer, 1995). Most of these species are rel-atively endemic, but their ranges show someregional overlap and associated taxa permit asubglobal recognition, for example in Baltica(Sweden), Avalonia (southeastern Newfoundland,New Brunswick, Welsh Borderlands), westernGondwana (Spain, Morocco), Siberia, the Al-tay^Sayan Mountains, and Kazakhstan. In west-ern Gondwana, Triangulaspis zirarii of Moroccohas its counterpart in the extremely similar Trian-gulaspis fusca from the OMZ, which is associatedwith Calodiscus schucherti, a species that also oc-curs in the Moroccan Sectigena Zone.Such assemblages are the earliest to show clear

faunal relationships between Avalonia and west-ern Gondwana. The top of the Callavia broeggeriZone in southeastern Newfoundland (regarded asa distinct Serrodiscus bellimarginatus Zone byFletcher, 1972) includes not only Serrodiscus bel-limarginatus and Hebediscus attleborensis, but alsoAcanthomicmacca species typical of the Sectigena

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Zone, the Marianian of the IC, and the Charlot-tenhof Formation in Saxo^Thuringia. It also in-cludes Mallagnostus (formerly Ladadiscus ; seeWhittington et al., 1997) llarenai, a species ¢rstdescribed from the OMZ. Very similar trilobites,including Acanthomicmacca, Calodiscus lobatus,Hebediscus attleborensis, Serrodiscus bellimargina-tus, and Triangulaspis annio, are known from var-ious levels in the Comley Limestones in the WelshBorderlands of England. Some eodiscoids, such asCalodiscus, Hebediscus, Serrodiscus, and Triangu-laspis, are shared with assemblages from the topof the Judomia and B. micmacciformis^ErbiellaZones on the Siberian Platform.The Lower Cambrian of southwestern Sardinia

has yielded a relatively diverse trilobite fauna witha pre-Sectigena age. Most of the species and evengenera are endemic, probably because they arelargely from lithologies incompletely studied insupposedly coeval successions from other partsof western Gondwana. Particularly well knownare the genera Dolerolenus, Giordanella, andMetadoxides. The few eodiscoids in these assem-blages are of little help for either biostratigraph-ical or biogeographical correlations because theyare also endemic. Another so-called Dolerolenusfauna has been described from the Ovetian ofthe IC (Sdzuy, 1987), an assemblage representingthe oldest diverse trilobite fauna known fromSpain. However, the trilobites are mostly de-scribed under open nomenclature because of theirpoor preservation. Nevertheless, the taxonomica⁄nities are su⁄cient to recognise that both areasshare a similar trilobite fauna and obviously werebiogeographically connected. Both may be re-garded as faunas particularly adapted to relativelyisolated, probably rimmed, carbonate platforms.In a monographic reappraisal of the Sardinian

trilobites, Pillola (1991a) described a number ofnew, endemic species and revised older species,mainly redlichioids. He assigned many of themto genera known almost exclusively from theSouth China Platform and proposed a strong pa-laeobiogeographical connection to that Cambriancontinent. However, trilobite faunas of similar ageand from similar facies are unknown from theplatforms supposed to have been situated betweenthose regions during the Lower Cambrian.

Pillola’s (1991a) revision partly a¡ected the no-menclature of trilobites from the Lower Cambrianof the Montagne Noire. Galloredlichia noiri wasassigned to the ‘Yangtze’ genus Eoredlichia, a ge-nus that occurs in relatively early trilobite assem-blages on the South China Platform such as thefamous Chengjiang Fauna. This assemblage canbe correlated with the Uper Issendalenian andUpper Ovetian. In the Montagne Noire, Gallored-lichia occurs together with Granolenus midi, a spe-cies also known from the Upper Ovetian of theCIZ.A Banian^Tissa¢nian faunal turnover is

marked by a relatively sharp change in the com-position of benthic communities, exempli¢ed by:(1) the appearance and massive increase of proto-lenine trilobites, (2) the sudden but apparentlystacked appearance of other new trilobite groups,and (3) the relatively abrupt disappearance of el-lipsocephaloid lineages. These strata with Lower^Middle Banian faunas are traditionally termed the‘Protolenus Zone’ and regarded as of Early Cam-brian age.Early Cambrian (Comley Series) trilobites are

known from Comley and the Rushton area inthe Welsh Borderland and at Nuneaton on theMidland Platform, together with a very few, in-cluding Hamatolenus and Pseudatops, from north-ern Wales. There has been little recent revision ofBritish Early Cambrian trilobites, and in view ofsigni¢cant recent work in Morocco, Spain andGermany, the generic placement of certain Britishtaxa, particularly of ellipsocephaloids, is nowopen to question. Thomas et al. (1984) listedsome 36 taxa from the Comley Series. If we ac-cept the names currently applied, their a⁄nities liemost strongly with trilobites from elsewhere inAvalonia. About 12 species are common to west-ern Avalonia, including some eight of eodiscoidsand species of Protolenus, Strenuella and possiblyof Callavia ; and a further 12 represent generaknown from Avalonia and the Taconic Belt ofNew York. In contrast, only two species and threeor four genera can be compared with Baltic fau-nas. The a⁄nities with marginal Gondwana ap-pear to be restricted to three species known fromthe OMZ ^ Calodiscus schucherti, Mallagnostusllarenai and possibly Serrodiscus serratus ^ and

