Ann. Rev. Earth Planet. Sci. 1989. 17:413-38 Copyright © 1989 by Annual Reviews Inc. All rights reserved

FAUNAL DYNAMICS OF

PLEISTOCENE MAMMALS

S. David Webb

Florida Museum of Natural History, University of Florida, Gainesville, Florida 326 1 1

Anthony D. Barnosky

Section of Vertebrate Fossils, Carnegie Museum of Natural History, 4400 Forbes Avenue, Pittsburgh, Pennsylvania 152 1 3

The abyss of time between the period at which North Europe was first covered with ice. when savages pursued mammoths and scratched their portraits with sharp stones in central France, and the present day, ever widens as we learn more abuut the events which bridge it.

T. H. Huxley (1896, p. 328)

INTRODUCTION

Tee Age mammals, the dramatis personae of this review, can be divided into three groups-megafauna, microfauna, and humans-each of which plays a fundamentally different role. Surely the most conspicuous actors were megafauna, wide-ranging herd animals such as mammoths and horses; yet they were in a sense "fall guys," taking the brunt of the late Cenozoic extinctions not only in North America but also generally at high latitudes. The microfauna were not, as once supposed, "bit players," for in the course of the Pleistocene they, like the meek, inherited the Earth. It was an obscure group of late-blooming primates, Homo sapiens, that stole the show as they came out of Africa. They created an ecological drama that still awaits the final curtain.

In the following pages we consider why this drama has played for the past several million years. The answer involves the three components of faunal dynamics: evolution, immigration, and extinction. Central to the discussion are the questions of whether or not Pleistocene extinctions

413 0084-6597/89/0515-0413$02.00

Ann

u. R

ev. E

arth

Pla

net.

Sci.

1989

.17:

413-

436.

Dow

nloa

ded

from

ww

w.a

nnua

lrev

iew

s.or

gby

Uni

vers

idad

e de

Bra

silia

on

04/2

8/14

. For

per

sona

l use

onl

y.

414 WEBB & BARNOSKY

and originations took place at ordinary rates, and how they fit into a background of changing late Cenozoic environments.

Despite our emphasis on the mammalian fauna of North America, we suggest that its dynamics can be understood only in reference to the whole continental ecosystem, and that the latter in turn can be comprehended only in the context of global change. We refer often to a system of bio­stratigraphic zones, North American land mammal ages (abbreviated as NALMAs), that have been recently revised in Woodburn ( 1 987).

FAUNAL DYNAMICS

Faunal dynamics operates within and among ecosystems. Because a given ecosystem provides finite resources, it seems likely that over time the species within that system will tend to maintain a dynamic equilibrium between extinctions and originations. During short intervals (ecological time), faunal equilibria balance immigrations against extinction, but over long intervals (geological time) evolutionary diversification also becomes significant. Because these processes appear differently at different time scales, the Pleistocene provides a crucial transition between ecological and geological perspectives. Diamond ( 1 984) followed the changing equilibria in bird and mammal communities through five different time scales: 10 yr, 100 yr, 1000 yr, "since the Pleistocene," and "over geo­logical time." Turnover rate for a given interval is the mean of extinc­tion rate (last appearances) plus origination rate (first appearances), both divided by interval duration. Originations in a given continent or region consist of new evolutionary branches (autochthons) and new immigrants (allochthons). The distinction between allochthons and autochthons usually can be made with confidence for the relatively complete records of the late Cenozoic. In general, the high origination rates of the late Cenozoic depend more heavily on immigrants than on new evolutionary products.

The fossil record of mammals appears more volatile than that of other groups. For example, Van Val en ( 1 974, p. 300) found that mammal species generally "become extinct more often than most organisms" and therefore that "epistandard origination rates are needed to replace the extinctions." Furthermore, in comparisons of mammalian faunal dynamics between various land mammal ages, the Pleistocene stands out as an interval of high turnover (Webb 1 969, Savage & Russell 1 983). When Kurten ( 1 985) measured turnover rates of mammalian species in the Quaternary, he found that they ranged from three times to an order of magnitude greater than comparable rates in the Tertiary.

On the other hand, Gingerich ( 1 983) empirically determined from numerous observations that evolutionary rates, and therefore turnover

Ann

u. R

ev. E

arth

Pla

net.

Sci.

1989

.17:

413-

436.

Dow

nloa

ded

from

ww

w.a

nnua

lrev

iew

s.or

gby

Uni

vers

idad

e de

Bra

silia

on

04/2

8/14

. For

per

sona

l use

onl

y.

PLEISTOCENE MAMMALS 415

rates as well, are inversely proportional to the intervals over which they are measured. If evolution or its stratigraphic record is episodic rather than uniform in rate, then measurements over shorter invervals will capture spurts of change that over longer intervals are averaged with slow or even negative rates. Gingerich ( 1987, p. 1055) recalculated allegedly high Pleistocene extinction rates as a percentage of total fauna and discovered that these rates did not exceed Tertiary background rates. Thus, "expla­nations involving climate and (or) human overkill no longer seem to be required" if Gingerich's Law alone accounts for high mammal turnover rates in the Pleistocene and very high rates in the late Pleistocene.

IMMIGRANTS

Climatic changes, ice sheet advances and retreats, sea-level changes, and tectonic events all have contributed to rearranging the habitats available to mammals through the late Pliocene and Pleistocene. The response of various species to these changes is manifested by latitudinal and altitudinal shifts in their geographic ranges as individuals immigrate into suitable environments. When they spread into new areas, immigrants become excel­lent biostratigraphic markers.

Late Cenozoic immigration rates of land mammals into North America were extraordinarily high. For comparison the highest Paleogene peaks were in the late Paleocene (Tiffanian NALMA with six genera) and in the late Eocene (Uintan NALMA with ten), both times of major climatic change (Prothero 1985, Webb, 1985a). Higher immigration peaks began to appear in the Neogene. About 15 land mammal genera came in at about 18 Ma and another 15 at 5 Ma (Hemingfordian and Hemphillian NALMAs, respectively). Each of these episodes appears to be associated with shifts toward more continental climates on land and also with major declines in marine isotopic temperatures and eustatic sea levels (Savin & Douglas 1985, Crowley & North 1 988, p. 99).

The acme of land mammal immigration into North America occurred in the Blancan NALMA at about 3 Ma, when at least 18 genera arrived from Asia and from South America. For the next 2 m.y., Pleistocene immigration, spanning the Irvingtonian and Rancholabrean NALMAs, continued at a very high level. If they are prorated over time, the Irving­tonian and Rancholabrean rates approach the Blancan mean rate of immi­gration and far exceed the average rate of any earlier immigration episodes (Webb 1985a). Detailed records of the acme of immigrations in the Blancan and younger intervals reinforce the view that climatic effects triggered the lively intermingling of faunas that are especially characteristic of the late Cenozoic.

Ann

u. R

ev. E

arth

Pla

net.

Sci.

1989

.17:

413-

436.

Dow

nloa

ded

from

ww

w.a

nnua

lrev

iew

s.or

gby

Uni

vers

idad

e de

Bra

silia

on

04/2

8/14

. For

per

sona

l use

onl

y.