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there are another 12 genera in common. The Brit-ish representatives of these genera are: Callaviacallavei and other species, Cobboldites comleyen-sis, Comluella platycephala (Morris, 1988, has

Strenuella (Comluella)), Condylopyge amitina, El-lipsocephalus (or Ourikaia?) heyi (see Geyer,1990b), Hamatolenus douglasi, Hebediscus attle-borensis, Latoucheia latouchei, Micmacca spp.,

Fig. 6. Global distribution chart illustrating presence (black) and absence (white) of Middle Cambrian trilobite genera reportedin the western Gondwanan margin (A), biodiversity patterns (B), and analysis of hierarchical Phi^Pearson similarity (C).

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Protolenus paradoxoides, Serrodiscus bellimargina-tus and other spp., and Triangulaspis annio. Ac-cording to Sdzuy (1961), possibly Andalusiana?(or Kjerul¢a?) granulata should also be included.The greatest resemblance is shown by faunas fromthe ‘Bre'che a' Micmacca’ in the basal MiddleCambrian in Morocco. Geyer and Malinky (1997)correlated this unit with the Protolenus Lime-stone of Comley, Shropshire (Cobbold’s unitAc5; see Rushton et al., 1999), which in Britainwas treated hitherto as late Early Cambrian.

5.2. Middle Cambrian

As discussed above for the Lower Cambriantrilobites of the western Gondwana margin, thedatabase from which the following analysis ismade is heterogeneous. This is due to the exis-tence of barren intervals, across the Leonian inthe Montagne Noire and the Middle^Late Lan-guedocian in Morocco and the OMZ, which pre-vent complete correlations (Fig. 6A). The Lower^Middle Cambrian boundary is not uniform overthe whole area, because it has been characterisedby di¡erent bioevents based on trilobite and acri-tarch appearances. The Lower^Middle Cambriantransition contains numerous barren stratigraphi-cal intervals and unconformities, so that the tran-sition (and not its boundary) will be describedbelow in two key areas with continuous sedimen-tation and fossil wealth: Morocco and the IC. Inthe present state of knowledge, two di¡erent bio-diversity patterns are recognised during Mid-Cambrian times: (1) a progressive decrease in di-versity in Morocco, and (2) a composite increase^decrease trend in the whole of southwestern Eu-rope. Consequently, during Early to Middle Cam-brian times, the area of maximum diversity mi-grated northeastwards; a result that may berelated to the southwestward drifting of the west-ern Gondwana margin.In general, two Middle Cambrian trilobite di-

versi¢cations are recognised (Fig. 6B): (1) a majormigratory radiation of trilobites during the Tissa-¢nian/Upper Bilbilian to Caesaraugustian whenmost families appeared without obvious ancestorsin the fossil record; and (2) a Late Languedociantrilobite diversi¢cation connected with transgres-

sive episodes and representing new migratory in-puts of recurrent trilobite genera. Both episodeswere punctuated by distinct faunal turnovers.Several lines of evidence have been put forward

that imply extinction events towards the end ofthe Early Cambrian. They point to the conceptof a widespread latest Early Cambrian biocrisis,as the culmination of a worldwide decline of tri-lobites and other taxa that commenced earlier inthe Early Cambrian and led to a global diachro-nous extinction (Debrenne, 1991; Brasier, 1995;Zhuravlev, 1995; AŁ lvaro et al., 1999). The termi-nal Lower Cambrian strata display a successionof extinction events related to the disappearancein the western Gondwana margin of microbial^archaeocyathan buildups and the establishmentof regressive conditions. A local event has beenidenti¢ed in the IC at the end of this tendency,termed the ‘Valdemiedes event’. Data from thetrilobites do not necessarily support a catastroph-ic extinction at this horizon, their record beingimperfect on account of barren intervals prior tothat event. The event represents the end of therelatively endemic, inner-platform survivors ofthe area, followed by an inshore migration ofmore pandemic, outer-platform faunas.It should be noted that during the Lower^Mid-

dle Cambrian transition the Moroccan successionshows a di¡erent pattern from that portrayed bysouthwestern Europe. In the Atlas ranges, the ter-minal Lower Cambrian^basal Middle Cambrianregressive^transgressive trends are represented bya marked coarse clastic input, regionally over-printed by volcanoclastic deposits. Two distinctunconformities exist, one within the sandstone^volcanoclastic unit (which is accordingly dividedinto the Tazlaft and Tatelt Formations), and an-other one at its top. The fossil record, however, isrich across both formations and suggests that apunctuated extinction did not take place. Al-though the faunal turnover reported across theBanian^Tissa¢nian transition signals a stronglydepauperate fauna in the basal Tissa¢nian Hupeo-lenus Zone, some species range across the bound-ary; this turnover pre-dates the ‘Valdemiedesevent’ of the IC, which roughly coincides withthe base of the Cephalopyge notabilis Zone. Thefossil poverty of the Hupeolenus Zone fauna can