4 1 6 WEBB & BARNOSKY

Besides their possible triggering effects, late Cenozoic climates also acted as ecological filters, restricting the biota that could pass over a land bridge. As indicated in Figure 1 , Beringian environments became progressively more stringent during the last several million years. Rich deciduous forests of the late Miocene gave way to increasingly evergreen-dominated savan­nas in thc Pliocene and early Pleistocene and finally to a unique steppe­tundra environment in the late Pleistocene (Wolfe 1 985). The mammals that passed between Holarctic continents reflect (and confirm) these changes; for example, mixed-feeding hipparion horses moved across Berin­gia in the late Miocene, browsing deer crossed in the Pliocene, and grazing equines and bovines traversed that region in the Pleistocene. The balance between directions of passage also shifted. Data summarized in Figure 1 from Tedford et al ( 1987) and Lundelius et al (1987) clearly indicate the increased weight of immigrations from Asia. Presumably this pattern reflects the extent of appropriate biomes in regions adjacent to Beringia.

Late Cenozoic immigration episodes were concentrated in pulses that depended on the opening of dispersal corridors. In the north, land had to bridge the Bering Strait, and one or more ice-free corridors of appropriate ecology had to offer passage into temperate latitudes. Opening of such corridors hinged on a complex interplay between the amount of water locked in glacial ice, isostatic rebound, ice sheet size, and worldwide climate. Similarly to the south, the inter-American corridor required plate tectonic closure of the Bolivar trough coincident with the establishment of appropriate savanna environments on the Isthmian land bridge before South America and North America could interchange their long isolated species (Marshall et al 1 979, Stehli & Webb 1 985).

Correlation of major late Cenozoic immigration events with the radio­metric time scale and the oxygen isotope curve suggests that requisite conditions often were fulfilled near the transition from glacial into inter­glacial times and vice versa. Key episodes were as follows: (a) 2.5 ± 0. 1 Ma, well dated in various western sections below the top of the Gauss magnetochron (i.e. before 2.48 Ma) and probably correlated with ter­mination of low sea level and the major cooling interval registered at about 2.6 Ma by oxygen isotope records; (b) 1 .9 ±O. 1 Ma, which corresponds to a warm spell recorded in oxygen isotope records; (c) 850,000 ± 25,000 yr ago, correlating with oxygen isotope Stage 22; (d) 400,000 ± 25,000 yr ago, correlating with glacial Stage 12; and (e) 1 50,000±25,000 yr ago, correlating with the upper half of glacial Stage 6 (Repenning 1984, 1987, Webb, 1 985b, Bradley 1 985, Shackleton & Opdyke 1 976, 1 977).

At 2.5 Ma (late Blancan NALMA) major immigration waves appeared from both Asia and South America. From Asia came such diverse mam­mals as an unknown bovine, Bretzia (extinct deer), Parailurus (extinct

Ann

u. R

ev. E

arth

Pla

net.

Sci.

1989

.17:

413-

436.

Dow

nloa

ded

from

ww

w.a

nnua

lrev

iew

s.or

gby

Uni

vers

idad

e de

Bra

silia

on

04/2

8/14

. For

per

sona

l use

onl

y.

PLEISTOCENE MAMMALS 417

MAMMALS ACROSS THE BERING SEA

Early Pleistocene

Pliocene

Figure 1 Numbers of land mammal genera that crossed Beringia from Asia (left) to North America (right), and vice versa, at successive late Cenozoic intervals from late Miocene (bottom) to late Pleistocene (top). Arrow widths proportional to numbers of genera. Chang­ing environments indicated by vegetational cartoons. See text for further discussion.

lesser panda), Lepus Gackrabbit), Cryptopterus (large flying squirrel), Pliopotamys (extinct muskrat), and two subgenera of Synaptomys (bog lemmings), Contemporaneous immigrants from South America include Glossotherium (ground sloth), Glyptotherium (large shelled edentate), Holmesina and Dasypus (large and small armadillos), Neochoerus (extinct capybara), Erethizon (porcupine), and Titanis (giant flightless bird) (Webb 1 985b, Lundelius et al 1 987, Repenning 1987). Triggered by almost sim­ultaneous reestablishement of the Bering land bridge as the avenue from Asia and the Isthmian land bridge as the land route from South America,

Ann

u. R

ev. E

arth

Pla

net.

Sci.

1989

.17:

413-

436.

Dow

nloa

ded

from

ww

w.a

nnua

lrev

iew

s.or

gby

Uni

vers

idad

e de

Bra

silia

on

04/2

8/14

. For

per

sona

l use

onl

y.

418 WEBB & BARNOSKY

this was the largest single immigration episode recorded in North Amer­ican land mammal history. Apparently it was facilitated by the onset of Northern Hemisphere glaciation and a major eustatic lowering of sea levels (Shackleton & Opdyke 1 977, Savin & Douglas, 1985, Webb 1985a, Repenning 1987).

Evidence for subsequent immigration events is most finely documented in the fossil record of micro tine (or arvicoline) rodents, e.g. the voles, lemmings, and muskrats. The importance of these animals as biostrati­graphic and paleoenvironmental indicators has been recognized for decades (Hibbard 1 944, Guilday & Bender 1 960, Nelson & Semken 1 970, Guilday et a1 1 978, Martin 1 979, Repenning 1 980). However, only recently have studies been undertaken that integrate taxonomy and first-appear­ance data of microtine species with a broad range of geologic and paleo­climatic data (Repenning 1 980, 1 984, 1 987, Fejfar & Heinrich 1 98 1 , Fejfar 1 983). Such multidisciplinary work has resulted i n a microtine­rodent biochronology that promises to be one of the most accurate ways of correlating pre-40,000-yr-old terrestrial deposits throughout North America and Europe (Repenning 1 987).

Microtine Holarctic biochronology divides the late Miocene, Pliocene, and Pleistocene epochs into ten micro tine zones, each of which demarcates an interval of time when certain assemblages of micro tine species char­acterize the stratigraphic record. The lower boundary of eight zones is recognizable as the first appearance of particular species in North America or Europe as they immigrated from their center of origin, which commonly was in Asia. (Recognition of the other two zones is facilitated by endemic evolutionary gradations.) The timing of each immigration event is dated independently by radiometric or paleomagnetic techniques. Five micro tine zones subdivide the North American Pleistocene and latest Pliocene: Blancan V (2.6 to 1.9 Ma), Irvingtonian I ( 1 .9 to 0.9 Ma), Irvingtonian II (900,000 to 400,000 yr ago), Rancholabrean I (400,000 to 1 50,000 yr ago), and Rancholabrean II ( 150,000 yr ago to the present) (Repenning 1 987).

Of course, other taxa besides micro tine rodents figure prominently in Pleistocene biochronology. The beginning of the Irvingtonian NALMA is marked by the immigration of taxa from both South America and Eurasia between 1.9 and 1.8 Ma. Among the South American interlopers are Mixotoxodon (rhinoceros-sized ungulate), Eremotherium and Nothro­theriops (large and small sloths), Myrmecophaga (anteater), and Desmodus (vampire bat) (Webb 1 985b). Eurasian immigrants that appear near the beginning of the Irvingtonian NALMA include Mammuthus (mammoth), Euceratherium and Soergelia (musk-oxen), Rangifer (caribou), Panthera onca (jaguar), Microtus and Clethrionomys (voles), Lemmus and Dicro-

Ann

u. R

ev. E

arth

Pla

net.

Sci.

1989

.17:

413-

436.

Dow

nloa

ded

from

ww

w.a

nnua

lrev

iew

s.or

gby

Uni

vers

idad

e de

Bra

silia

on

04/2

8/14

. For

per

sona

l use

onl

y.