PALAEO 3071 9-5-03


largely be attributed to unfavourable living con-ditions during the pronounced input of terrig-enous clastics and volcanogenic components. Bycontrast, the C. notabilis Zone includes one of themost diverse trilobite assemblages in the worldknown from this time. Its base is regionally placedwithin the clastic blanket so that the diversi¢ca-tion can be proved to be a true radiation ratherthan merely created by a favourable diversity ofecological niches. Nevertheless, the peak diversitywas not reached until the establishment of a num-ber of diverse facies conditions on the basal Mid-dle Cambrian platforms. As noted above, thestrata with Lower^Middle Banian faunas are cor-related with the ‘Protolenus Zone’ in areas such aseastern Avalonia (England and Wales), westernAvalonia (southeastern Newfoundland and NewBrunswick) and Baltica, and are regarded as ofEarly Cambrian age.The earliest Mid-Cambrian faunal recovery can

be regarded as an adaptative radiation of an im-migrant trilobite community that progressively in-vaded the western Gondwanan margin from deep-er-water environments. The Tissa¢nian/Leonianimmigration is characterised by the stepwise ap-pearance of relatively cosmopolitan trilobites,such as the acrocephalitids, agraulids, corynexo-chids, conocoryphids, paradoxidids, solenopleur-ids and agnostoids (condylopygids, phalacromidsand spinagnostids). The observed pattern of di-versi¢cation was not facies-controlled because tri-lobites appeared in a wide diversity of facies andenvironments. Some trilobite families crossed theLower^Middle Cambrian transition, although de-creasing sharply in diversity, such as the ellipso-cephaline and protolenine trilobites. Subsequent-ly, the western Mediterranean area apparentlyoperated as a centre of evolutionary radiation(or a site of speciation), from which the specieswere able to spread outward into adjacent plat-forms, in some cases by sympatric speciation (AŁ l-varo and Vizca|«no, 2001). As a result, a relativebiogeographical concordance of evolutionary pat-terns is virtually complete for some trilobite fam-ilies, and there are no apparent extrinsic barriersseparating the sibling species. Some Caesaraugus-tian benthic species evolved in this area andproved capable of migrating to colonise areas

from Newfoundland and Rhode Island to Turkey(e.g. Badulesia tenera). There were no physicalbarriers preventing trilobite dispersal, which wasfavoured by uniform sea£oor conditions (open-platform, muddy o¡shore substrates), and lowtemperature-gradients. Diversity increased andspecies proliferated during Caesaraugustian times.The acme is associated with transgressive condi-tions and subsequent connection of platforms,and represents the peak in diversity of trilobitefamilies, genera and species during the MiddleCambrian.A major reduction in the number of trilobite

genera occurred across the Caesaraugustian^Lan-guedocian transition, at a time of widespreadcoarse-grained terrigenous input associated witha well-documented regressive trend. The disap-pearance of trilobite taxa is related to the dis-appearance of their habitats. This diachronousdecline was catastrophic for some previouslysuccessful trilobites, such as the solenopleuropsi-nae (except the genus Sao). This biocrisis is not amajor extinction at familial level for trilobites, butrepresents a signi¢cant generic turnover. The fau-nal turnover coincides with a rapid progradingand shoaling that produced widespread areas ofcoarse-grained sandstones and, therefore, seemsto re£ect geographically extensive environmentalchanges. In Baltica there is a regression at thelevel of the late Middle Cambrian AndrarumLimestone (Nielsen, 1996), and this is recognisedin the southern British Isles and Newfoundland asan ‘Andrarum Limestone regression’, with the lae-vigata Zone above. It is unlikely that the last Lan-guedocian fauna is as young as the L. laevigataZone (see Fig. 2); paradoxidids appear to be ex-tinct by then in Baltica and a new fauna occurswith trilobites of Upper Cambrian aspect, such asAgnostus, Andrarina, and Proceratopyge.The Late Languedocian trilobite diversi¢cation

is well documented in the southern MontagneNoire, Sardinia, and Saxo^Thuringia, where it ischaracterised by an abrupt increase in genera andfamilies. The appearance of this new assemblageof trilobites represents an immigration event re-lated to transgressive pulses. Episodically, o¡-platform trilobite taxa migrated towards the innerplatform, leading to a strict facies control on the

PALAEO 3071 9-5-03


lithofacies di¡erentiation, and coinciding with theestablishment of suitable muddy o¡shore sub-strates. However, the diversity of the new groupsis less than in the previous radiation. Dispersaltook place to some extent, but the presence ofsome provincialism suggests that species exchangewas selective, and that the oceanic areas may haveserved as partial barriers to migration. Late Lan-guedocian trilobites include typical genera of theLeonian^Caesaraugustian diversi¢cation (such asthe conocoryphids and paradoxidids) and newAsian invaders, such as the genera Abharella,Chelidonocephalus, Derikaspis and Dorypyge (seeAŁ lvaro et al., 1999, and references therein). Thisin£uence illustrates the immigration of faunasfrom the northern Gondwanan margin, whichwent on during Late Cambrian times (see below).However, the percentage of pandemic species de-creased across the Languedocian, and geographicdistribution also decreased because of successiveregressive pulses.The Middle Cambrian of the British Isles (St.