PLEISTOCENE MAMMALS 419

stonyx (lemmings), Canis lupus (wolf), Mustela erminea (ermine), and Lutra (otter). The Irvingtonian ends with the first appearance of Bison (bison) from Eurasia, whose presence is often taken to indicate the Rancholabrean NALMA (Lundelius et al 1 987).

EVOLUTION

Morphologic Evolution

Pleistocene mammals provide information about evolutionary processes that is not available from other parts of the geological record or from modern mammals. To a large measure this is because the Pleistocene, when combined with the Holocene, bridges the gap between geological and ecological time, thereby making it possible to study morphological change on a time scale that will reflect any links or separations between macroevolutionary and microevolutionary processes. Furthermore, many of the skeletal traits that characterize species of modern mammals can be readily identified. Therefore traits that are clearly diagnostic of biological species can be traced through time in extant species and in closely related extinct ones. Also, much is known about the ecology and preferred habitats of many living mammalian species, and that environmental perturbations associated with Quaternary climatic changes affected mammalian habitats (see papers in Wright 1983, Porter 1 983, Velichko 1983, Ruddiman & Wright 1 987). Within the paleoenvironmental framework now available, it becomes possible to test for correlations between morphological change, environmental change, and various models of evolution. In addition, the likelihood that an apparent morphological change at a particular locality is attributable to immigration of a population (or species), rather than to in situ evolution, can be assessed. Finally, samples of Pleistocene and Holocene mammals commonly are larger and more widespread than those of earlier times, and those younger than about 50,000 yr can be dated with relatively great precision by radiocarbon methods.

How these special qualities of the Quaternary mammalian record have contributed to the study of evolution was reviewed recently by Barnosky (1987). Published data on 98 species from North America, Europe, and Africa were used to test the predictions of two evolutionary models­phyletic gradualism and punctuated equilibrium. Of 760 articles culled from the literature on evolution in the Pleistocene and Holocene, about 30 presented documentation necessary to conclusively test some aspect of the models. Representative examples of studies that provide abundant data for testing the predictions include Guilday et al (1978) and Jones et al ( 1984) on shrews, Harris & White ( 1 979) on pigs, Maglio ( 1973) on elephants, Rensberger et al (1984) on squirrels and voles, Stuart (1982) on

Ann

u. R

ev. E

arth

Pla

net.

Sci.

1989

.17:

413-

436.

Dow

nloa

ded

from

ww

w.a

nnua

lrev

iew

s.or

gby

Uni

vers

idad

e de

Bra

silia

on

04/2

8/14

. For

per

sona

l use

onl

y.

420 WEBB & BARNOSKY

voles, Vrba ( 1984) on bovids, and Wolpoff ( 1 984) on Homo erectus. Additional useful studies include Hibbard & Zakrzewski ( 1967, 1972) on voles; Kurten ( 1968) on hamsters and several other species; L. D. Martin ( 1979, 1984) on voles, muskrats, and dirk-tooth cats; R. A. Martin (1984) on cotton rats; and Nelson & Semken ( 1970), Zakrzewski ( 1974), and Barnosky (1985) on muskrats.

Examination of these and other data indicates that morphological pat­terns consistent with both punctuated equilibrium and phyletic gradualism are recognizable within the Quaternary; hence, it is difficult to dismiss punctuated equilibrium solely on the basis that paleontologists look on the wrong time scale. However, both patterns are widely represented. In cases where the majority of the predictions arising from the two models can be tested, the favored model seems to correlate with higher taxonomic group or with the nature of the prediction being tested. For example, punctuated equilibrium seems the rule in shrews, alcelaphine bovids, squir­rels, and hamsters, whereas phyletic gradualism seems more typical of voles, elephants, pigs, impalas, Homo erectus, and probably muskrats and dirk-tooth cats. Thus, phyletic gradualism seems to occur more frequently if one considers only higher taxonomic groups. But if one considers total numbers of species, the punctuated equilibrium model is supported more frequently. The most interesting questions to be answered are not whether both patterns exist in the Quaternary-apparently both do-but why certain taxa seem to be characterized by one or the other mode. Why, for example, is there no obvious correlation between evolutionary rate and generation time, contrary to Kurten's ( 1 985) suggestion that higher turn­over rates appear in mammals with shorter generation times.

Various regions within North America, especially the Appalachians, the Great Plains, and the Southwest, now have built up networks of dated Quaternary mammal sites, making it possible to trace phenotypic changes in morpho species through time and space (Lundelius et al 1 987, Graham & Mead 1 987, Graham et al 1 987). Such studies are still in their infancy, but among the most promising are those that utilize digitized images of teeth (on which fossil-mammal taxonomy is largely based) to quantify subtle changes in shape. Applicable techniques include those based on fractal geometry and originally developed for sedimentological studies (Kennedy & Lin 1986, Plotnick 1 986), those comparing linear differences between profiles at multiple points (Walker 1 984), those relying on com­parisons of irregular shapes to regular geometric forms (Schmidt-Kittler 1 986), and those that transform directional data into histograms (Rensberger et aI1984).

For example, Microtus pennsy/vanicus (the meadow vole) is recognizable by its third upper molars in fossil sites that range from the Holocene back

Ann

u. R

ev. E

arth

Pla

net.

Sci.

1989

.17:

413-

436.

Dow

nloa

ded

from

ww

w.a

nnua

lrev

iew

s.or

gby

Uni

vers

idad

e de

Bra

silia

on

04/2

8/14

. For

per

sona

l use

onl

y.

PLEISTOCENE MAMMALS 42 1

to 400,000 yr ago. Intraspecific variation in the shape of M. pennsylvanicus molars is pronounced across its modern geographic range, which extends from Alaska and Labrador into the northern half of the United States (Guilday 1 982, Davis 1987). To identify whether the variation is random with respect to space and time, digitized images of third molars from fossil and recent populations were subjected to algorithms that quantify aspects of size and shape (for example, length, width, area, perimeter, and approxi­mation to a perfect circle). The analysis shows that fossil samples from Virginia resemble modern ones from the same state. In contrast, modern samples from Alaska differ from both the modern and fossil ones in Virginia. These data suggest that in most respects thc third molars of the meadow vole have not changed systematically or unidirectionally over at least the past 30,000 yr, and that the main source of intraspecific variation is geographic rather than temporal isolation.

Conventional meristic data also elucidate interesting evolutionary relationships, as illustrated by Churche & Pinsoif's ( 1987) study of dinal variation in Cervalces, the extinct tundra moose. They showed that the length of its anterior antler beam and the vertical orientation of its tines increased from New Jersey northwestward into Beringia and beyond that into temperate latitudes of Eurasia.

Molecular Evolution

Tests of evolutionary models must be based on a firm understanding of the phylogenetic relationships among taxa. Generally paleontologists study only the hard parts (skeletons and teeth) of animals to infer these phylogenies, since soft parts are not preserved in the fossil record. However, because Pleistocene genera and species often have extant rep­resentatives, it is possible to test the phylogenies inferred from hard parts against those inferred independently from biogeography, from soft parts, and, very significantly, from molecular data obtained from the actual genome.

The application of molecular data to evolutionary rates is a relatively recent innovation that relies on the assumption that the proteins com­posing genes accumulate amino-acid substitutions at a rate that is sto­chastically constant. If that assumption is true, the biochemical differ­ence between two taxa should be very small when the taxa first split from a common ancestor and should increase at a uniform rate through time (King & Jukes 1969, Nei 1 97 1 ). Therefore, when averaged over a sufficient number of genetic loci and calibrated by fossil evidence, the protein changes can be used as an "evolutionary clock" that records the amount of time elapsed since the taxa shared a common ancestor (Langley & Fitch 1974, Dobzhansky et aI1 977, Lowenstein 1 986).

Ann

u. R

ev. E

arth

Pla

net.

Sci.

1989

.17:

413-

436.

Dow

nloa

ded

from

ww

w.a

nnua

lrev

iew

s.or

gby

Uni

vers

idad

e de

Bra

silia

on

04/2

8/14

. For

per

sona

l use

onl

y.

422 WEBB & BARNOSKY

This technique has been used primarily to elucidate relationships at higher taxonomic levels (e.g. among orders of mammals) (Benton 1988). Chaline & Graf ( 1988), however, illustrate how biochemical studies can be applied at a finer scale of taxonomic resolution for Pleistocene mammals. They compared a microtine rodent phylogeny constructed from fossil and recent teeth with information about protein variation in extant species. The two data sets were congruent in several respects. For instance, both confirm that the family to which voles and lemmings belong, the Arvicolidae, is a natural taxonomic group and suggest early separation into taxonomic tribes, the Dicrostonychini (lemmings), Lemmini (lemmings), Clethrionomyini (voles), and Microtini (voles). Within Microtini, Phen­acomys (heather vole) appears to have been a distinct lineage for more than 4 m.y. On the other hand, all analyzed species broadly included in Microtus (including Pitymys and Pedomys as subgenera) on morphologic criteria appear to have shared a recent common ancestor according to the biochemical analysis. But the biochemical data contradict some views previously established on morphological grounds-for example, that Microtus (Pitymys) pinetorum (pine vole) either is a Nearctic member of the Eurasian Pitymys group or represents a lineage quite distinct from Microtus. Chaline & Graf aptly conclude that using both molecular and paleontological methods in concert is the most powerful approach to phylogenetic analysis.

In ideal cases, mitochondrial DNA can be extracted from exceptionally preserved specimens of extinct species, which thereby provides a direct means of comparing genotypes with extant taxa. For example, the zebra­like Equus quagga (quagga), a horse once populating Africa near the Cape of Good Hope, was hunted to extinction by 1880. DNA sequences extracted from muscle tissue in museum collections were cloned and com­pared with those of the extant horse Equus caballus, the plains zebra E. burchelli, and the mountain zebra E. zebra hartmanii (Hughes 1988). Only two nucleotide sequences separate the extinct quagga from E. burchelli. Previously, morphological criteria were used to postulate a closer relation­ship between the quagga and E. caballus. Now it appears that these morphological similarities-short ears and an isthmus on the lower molars-indicate convergence toward similar adaptive strategies rather than close evolutionary affinities.

The work with "fossil" DNA from the quagga opens exciting possi­bilities for analyzing frozen or mummified remains of extinct Pleistocene animals like mammoths and bison from Alaska and Siberia. Efforts at extracting and cloning DNA from such specimens, however, have not met with success yet.

Ann

u. R

ev. E

arth

Pla

net.

Sci.

1989

.17:

413-

436.

Dow

nloa

ded

from

ww

w.a

nnua

lrev

iew

s.or

gby

Uni

vers

idad

e de

Bra

silia

on

04/2

8/14

. For

per

sona

l use

onl

y.

EXTINCTIONS

PLEISTOCENE MAMMALS 423

Patterns of extinction during the late Cenozoic are notable for their com­plexity and apparent rapidity. Rates of faunal turnover among terrestrial mammals reach their maxima and, depending on the time intervals over which they are calculated, may be an order of magnitude greater than those recorded for earlier intervals (Savage & Russell 1983, Webb 1984b, Kurten 1 985). Radiocarbon age determinations and other chronostrati­graphic methods have greatly improved the precision of dating; likewise, cumulative collecting activities, including waterscreening for microverte­brates, have vastly augmented the record of fossil mammal extinctions throughout the world.

Such advances still have not identified the cause of the extinctions. The debate that rages today is about as old as recognition of "the Ice Ages" themselves; and from its inception, it has pitted human disturbance against climatic change (Owen 1846, Lyell 1863). In their influential book on Pleistocene mammals of North America, Kurten & Anderson (1980, p. 363) recognized that a mosaic of environmental changes "reduced or weakened populations making them vulnerable to environmental pressures, including man, the hunter, who probably delivered the coup de grace to some of the megafauna between 1 2,000 and 9,000 years ago."

The seemingly refractory nature of the "Pleistocene extinction problem" has inspired a search for more rigorous methodologies and intensified efforts to gather relevant data. Carbon dates have been more carefully scrutinized and a rating scale introduced (Meltzer & Mead 1985). Par­ticipants in the debate have produced more closely reasoned discussions with falsifiable hypotheses (Martin & Klein 1984). And above all, the "extinction problem" has galvanized Quaternary stratigraphers and ver­tebrate paleontologists to search for more detailed sequences in key areas (e.g. Graham et aI1987).

Several conclusions emerge from recent studies of late Cenozoic extinc­tions. One is that the most heavily impacted species were large herbivores, herding ungulates ("hoof-bearers"). Large carnivore species that fed upon them also experienced deep cuts in diversity, but as befits their place at the top of the food chain, there were far fewer predator species to be lost (Anderson 1984). Late Pleistocene extinctions are also commonplace in birds, many of them having been predators or probable scavengers (Grayson 1977, Steadman & Martin 1984). The advent of accelerator mass spectrometer dates, permitting milligram samples to be accurately dated, shows that the near extinction of condors from North America was also a late Pleistocene event, presumably linked to the extinction

Ann

u. R

ev. E

arth

Pla

net.

Sci.

1989

.17:

413-

436.

Dow

nloa

ded

from

ww

w.a

nnua

lrev

iew

s.or

gby

Uni

vers

idad

e de

Bra

silia

on

04/2

8/14

. For

per

sona

l use

onl

y.

424 WEBB & BARNOSKY

of the megafaunal species that it scavenged (Emslie 1 987, Steadman & Miller 1987).

Secondly, it is increasingly clear that the late Cenozoic extinction epi­sodes are complexly distributed through time and space. The land mam­mals of North America experienced six major extinction pulses during the late Cenozoic. The largest, in the mid-Hemphillian NALMA (ca. 6 Ma), records last appearances of 62 genera. The next largest, in the late Rancholabrean (ca. 1 0,000 yr ago), lost 43 genera. And the third largest, in the late Blancan (ca. 1 .9 Ma), apparently took 35 genera (Webb 1 984b).

A key question concerns the timing of various extinction waves on various continents. Figure 2, based on data from Savage & Russell ( 1983), summarizes the late Cenozoic decline of ungulate genera on five continents. In Europe and South America the pattern of successive declines is remark­ably similar to that in North America. Sub-Saharan Africa and tropical Asia were less severely impacted in the late Pleistocene. One notes, however, that if bovids are set aside, the decline of ungulates in Asia (and even Africa) closely parallels that in Europe and North America.

DECLINE OF THE UNGULATES BY CONTINENTS (GENERA)

EUROPE �--� ----�----�

Late

genera Othe;:W�$;WOVidae

Figure 2 Late Cenozoic ungulate extinctions on five continents. All major groups declined except for Bovidae. Note extreme declines in Europe and North America. Genera tabulated after Savage & Russell ( 1983).