David’s Series) is in the lower part generally de-veloped as arenaceous clastic deposits, and inShropshire (and possibly elsewhere also, thoughthe evidence is not so clear) they overlie a breakin the succession. These deposits contain a re-stricted fauna of agnostoids, paradoxidids and afew other taxa such as Bailiella. Higher up, darkmudstones (the classical ‘Menevian Beds’) arewidespread. They contain a richer fauna of agnos-toids, paradoxidids and various polymerid trilo-bites, commonly including blind genera. Thesedark beds are overlain by a hiatus that in mostplaces is evinced by erosion during the marinelow-stand corresponding to the Solenopleura(now Erratojincella) brachymetopa Zone (Rush-ton, 1978; Nielsen, 1996). Thomas et al. (1984)listed 106 named St. David’s taxa, and more re-cent work has added a few more: in St. Tudwal’sPeninsula, northern Wales, Young et al. (1994)¢gured forms derived from the brachymetopaZone, namely Bailiaspis glabrata (as a senior syn-onym of B. nicholasi), Dolichometopus cf. svecicus,Linguagnostus aristatus, and fragments of Acro-cephalites, Dorypyge and Centropleura, whilstBridge et al. (1998) have ¢gured Luhops expect-

ans, L.? pugnax (Clarella in Morris, 1988) andParadoxides abenacus. The faunas are mostclosely related to those of Baltica, with which 40species are in common, and a further 14 at genericlevel, and with western Avalonia (mainly New-foundland), with which 34 species are in commonand a further fourteen at the generic level. Resem-blance with the Gondwanan margin is consider-ably lower: there are eight species in commonwith Bohemia and Saxo^Thuringia, namely Con-dylopyge rex, Eccaparadoxides pusillus, Luhopsexpectans, Peronopsis integra(?), Phalacroma bi-bullatum, Phalagnostus nudus, Pleuroctenium gra-nulatum granulatum, and Plutonides (Hydrocepha-lus) hicksii. Further genera in common appear toinclude Acadoparadoxides, Agraulos, Ctenocepha-lus, Dawsonia? (if Metadiscus is a synonym), Ho-locephalina, Hypagnostus, Paradoxides, Ptychag-nostus, and Ptychoparia? There are about sixspecies in common with the Iberian Peninsula,namely Agraulos longicephalus, Condylopyge cari-nata, C. rex, Paradoxides davidis?, Peronopsis fal-lax?, Plutonides hicksii, and, from Sardinia, Ano-polenus henrici? (Loi et al., 1995). The only Britishmember of the important peri-Gondwanan familySolenopleuropsidae is Solenopleuropsis (Manuble-sia) variolaris, which occurs in the punctuosusZone. This assessment shows once again that theagnostoid trilobites are the most widely dispersedof taxa, with Condylopyge rex being recordedfrom each of the regions considered here. Theresemblance of the British (eastern Avalonia),Newfoundland (western Avalonia) and Baltic fau-nas rests largely on the similarity of their agnos-toids, and doubtless the relative paucity of peri-Gondwanan agnostoids is a hindrance to inter-regional correlation. Avalonia and Baltica showagnostid faunas intermediate between those ofLaurentia and parts of the Gondwanan margin(Conway Morris and Rushton, 1988). Some spe-cies of paradoxidids are also relatively widely dis-tributed, although their larger size and commonlyfragmentary preservation sometimes makes fordi⁄culties in collection and identi¢cation. Thepolymerid trilobites such as conocoryphids andptychopariids appear much more frequently tobe endemic.

PALAEO 3071 9-5-03


5.3. Upper Cambrian

Upper Cambrian trilobites are very rare in thewestern Gondwanan margin, compared with thewealth recorded in the Lower and Middle Cam-brian, and occur in a variety of lithofacies associ-ations ranging from inner platform sandstonesand shales to outer platform and slope carbon-ates. Temperate-water faunas were less diverseand more impoverished than Lower^Middle Cam-brian ones. In contrast, British faunas containaround 100 taxa that represent the whole UpperCambrian.

5.3.1. The Iberian MassifLate Cambrian trilobites occur in the WALZ,

DM and IC. The occurrence of Late Cambriantrilobites in the DM was ¢rst reported by Colchen(1967) who determined the presence of Chuangiaand Prochuangia in the lower member of the Na-jerilla Formation. These trilobites were subse-quently described and revised by Shergold et al.(1983), who recognised two trilobite associations:(1) an oldest one comprising four taxa (aphelas-pidine, leiostegiid?, Maladioidella colcheni, andLangyashania felixi) ; and (2) a younger assem-blage, from the second member of the NajerillaFormation, containing only two taxa (pagodiineand a solenopleuroidean? a¡. Lajishanaspis).These determinations support Colchen’s conten-tions of an Asian in£uence, e.g. Maladioidellahas a distribution that extends from Spain tonorthern China (Rushton and Hughes, 1996;Jago, in Brock et al., 2000).In the IC, the ¢rst Late Cambrian trilobites