Ann

u. R

ev. E

arth

Pla

net.

Sci.

1989

.17:

413-

436.

Dow

nloa

ded

from

ww

w.a

nnua

lrev

iew

s.or

gby

Uni

vers

idad

e de

Bra

silia

on

04/2

8/14

. For

per

sona

l use

onl

y.

PLEISTOCENE MAMMALS 425

The marine biota was far less frequently affected by extinction events of the late Cenozoic than was the terrestrial biota. The principal extinction episode came during the Pliocene, when molluscan faunas experienced heavy losses (Stanley 1986). Virtually no marine extinctions occurred dur­ing the Quaternary itself (Vermeij 1 985). The major marine extinctions in the northwestern Atlantic may have coincided with the onset of Northern Hemisphere glaciation and inauguration of the Gulf Current (Stanley 1986).

These late Cenozoic extinction episodes may be correlated with intervals of rapid climatic change, especially with glacial terminations (Bonifay 1980, Webb 1984b, Vrba 1984). A crucial test comes from Australia, where major extinctions of megafauna are clustered at about 20,000 yr B.P., a time of decreasing climatic equability and approximately 20,000 years after the earliest records of human activity (Bowler 1 976, Lundelius 1 983). Likewise in Ireland the only large ungulate, Mega/oeeras (the giant Irish elk), became extinct coincident with the onset of the Younger Dryas cold spell about 10,600 yr B.P. but more than 1000 years before the first humans arrived in Ireland (Barno sky 1 986).

Such suggestions, however, require detailed stratigraphic sections and precise correlations. Records of mass extinctions are interpretive con­structs drawn from presumed last records of individual taxa, which are themselves constructs of local range zones. Consider, for example, the stratigraphic range of the North American proboscidean genus Stego­mastodon. It has been widely recognized as an index of the Blancan and counted as one of the late Blancan extinctions (Lindsay et aI1987). Recent records from several states, however, show that Stegomastodon sur­vived into the early Irvingtonian (Lundelius et al 1987). This constitutes a range extension of at least 0.2 m.y., which is somewhat surprising in view of the large body size and therefore presumed preservability of Stego­mastodon. Such changes serve as a reminder that the apparent strati­graphic ranges of fossils are subject to continual extension both upward and downward.

Human Impacts

All but the final, latest Pleistocene extinction episode are widely accepted as climatically induced. Even in Africa and other continents where man had a long late Cenozoic history, most anthropologists envision earlier human populations as living in balance with nature. In the late Pliocene and early Pleistocene, Homo habilis used stone tools to obtain meat but probably as a scavenger rather than a hunter. By 1.6 Ma, Homo ereetus tribes evidently had developed efficient hunting techniques; indeed, Walker (1984) has suggested that their range extension from Africa into Europe and Asia resulted from the broadened hunting strategy of a large carnivore.

Ann

u. R

ev. E

arth

Pla

net.

Sci.

1989

.17:

413-

436.

Dow

nloa

ded

from

ww

w.a

nnua

lrev

iew

s.or

gby

Uni

vers

idad

e de

Bra

silia

on

04/2

8/14

. For

per

sona

l use

onl

y.

426 WEBB & BARNOSKY

Concepts such as "overkill" and "blitzkrieg" are reserved for the hunting practices of advanced (Cro-Magnon) members of our own species, Homo sapiens, only in the last 40,000 yr. This is approximately the time of explosive technological and cultural advances in human populations as recognized by field archeologists; it is also about the time proposed by the "Eve hypothesis" of human relationships derived from mitochondrial DNA (Cann et al 1987).

The overlapping range zone of paleo-Indians and extinct megafauna (hereafter the PIM Zone) is a remarkably brief interval in the latest Pleis­tocene of North America. Widely accepted dates for classic Clovis sites range from about 10,500 to about 11,500 yr B.P. The lower limit of the PIM Zone, however, may extend down as early as 13,000 yr B.P., or at least is open to new efforts to extend it downward (West 1983, Haynes 1984). If the ice-free corridor from Alaska through British Columbia was open as early as the mid-Wisconsinan, the continued search for pre-Clovis cultures in the New World remains credible (Bobrowsky 1988).

A fundamental problem with the debate regarding latest Pleistocene extinctions in North America is that both human-caused and climate­induced extinction models predict the megafaunal demise a little before 10,000 yr B.P. It would greatly sharpen the debate if some opposite pre­dictions were generated by the competing models. For example, if paleo­Indians spread across Beringia and thence southward, whereas organisms following the retreat of periglacial habitats trended northward, then crucial tests could be devised. Such tests are within the reach of late Pleistocene data, for their chronological precision and geographic coverage greatly exceed those of any earlier parts of the geological record.

Of utmost importance are data regarding human populations, their density, their distribution, and, above all, the evidence for their having hunted and butchered various megafaunal populations. There is no a priori reason to expect such evidence to be more recondite than other kinds of fossil evidence; indeed, lithic remains, being inherently more durable than bones, have a better chance of preservation and may even lead the way to osteological remains. Increasing numbers of extinct mammalian remains thought to represent human prey have been discovered in about 50 paleo­Indian sites in North America (West 1983, Meltzer & Mead 1985, Graham et al 1987). In the Great Lakes area, about half of the latest Pleistocene mastodont sites studied by Fisher ( 1 987) were implicated by their taphon­omy as sites invaded by man, and the mastodont deaths occurred in the fall before the winter took the other half naturally. On the other hand, only 5 of 37 large vertebrate genera that disappeared in the late Pleistocene of North America (mammoths, mastodonts, horses, camels, and giant tortoises) have been documented as butchered or killed in one or more

Ann

u. R

ev. E

arth

Pla

net.

Sci.

1989

.17:

413-

436.

Dow

nloa

ded

from

ww

w.a

nnua

lrev

iew

s.or

gby

Uni

vers

idad

e de

Bra

silia

on

04/2

8/14

. For

per

sona

l use

onl

y.

PLEISTOCENE MAMMALS 427

sites. The three megafaunal species most frequently recovered from kill sites in western North American are Antilocapra americana, Bison antiquus, and Mammuthus columbi. The first two species were extensively hunted by mass-kill techniques (e.g. "buffalo jumps"), yet they are the very ones that survived (Frison 1987).

Thus a half century of stratigraphic study of the PIM Zone in North America has demonstrated clearly that humans hunted several species of large game and probably administered the coup de grace to some (Kurten & Anderson 1 980). The record does not demonstrate convincingly that humans exterminated any megafaunal populations. More demographic and taphonomic studies at both kill and nonkill sites are urgently needed (King & Saunders 1984, Shipman et al 1984, Haynes 1984).

A tacit corollary of the "overkill" hypothesis has been that all megafau­nal species whose extinction occurred in the PIM Zone were exterminated by man. Possibly the burden of proof falls on the overkill side of the question. For a given species, it takes only one unequivocal tool cut on one bone to moot the hypothesis of environmentally induced extinction. Whereas 32 genera of the extinct late Pleistocene megafauna have been collected uniquely in nonhuman contexts, only five genera are directly implicated as prey species. Frison (1987, p. 214) summarizes the situation as follows: "If Pleistocene extinctions resulted from human overkill it is highly unlikely that the evidence for this amount and kind of human activities could continue to escape detection."

Ultimately the remarkable resolving power of radiocarbon dating may be expected to resolve the late Pleistocene extinction debate. In a critical review of the corpus of megafaunal radiocarbon dates in North America, Meltzer & Mead (1985) accepted 307 dates from 163 sites. These data "demonstrate the extinction process was complete by as late as 10,000 B.P. and possibly as early as 10,800 B.P., at least for the following well-dated genera: Camelops, Equus, Mammut, Mammuthus, Nothrotheriops, and Panthera leo atrox. Other genera of extinct mammals are not reliably dated in sufficient quantities to adequately determine their terminal dates" (Meltzer & Mead 1 985, p. 166). A few genera, including Cervalces, Eucer­atherium, and Symbos, are known only from older dates. Grayson (1987a) suggests also that a number of genera lacking dates, including Eremother­ium, Glyptotherium, Hydrochoerus, Palaeolama, Tetrameryx, and Tre­marctos, are undated because they did not survive late enough. The basis of this proposal is that the statistical mode of usable radiocarbon dates assembled by Meltzer & Mead (1985) falls sharply between 11,500 and 1 1,000 yr B.P. and is thus biased in favor of latest Pleistocene dates. This may reflect both nature's own bias toward preserving latest Pleistocene sites, notably in such ephemeral sites as cave floors and recently deglaciated

Ann

u. R

ev. E

arth

Pla

net.

Sci.

1989

.17:

413-

436.

Dow

nloa

ded

from

ww

w.a

nnua

lrev

iew

s.or

gby

Uni

vers

idad

e de

Bra

silia

on

04/2

8/14

. For

per

sona

l use

onl

y.

428 WEBB & BARNOSKY

areas, and also human bias toward dating late sites. A new generation of tandem accelerator mass spectrometry dates, coupled with more detailed predictions and more thorough sampling of older sites, promises to resolve the long-standing debate about late Pleistocene extinctions (Grootes 1983, Mead & Meltzer 1985).

Climatic Impacts

In order to pursue the climatic hypothesis of late Pleistocene extinctions, detailed predictions about distributions of species through time and space are required. And finer grained records of late Pleistocene faunas will be needed to test these predictions. Even now, there are many indications that not only extinct megafi;tuna but also more sensitive microfauna responded differently in different regions during the Quaternary.

A relatively coarse-grained example of different faunal effects in different regions is the tropical survival of "extinct" taxa. Capybaras, spectacled bears, tapirs, llamas, peccaries, and such cats as jaguars, ocelots and margays, were lost from north temperate latitudes hut survive in the neotropical realm (Webb 1984b, 1985). The most striking example is the discovery of a large peccary, very closely related to the extinct Platygonus, living in the Gran Chaco of South America (Wetzel et aI1975). Still other late Pleistocene casualties in North America, such as horses, camels, saiga antelopes, elephants, and cheetahs, live on in Old World refuges, especially in the tropics (Reed 1970).

In Beringia, critical changes in paleoecology and paleogeography have been linked with late Pleistocene extinctions. The mere fact that sea level rose and severely reduced the habitable area may have produced extinction of saiga antelopes on the North American side and musk-oxen on the Asian side. Major climatic and vegetational changes at the end of the Pleistocene, including wetter winters, reduction of grassland, and expan­sion of less palatable taiga, have been linked to collapse of the "Mammoth­steppe Biome." These conclusions are supported by most students on the North American side (e.g. Guthrie 1984) and the Asian side (e.g. Vereshchagin & Baryshnikov 1984).

It is expected that if climatic change adversely affected a species, it would act at somewhat different times in various parts of that species range. Such stratigraphically sequenced records of distributional changes are particularly desirable but will require large arrays of well-dated and widely dispersed sites. The first such instance was developed by Hibbard ( 1960), who pointed out that large tortoises (Geochelone) retreated incrementally southward. Last known in the Great Plains during the last interglacial, they survived in Florida and Central America into the last glacial, and they still live on the Galapagos Islands. Extinction of Nothrotheriops

Ann

u. R

ev. E

arth

Pla

net.

Sci.

1989

.17:

413-

436.

Dow

nloa

ded

from

ww

w.a

nnua

lrev

iew

s.or

gby

Uni

vers

idad

e de

Bra

silia

on

04/2

8/14

. For

per

sona

l use

onl

y.

PLEISTOCENE MAMMALS 429

(Shasta ground sloth) is very well dated in western North America (at about 11,000 yr B.P.). In the east, however, it had disappeared from Florida and the Gulf Coastal Plain by the end of the Irvingtonian (McDon­ald 1985), at about the same time that jackrabbits and pronghorn antelopes disappeared from this region, and it had apparently retreated westward from Texas before the late Rancholabrean. In the other direction, glypto-

. donts, capybaras, and round-tailed muskrats retreated to the southeast during the course of the Pleistocene, which suggests progressively more severe effects of cooler winters (Webb 1977, 1984b).

Another avenue is to examine detailed morphological, meristic, and demographic data within a species shortly before its extinction. For exam­ple, King & Saunders (1984) recognize divergent forest versus prairie dental morphs in American mastodonts shortly before their extinction. Kurten (1965) indicated a size decrease in Floridian late Pleistocene saber­cats. And Barnosky (1985, 1986) found disproportionately small antlers in the latest populations of Irish elk in Ireland. Fossil mammal samples from the tar pits at Rancho La Brea (Los Angeles, California) range over about 25,000 yr and offer an ideal series by which to study samples of various terminal trends (Akersten et al 1983). In the Pliocene the deerlike Pediomeryx and the small hipparion horse Pseudhipparion each experi­enced remarkably rapid increases in hypsodonty immediately before extinction (Webb 1983, Webb & Hulbert 1986). Such trends may reflect strong natural selection and thus reveal at the population level a kind of "final agony."

Late Pleistocene mammal communities of North America differ from those that preceded them in that they contain a large number of extant species. Possibly these extant species may shed more light on extinction causes than the extinct species themselves. It is noteworthy that the geo­graphic ranges of most extant taxa were different in the Pleistocene than now. Species that today are strictly tundra inhabitants lived side by side with species that now occur only in temperate woodlands or prairies (Lundelius et al 1987). Similar altered Pleistocene distributions are also common in reptiles and amphibians (Brewer 1985, Holman & Grady 1987).

The extant species in many late Pleistocene faunas appear to be eco­logically incompatible among themselves by present biogeographic stan­dards. They have been called "nonanalogue communities" and "dis­harmonious faunas" (Lundelius et al 1983). As Graham & Mead (1987, p. 371) emphasize, this latter term does not "mean that the organisms were not in harmony with prevailing Pleistocene environments. [Rather,] disharmonious biotas were apparently maintained by . . . climates that have no modern analogues."

Ann

u. R

ev. E

arth

Pla

net.

Sci.

1989

.17:

413-

436.

Dow

nloa

ded

from

ww

w.a

nnua

lrev

iew

s.or

gby

Uni

vers

idad

e de

Bra

silia

on

04/2

8/14

. For

per

sona

l use

onl

y.

430 WEBB & BARNOSKY

Testing Climate Models

For three decades North American Quaternary specialists have sought to relate the changing distributions of late Pleistocene vertebrates to climatic features of that interval. In his seminal Presidential Address to the Michi­gan Academy of Science, Hibbard (1960) proposed that latest Pleistocene faunas from the midcontinent reflected cooler summers and warmer win­ters than at present. Likewise, Axelrod (1967) suggested that a sudden decline in climatic equability had triggered late Pleistocene large mammal extinctions. Most recently, the detailed climatic signal derived from fossil mammal faunas has been compared with results of paleoclimatic model simulations.