were determined by Sdzuy (in Josopait, 1972),and subsequently described by Shergold andSdzuy (1991). These occur both in the middlepart of the Valtorres Formation and the lowerpart of the overlying Valconcha¤n Formation.The faunal association includes agnostoid, aphe-laspidine a¡. Aphelaspis rara (OrIowski) sensu Z] y-lin¤ska (2001), Elegantaspis cf. beta, Parachangsha-nia sp., Pseudagnostus sp., Punctaspis? schmitzi,solenopleuroidean, and Valtorresia volkeri. TheUpper Cambrian assemblage is overlain in theuppermost Valconcha¤n Formation by a sandstonebed containing Pagodia (Wittekindtia) alarbaensis

and olenid fragments now thought to represent aspecies of Jujuyaspis that would indicate an ear-liest Ordovician age. It is possible that the occur-rence of P. (Wittekindtia) in the DM is also of thisage. Both there and in the IC, this subgenus isassociated with the echinoderm-like Oryctoconuslobatus Colchen and Ubaghs, 1969. By virtue ofthe aphelaspidoid morphologies in the IC, andMaladioidella in the DM, the trilobites of north-ern Spain are considered to have an Iverian ageaccording to the Australian standard and a Sun-waptan age on the Laurentian timescale.

5.3.2. Montagne NoireLate Cambrian trilobites occur in the Val

d’Homs Formation (Ferrals-les-Montagnes).They were ¢rst described by Feist and Courtessole(1984), who recognised two species referred toProchuangia gallica and Bergeronites latifalcatus.Subsequent collecting from a limestone lens closeto the base of the formation has revealed the pres-ence of further taxa (Shergold et al., 2000):Abharella sp., Ammagnostus (A.) a¡. sinensis, Kor-magnostus? sp., Olentella cf. africana, Palaeadoteslatefalcata, Paraacidaspis ultima, Proceratopyge(P.) spp., Prochuangia gallica, Shengia cf. spinosa,and Stigmatoa courtessolei. The age of this faunais judged to be similar to, but slightly older than,that characterised by Maladioidella in the DM.Many of its elements have a widespread distribu-tion along the Gondwanan margin (Shergold etal., 2000), and the agnostoids and Proceratopygeextend even to Laurentia. However, the Ferralsassociation shows greatest relationship withsouthern and central China, Australia and Ant-arctica.Elsewhere in the Montagne Noire, an assem-

blage containing species of Maladioidella, On-chonotellus, Probilacunaspis, Proceratopyge, Pro-chuangia, and a pseudagnostinid occurs incarbonates of the Val d’Homs Formation to thesouthwest of Coulouma. Once more, its composi-tion is dominated by Chinese genera. This assem-blage is the youngest Cambrian fauna found sofar in the Montagne Noire. The earliest Ordovi-cian is marked traditionally by the appearance ofProteuloma geinitzi described by Sdzuy (1958)from reddish limestones just below the base of

PALAEO 3071 9-5-03


the La Dentelle Quartzite in the vicinity ofCombes de Barroubio.

5.3.3. SardiniaAlthough they have not been systematically

documented, Late Cambrian trilobites have beennoted and illustrated in the upper part of theCabitza Formation in the Cabitza area and northof Domusnova (Loi et al., 1995, 1996). They oc-cur in ¢ne sandstones and siltstones probably rep-resenting a tide-dominated deltaic environment.Illustrated trilobites (Loi et al., 1995, 1996) arefrom faunal assemblage CAB 5b, which containsMacropyge sp., Maladioidella cf. colcheni, Micrag-nostus cf. haudei, Niobella cf. primaeva, Onchono-tellus? amsassensis, Proceratopyge sp., a leioste-giid a¡. Pagodia, a eulomid, and a calymenid.At Monte Cani this assemblage is succeeded bygreen siltstones containing (CAB 6) Rhabdinopora£abelliformis (Pillola and Gutie¤rrez-Marco, 1988;Pillola and Leone, 1993), Proteuloma geinitzi (Pil-lola, 1991b) and Oryctoconus cf. lobatus. Theyounger assemblage is clearly of earliest Trema-docian age.

5.3.4. MoroccoTwo Late Cambrian trilobite taxa have been

documented from the upper member of the JbelLmgaysmat Formation, in the Foum Zguid re-gion (Destombes and Feist, 1987). Olentella afri-cana and Seletella latigena represent the ¢rstUpper Cambrian occurrence in North Africa.Both genera occur in the Sakian, Aphelaspis^Ku-jandaspis Zone, in central Kazakhstan (Ivshin,1955, 1956, 1962). Olentella cf. africana has alsobeen described from Ferrals-les-Montagnes in theMontagne Noire (Shergold et al., 2000). The fos-siliferous Upper Cambrian is overlain by a dis-cordance followed by shales containing Rhabdino-pora £abelliformis.