The key comparisons concentrate on full glacial to late glacial faunas in northcentral United States within a few hundred kilometers of the Laurentide ice sheet between the Rocky Mountains and the western slope of the Appalachians. They have been used to argue for more equable climates between 18,000 and 13,000 yr ago (Graham & Mead 1987, Wright 1987). "Equable" is used here in the sense of less temperature difference than now exists between summer and winter, even though mean annual temperatures were cooler. Climatic equability is based on the occurrence of boreal and arctic-tundra species (such as arctic shrews, lemmings, voles, and ground squirrels) in the same sites as species that now inhabit grass­lands or deciduous woodlands (for instance, prairie voles, sagebrush voles, the thirteen lined ground squirrel, and the eastern chipmunk). Reptiles and amphibians frequently are present as well. The interpretation that the temperature difference between summer and winter was less than today is based on the observation that animals that are now retracted northward are apparently limited by hot summer temperatures, whereas those that are now confined southward and eastward are probably limited by cold winter temperatures. Only under more equable (less extreme) conditions could such opposite groups have extended their ranges into the same geographic area.

Paleoclimatic model simulations predict similar conditions. Model results elaborated by Kutzbach (1987) suggest that in northcentral United States between 18,000 and 15,000 yr ago, July temperatures were 7-10°C cooler, and January ones 5-lOoC cooler, than at present. The height and extent of the Laurentide ice sheet caused the westerly jet stream to split. Consequently storm tracks were directed south of the southern margin of the ice. Rainfall was less, but effective moisture (precipitation minus evaporation) increased relative to today. At the surface 18,000 yr ago, a glacial anticyclone centered over the ice sheet. The anticyclonic circulation brought surface easterlies or weak westerlies south of the Laurentide ice

Ann

u. R

ev. E

arth

Pla

net.

Sci.

1989

.17:

413-

436.

Dow

nloa

ded

from

ww

w.a

nnua

lrev

iew

s.or

gby

Uni

vers

idad

e de

Bra

silia

on

04/2

8/14

. For

per

sona

l use

onl

y.

PLEISTOCENE MAMMALS 431

sheet, particularly in winter, whereas winter winds today are strong westerly. As the air descended along the margin of the ice sheet, it was adiabatically warmed, which modified winter temperatures locally (Kutz­bach & Wright 1985, Wright 1987). This feature counteracted solar radi­ation values that increased in summer and decreased in winter by 13,000 yr ago, in contrast with essentially modern values at 18,000 yr ago (Kutzbach 1987).

The congruence between predictions derived from fossil mammal data and paleoclimatic simulations suggests that both reconstruction methods are robust. On the other hand, it must be noted that a cooler, more equable climatic regime is indicated only for northcentral United States near the ice margin, and only between 18,000 and 13,000 yr B.P. Disharmonious faunas occur at many other locales and date from at least 400,000 yr ago into the Holocene, but they involve other sets of taxa and can be expected to reflect other climatic regimes (Graham et aI1987). In this light, study of disharmonious faunas-where and when they occur and what local climatic signal they reflect-holds great promise both as a climatic tool and as a test of hypotheses derived from paleoclimatic model simulations. The value of this approach is enhanced in areas where fossil mammals occur commonly and other paleoclimatic proxies are absent (Graham et al 1987).

In the southwestern United States it has long been suspected that many late Pleistocene extirpations were related to the onset of drier and less equable conditons there (Axelrod 1967). Patterson's (1984) work on pres­ently disjunct mountaintop communities of small mammals in the Great Basin forcefully indicates the extent of upslope retreat since the onset of arid climates in the late Pleistocene. For some species, such as Phenacomys intermedius (heather vole), evidence from archaeological sites helps date the progress of that retreat (Grayson 1981, 1987b). The best direct evidence of changing plant and animal distributions in this region during late Pleistocene and early Holocene time comes from packrats (and occasion­ally porcupines), for their middens provide over 1000 dated samples recording (very generally) an upslope retreat of mesic biotas in the south­western region (Van Devender & Spaulding 1979, Spaulding et al 1983). Confirmation of increasing aridity also comes from the retreat of late Pleistocene lakes in the Great Basin (COHMAP Members 1988).

In many cases, perhaps most, the correlation between mammals and climate is a second-order one linked by vegetation. Clearly climatic data are most effectively compared with data on Pleistocene vegetation (Bryant & Holloway 1985, Webb et al 1987, Barnosky et al 1987, COHMAP Members 1988). Pollen data show that between 18,000 and 12,000 yr ago vegetation in northcentral United States was dominated by communities

Ann

u. R

ev. E

arth

Pla

net.

Sci.

1989

.17:

413-

436.

Dow

nloa

ded

from

ww

w.a

nnua

lrev

iew

s.or

gby

Uni

vers

idad

e de

Bra

silia

on

04/2

8/14

. For

per

sona

l use

onl

y.

432 WEBB & BARNOSKY

rich in spruce and sedges, in assemblages for which few modern analogues exist (Webb et al 1 987). The general picture is of a spruce parkland, perhaps most similar to that presently found in the southern part of the Ungava Peninsula in northern Quebec.

The full-glacial faunas of northcentral United States agree with the vegetational reconstructions in that the mammal communities are also without modern analogue and include a significant boreal, tundra, and grassland component. For example, Wright ( 1987) recognized the dis­tinctive spruce forests of the northern Great Plains as more open than living spruce forests; in the same time and place, Rhodes ( 1984) interpreted a distinctive fauna as inhabiting "boreal grasslands. " The ca. 17,000 yr old Moscow Fissure fauna from near the ice front in Wisconsin contained the collared lemming (modern distribution in tundra), the yellowcheek vole and heather vole (boreal), and the northern pocket gopher and western jumping mouse (grass/forb habitats) (Foley 1 984, Graham & Mead 1987). Modern biogeographic ranges suggest that redistribution of the mammal species may have tracked subsequent vegetational changes. The yellow­cheek and heather voles retreated northward with the boreal forest, whereas the northern pocket gopher and western jumping mouse followed the sedges and grasslands west [compare range maps in Wcbb et al ( 1987) with those in Hall ( 1980)].

That disharmonious faunas bear both a climatic and a vegetational signature suggests that one of the most important tasks in Quaternary mammalogy involves sorting out the links between climate, vegetation, and mammal distributions. Data are now available to undertake such work in the form of climatic models, regional syntheses of paleobotanical data, and compilations of fossil mammal data (e.g. papers in Bryant & Holloway 1985, Ruddiman & Wright 1 987, Graham et al 1987). What remains is to recognize the specifics of faunal disharmonies in different parts of the continent and to test resultant predictions against inde­pendently derived vegetational and climatic models. Innovative analytical techniques have recently become available to do this-for example, the use of response functions (Bartlein et al 1986) to place the distribution of species in climate-space and the use of graphic information systems (GIS) to assemble and array data digitally (GIS 1 987, Madigan et aI 1 988).

CONCLUSIONS

The late Cenozoic record of mammals in North America provides a data­rich framework in which to study faunal dynamics-that is, patterns of immigration, evolution, and extinction. Especially during the last 50,000 years, for which cven small bone fragments may now be expected to

Ann

u. R

ev. E

arth

Pla

net.

Sci.

1989

.17:

413-

436.