5.3.5. British IslesThe Late Cambrian (Merioneth Series) trilo-

bites have been found in England and Waleswherever rocks of that age are exposed, and allthe main zones are proved. In the lower Merio-neth Series there are commonly muddy, silty andsandy beds with Homagnostus and a few species

of Olenus. The higher Merioneth is developedeverywhere as black mudstones related to the ole-nid biofacies and contains a considerable varietyof olenids, admixed with a small number of ag-nostoid and other non-olenid genera. A numberof new records have appeared since the publica-tion of Thomas et al. (1984): Allen and Jackson(1985) recorded the presence of Niobella homfrayipreciosa, Parabolina acanthura, P. cf. angusta, andParabolinella contracta ; Hughes and Rushton(1990) transferred ‘Dikelocephalus’ discoidalis tothe ceratopygid genus Cermatops ; Cope andRushton (1992) described previously unknownfaunas from Llangynog, south Wales, that includeCtenopyge (C.) linnarssoni ; Howells and Smith(1997) recorded the presence of Loganellus sp.,Maladioidella abdita (‘Conokephalina’ in Morris,1988), Olenus cf. solitarius, Parabolina a¡. mober-gi and P. (Neoparabolina) lobata praecurrens ;Bridge et al. (1998) noted Ctenopyge (C.) pectentenuis from a borehole.Of these trilobites, the olenids are widely dis-

tributed where the appropriate dysaerobic facies ispresent, and some others, such as agnostoids (es-pecially Glyptagnostus reticulatus), Proceratopygeand other ceratopygoids, Irvingella and Maladioi-della (Rushton and Hughes, 1996) are widely dis-tributed in a variety of shelf environments. InBritain other rare non-olenid, non-agnostoidtaxa include species of Acanthopleurella, Aphelas-pis (Z] ylin¤ska, 2001), Araiopleura, Conophrys, Cy-clolorenzella, Eoasaphus, Loganellus, Modocia,Niobella, Parabolinoides, Proteuloma, Richardso-nella, and Schmalenseeia.Thomas et al. (1984) listed 86 named taxa and

the above-mentioned works added nine more. Ofthese, 61 are identi¢ed with, or compared with,species from Baltica, and a further 10 are compa-rable at the generic level. The Upper Cambrianfaunas of western Avalonia appear to be less fullyknown, but, out of a total of about 20 speciesrecorded by Martin and Dean (1988), there are13 species in common with British faunas andfour or ¢ve additional shared genera.The Upper Cambrian in the peri-Gondwanan

region contains a number of faunas (discussedabove), but, presumably because the proli¢c ole-nid biofacies is not well developed, there are rel-

PALAEO 3071 9-5-03


atively few taxa in common with Britain. For ex-ample, there is nothing in British faunas to matchthe oriental genera recorded from the MontagneNoire, and with the transfer of Olentella rara ofRushton (in Allen and Jackson, 1985) to Aphelas-pis (Z] ylin¤ska, 2001), there is no genus in commonwith the Moroccan fauna. Upper Cambrian out-crops from Spain (Shergold et al., 1983) have onlyyielded Maladioidella, and possibly an undeter-mined aphelaspidine that can be compared withtaxa in British faunas, while a fauna from Sardi-nia only has a few more comparable taxa, namelyMaladioidella, Proceratopyge, an early form ofNiobella and some unspeci¢ed olenids (Loi etal., 1995).

5.3.6. TurkeyLate Cambrian trilobites have been recorded

from the Taurus and Amanos Mountains insouth^central Turkey. In the northeast, they oc-cur in the Seydisehir Formation of the Sultan Dagfl(Shergold and Sdzuy, 1984; Dean et al., 1993), onthe Tu«lu«ce Tepe section. Trilobites described byShergold and Sdzuy (1984) include Homagnostus?sp., Maladioidella kelteri, Pseudagnostus cf. cyclo-pyge, Pseudagnostus sp., and undetermined pty-choparioids and solenopleuroideans. Dean et al.(1993) also illustrated specimens of Homagnostusand M. kelteri. All these assemblages are LateCambrian in age, which may well correlate withthe Maladioidella-bearing horizons of Sardinia,Montagne Noire, and northeastern Spain.In the Sultan Dagfl, this fauna is succeeded by a

younger association (Shergold and Sdzuy, 1984)containing Koldinioidia (K.) cf. sulcata, Macro-pyge cf. taurina, Micragnostus haudei, Niobellacf. primaeva, Onchonotellus a¡. amsassensis, Para-koldinioidia cf. gibbosa, Proteuloma cf. geinitzi,and Rhaptagnostus? sp. This was assumed byShergold and Sdzuy (1984) to indicate an EarlyTremadocian age since elements of the associationare traditionally considered to be of this age andhave a wide distribution in the Franconian Forestarea (Germany), the Montagne Noire, the CzechRepublic, southern Kazakhstan, southern Siberia,and the Xinjiang Province in China. However, allof the generic taxa have latest Cambrian origins,except perhaps Macropyge (although the genus is

noted to occur in the Sardinian CAB 6 fauna,though not illustrated). Micragnostus, Niobellaand Parakoldiniodia, for example, are related totaxa described from the Acerocare Zone (Rush-ton, 1982), which Landing et al. (1978) regardas correlatable with the Cordylodus proavus con-odont Zone. Given the current acceptance of theposition of the Cambrian^Ordovician boundary(Cooper and Nowlan, 1999) at the ¢rst appear-ance of the conodont Iapetognathus £uctivagusNicoll et al., 1999, associations correlated withthe C. proavus Zone are terminal Cambrian age,or Datsonian in the Australian standard (see alsothe discussion of the Sardinian CAB 6 assemblageby Loi et al., 1995; and the correlations proposedby Geyer and Shergold, 2000). Accordingly, itmay be necessary to reassess the Sultan Dagfl‘Tremadocian’.Finally, in southeastern Turkey, Late Cambrian

trilobites have been reported by Dean et al. (1981)and Dean and Monod (1997) from the SeydisehirFormation of the Samur Dagfl area, on the YaylaTepe section. Trilobites reported from here in-clude species of Chuangia, Drepanura, and Pro-chuangia, overlain by younger assemblages char-acterised by Alborsella, Niobella, Pagodia (Witte-kindtia), and Saukia, all indicating a⁄nities withnorthern Iran.

6. Biogeographical cluster analyses

A matrix tabulating all genera and speciesknown from the Lower and Middle Cambrianof Morocco, Ossa^Morena, the Cantabro^IberianBasin, Montagne Noire, Sardinia, Saxo^Thurin-gia, the Barrandian area and Turkey (the latteronly for the Middle Cambrian) has provided sig-ni¢cant information (Figs. 5C and 6C). Phyloge-netic biogeography (Lieberman, 2002) is notavailable for the Cambrian trilobites of the west-ern Gondwanan margin due to the scarcity ofcladistic analyses. However, the data reportedhere (114 genera and 249 species for the LowerCambrian, and 113 genera and 442 species for theMiddle Cambrian) were analysed using a stan-dard method of linkage cluster analysis, namelyanalysis of hierarchical Phi^Pearson similarity,

PALAEO 3071 9-5-03


which produces dichotomising patterns that areeasier to interpret than multivariate analyses. Per-centage similarity in a single linkage cluster anal-ysis provides an approximate key to robustness ofbranching nodes within the constraints of para-metric analysis. Trilobite faunas from the di¡erentbasins and platforms are associated in terms oftheir shared genera and species. The analysis ofsimilarity produces bifurcating dendrograms read-ily interpreted in terms of biogeography. The dis-advantage of this method is that biofacies pat-terns are not considered, and that the totaldiversity of each platform strongly a¡ects the per-centage of shared taxa. Endemic taxa were elimi-nated from the database before they were enteredin the data matrix because they may representlocal ecological conditions, lack of marine con-nections to neighbouring platforms, or samplingbiases. Further research might extend the knownbiogeographical distribution of some taxa but itseems unlikely that such new data would seriouslymodify the basic patterns discussed in this paper.

6.1. Lower Cambrian

The Lower Cambrian shallow subtropical ma-rine biota exhibits the greatest diversity patternsin the whole Cambrian. One of the most interest-ing features of the Lower Cambrian is the wide-spread development of endemic species. The Low-er Cambrian palaeogeographical complexities ofthe carbonate and mixed platforms, episodicallyrelated to the development of microbialârchaeo-cyathan reefal complexes, o¡ered various oppor-tunities for the isolation of populations and sub-sequent allopatric speciation. Many subtropicalendemic trilobites assemblages are essentially re-lated to reef communities. Trilobite similarity ex-hibits two major clusters (Fig. 5C): (1) the south-ern British Isles, Morocco, and the OMZ; and (2)Saxo^Thuringia, Montagne Noire, the CIZ, theCantabroÎberian Basin, and Sardinia. The closerelationship of the OMZ with the entity ‘easternAvalonia^Morocco’ is signi¢cant because it couldre£ect easier reproductive communication in thesouthwesternmost part of the area studied due toopen oceanographic conditions and a possible set-ting of the OMZ closer to Morocco than that

illustrated in palaeogeographical maps. Trilobitefaunas show a marked change in distribution pat-terns across the Lower^Middle Cambrian transi-tion; this is attributed to a major palaeogeograph-ical reorganisation, in which the endemic patternsdisplayed by the Lower Cambrian trilobite faunaended diachronously and were followed by a pe-riod of relative sedimentary homogeneity, with anopen connection of platforms that allowed immi-gration of trilobites.

6.2. Middle Cambrian

Trilobite similarity shows two main clusters(Fig. 6C): (1) Balticaêastern Avalonia, and (2)the rest of the basins discussed here. This alsoshows strong support for the presence of twoMid-Cambrian biogeographical entities withinthe Mediterranean subprovince of Sdzuy (1972),distinctly di¡erentiated from eastern Avaloniaand Baltica. Therefore the relative Baltic a⁄nityof the Middle Cambrian faunas from thePr›|¤bram^Jince Basin (Havl|¤c›ek, 1999) does notsupport di¡erentiation of a Middle Cambrian Pe-runica terrane. Another important biogeographi-cal relationship with Bohemia is indicated by aMiddle Cambrian trilobite assemblage found inthe Ptychagnostus atavus Zone of the CarolinaSlate belt (eastern United States). The latter isconsidered as an exotic terrane likely accreted toNorth America during the Early or Middle Pa-laeozoic, although an alternative possibility hasbeen proposed placing this fauna in cool-waterenvironments along the periphery of Laurentia(Samson et al., 1990).The variability of the biogeographical distribu-

tion documented in the studied populations is di-rectly related to species diversity. Decreasing pro-vinciality of trilobites during the Caesaraugustianappears to have been accompanied by an increasein total diversity. The development of regionalregressions induced habitat di¡erences on a localand regional scale favouring ‘microallopatric’ spe-ciation.Regarding the British Cambrian faunas, they

have their closest a⁄nity to those of the Gond-wanan margin at the end of the Early Cambrian(or earliest Mid-Cambrian in Morocco) and this

PALAEO 3071 9-5-03


a⁄nity declines through the Middle and LateCambrian. Through the same period the BritishCambrian faunas retain considerable similaritieswith western Avalonian faunas (those of east mar-itime Canada and eastern United States, e.g. Mas-sachusetts, New Brunswick, Newfoundland, etc.)and become progressively more similar to Balticfaunas.

6.3. Upper Cambrian

A cluster analysis of the Upper Cambrian tri-lobite faunas is not signi¢cant because of theirlow diversity patterns in areas other than Britain.In addition, trilobite biofacies possibly occur atonly three stratigraphical levels: (1) the Idameanor Steptoean (IC, Montagne Noire, Morocco),characterised by aphelaspidoid morphologies; (2)the Iverian or Sunwaptan (DM, Montagne Noire,Sardinia, western Turkey), characterised by theoccurrence of Maladioidella ; and (3) the Datso-nian, of Cordylodus proavus Zone age. If the ageof strata with Niobella, Macropyge, Pagodia (Wit-tekindtia), and Parakoldinioidia can be ¢rmly es-tablished as of Cordylodus proavus Zone age, thenthat biofacies becomes the third of latest Cambri-an age. Some support for this assumption comesfrom conodonts recovered by Oº zgu«l and Gedik(1973) from the Seydisehir Formation in the cen-tral Taurus Mountains, which by Australian stan-dards are terminal Cambrian. Strata with Orycto-conus also support this assignment, since theyoccur in association with the Acerocare RegressiveEvent (Erdtmann, 1986; Nicoll et al., 1992; Loi etal., 1995, 1996), a global sea-level £uctuationclose to the currently adopted CambrianÔrdovi-cian boundary. Most of the Upper Cambrian fau-nas described from western Gondwana containelements interpreted as of central and southeast-ern Asian provenance.

7. Conclusions and further questions

One process can be considered primarily re-sponsible for the northeastward migration of thebiodiversity centre (based on trilobites) on thewestern Gondwanan margin during the Cambri-

an: the cooling associated with the southwestwarddrift of the continental margin. Diverse LowerCambrian trilobite assemblages are found in theshallow waters of subtropical areas, as studied inthe Souss Basin of Morocco. The progressivechange from relatively endemic to cosmopolitantrilobite assemblages across the Lower^MiddleCambrian transition can be related to threemain events: (1) the disappearance of microbialârchaeocyathan buildups and neighbouring peri-reefal environments, (2) the relative uniformity ofmuddy o¡shore substrates related to a regional(eustatic?) early Mid-Cambrian transgression,and (3) the disappearance of signi¢cant temper-ature gradients on the Southern Hemisphere.The peak of Middle Cambrian trilobite diversityprogressively migrated from Morocco to south-western Europe (CantabroÎberian, MontagneNoire, Saxo^Thuringia and Barrandian areas),although these subtropical- to temperate-waterplatforms did not reach the richness reported inthe Lower Cambrian of the Souss Basin. Finally,Upper Cambrian temperate-water faunas are dra-matically impoverished compared with the pre-vious wealth, except in the southern British Isles,where the proli¢c olenid biofacies developed, ex-hibiting a strong biogeographical relationshipwith Baltica.Ocean currents played a signi¢cant role in the

regional distribution of benthic communities. TwoCambrian unidirectional circulation trends seemto have been in operation: (1) in the Early toMid^Cambrian, a western peri-Gondwanan cur-rent with a SW^NE trend extended from polarareas to lower latitudes and favoured interchangewith benthic communities of Baltic a⁄nity; (2) inthe Late Cambrian a NE^SW longshore current£owed towards higher latitudes and seem to haveinvolved organisms of Asian a⁄nity. However, itremains a question whether the dispersal of sub-tropical, Australo^Sinian trilobites into temperateor cold seas of the Southern Hemisphere can beattributed to transport of southward-drifting cur-rents? Or did the migration take place in a direc-tion opposite to that of the main northeastward£ow of surface waters?Numerous questions are still open for future

discussion that will bene¢t from a multidiscipli-

PALAEO 3071 9-5-03


nary approach. This paper constitutes only onenew step in the establishment of Cambrian palaeo-biogeographical models. The palaeogeographicalmaps and biogeographical interpretations ex-pressed here are intended to stimulate critical re-marks from specialists of other fossil groups andgeological disciplines, and to be tested further inthe light of new data.

Acknowledgements

The authors thank warmly the proposals anddiscussions of Per Ahlberg (Lund), Pete Palmer(Boulder), and the editing work of Alain Blieck(Lille), which have helped to complete and im-prove a previous manuscript. This paper is a con-tribution to Project ATI 15-52.

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