Dow

nloa

ded

from

ww

w.a

nnua

lrev

iew

s.or

gby

Uni

vers

idad

e de

Bra

silia

on

04/2

8/14

. For

per

sona

l use

onl

y.

PLEISTOCENE MAMMALS 433

yield radiocarbon dates (thanks to tandem accelerator mass spectrometry systems), it is potentially possible to answer complex questions in a finer grained manner than for earlier periods of Earth history. Thus, the Pleis­tocene provides a crucial scale transition from ecological to geological perspectives.

At various times during the late Cenozoic, rates of immigration, evo­lution, and extinction reached very high levels. The highest Pleistocene rates of evolution and extinction may be inflated compared with Tertiary rates simply by virtue of the short time intervals over which they are measured. On the other hand, the broad spectrum of punctuated and gradual evolutionary tempos found in the Pleistocene is of great interest on its own terms. It offers a substantial field for comparative evolutionary studies that bridge the geological and ecological scales.

Both evolutionary rates and extinction rates recorded among late Pleis­tocene mammals often fall well above rates known during earlier Ceno­zoic intervals. Is this simply because the intervals over which they must be averaged are short? Since extinctions represent last appearances, which are by definition instantaneous in time, there may be episodes in the past, such as the very large extinction set in the mid-Hemphillian, that may have been just as dramatic as the late Pleistocene crash. This proposition cannot be tested because it is impossible with present chronostratigraphic methods to show whether pre-late Pleistocene extinctions were amassed during an interval on the order of a thousand years.

With respect to immigration rates of mammal genera into North Amer­ica, it is quite clear that late Cenozoic rates considerably exceed those known from earlier mammal ages. Indeed, the absolute rates for the Blancan, Irvingtonian, and Rancholabrean NALMAs exceed those of earlier intervals, even though the late Cenozoic intervals are relatively short. The late Blancan is an extremely active interval, with high levels of interchange between North America and Asia on the one hand and North and South America on the other.

Among episodes of rapid faunal turnover in the history of life, two rather different extremes can generally be distinguished. At one end are extinction-driven (E-type) episodes, and at the other are immigration­driven (I-type) episodes. Pleistocene mammal faunas are notable not only for their extremely high turnover rates but also for combining E-type and I-type high turnover rates (Savage & Russell 1983, Webb 1 984a). Translating these observations beyond faunal dynamics, we see that such extreme volatility must be rooted in the extraordinarily rapid changes of Pleistocene environments.

We are fortunate to stand so close to such an unusual chapter in Earth history. The detailed record of late Cenozoic land mammals provides a

Ann

u. R

ev. E

arth

Pla

net.

Sci.

1989

.17:

413-

436.

Dow

nloa

ded

from

ww

w.a

nnua

lrev

iew

s.or

gby

Uni

vers

idad

e de

Bra

silia

on

04/2

8/14

. For

per

sona

l use

onl

y.

434 WEBB & BARNOSKY

useful proxy of environmental changes over the past several million years. Of special interest are events at the end of the last glacial epoch and the transition from them to present and future events. In this part of the fossil record we have a unique combination of tight chronologic and geographic control, as well as abundant paleoecological information, within a time span long enough to link patterns of past, present, and future faunal dynamics.

ACKNOWLEDGMENTS

We wish to thank the many colleagues who have helped us formulate these ideas-in particular, Cathy W. Barnosky, Ernest L. Lundelius Jr., Charles Repenning, William A. Watts, Dale A. Guthrie, Russell W. Graham, Pat Bartlein, and Richard C. Hulbert Jr. Our own Pleistocene mammalian studies have been supported by grants from the National Geographic Society and the National Science Foundation (grants no. EAR-8615373 and 8708045). This article is University of Florida Contribution in Paleo­biology No. 340.

Literature Cited

Akersten, W., Shaw, C., Jefferson, G. 1983. Rancho La Brea: status and future. Paleo­biology 9: 2 1 1 - 1 7

Anderson, E. 1984. Who's who i n the Pleis­tocene: a mammalian bestiary. See Martin & Klein 1984, pp. 40-89

Axelrod, D. I. 1967. Quaternary extinctions of large mammals. Univ. Calif. Publ. Bioi. Sci. 74: 1--42

Barnosky, A. D. 1985. Late Blancan (Plio­cene) microtine rodents from Jackson Hole, Wyoming: biostratigraphy and bio­geography. J. Vertebr. Paleontol. 5(3): 255-71

Barnosky, A. D. 1986. "Big game" extinc­tion caused by late Pleistocene climatic change: Irish elk (Megaloceros giganteus) in Ireland. Quat. Res. 25: 1 28-35

Barnosky, A. D. 1987. Punctuated equi­librium and phyletic gradualism: some facts in the Quaternary mammalian record. In Current Mammalology, ed. H. H. Genoways, 1: 107-47. New York: Plenum.

Barnosky, C. W., Anderson, P. M . , Bartlein, P. J. 1987. The northwestern U.S. dur­ing deglaciation: veg..:tation histury and paleoclimatic implications. See Ruddi­man & Wright 1987, pp. 289-32 1

Bartlein, P. J., Prentice, I . C., Webb, T. III. 1 986. Climatic response surfaces from pol­len data for some eastern North American taxa. J. Biogeogr. 1 3: 35-57

Benton, M. J. 1988. The relationships of the major groups of mammals: new approaches. Trends Ecol. Evol. 3(2): 40-45

Bobrowsky, P. T. 1988. Ice-free conditions in the northern Canadian cordillera at 18 ka and timing of the late Wisconsi­

nan. Am. Quat. Assoc. Program Abstr., Amherst, Mass., p. 108 (Abstr.)

Bonifay, M. R. 1980. Relation entre les don­nees isotopiques, oceaniques et l'histoire des grandes faunes Europeennes Plio­Pleistocenes. Quat. Res. 1 4: 251-62

Bowler, J. M. 1976. Aridity in Australia: age, origins and expression in aeolian land forms and sediments. Earth Sci. Rev. 1 2 : 297-3 1 0

Bradley, R. S. 1985. Quaternary Paleo­climatology. Winchester, Mass: Allen & Unwin. 330 pp.

Brewer, D. J. 1985. Herpetofaunas in the late Pleistocene: extinctions and extralimital forms. See Mead & Meltzer 1985, pp. 3 1 -52

Bryant, V. M. Jr., Holloway, R. G., eds. 1985. Pollen Records of Late-Quaternary North American Sediments. Dallas: Am. Assoc. Stratigr. Palynol. Found.

Cann, R. L., Stoneking, M . , Wilson, A. C. 1987. Mitochondrial DNA and human evolution. Nature 325: 3 1-36

Chaline, J., Graf, J. 1988. Phylogeny of the

Ann

u. R

ev. E

arth

Pla

net.

Sci.

1989

.17:

413-

436.

Dow

nloa

ded

from

ww

w.a

nnua

lrev

iew

s.or

gby

Uni

vers

idad

e de

Bra

silia

on

04/2

8/14

. For

per

sona

l use

onl

y.

Arvicolidae (Rodentia): biochemical and paleontological evidence. J. Mammal. 69(1): 22-33

Churcher, C. S., Pinsoff, J. D. 1987. Vari­ation in the antlers of North American Cervalces (Mammalia, Cervidae): review of new and previously recorded specimens. J. Vertebr. Paleontol. 7: 373-97

COHMAP Members. 1 988. Climatic changes of the last 1 8 ,000 years: obser­vations and model simulations. Science 241 : 1 043-52

Crowley, T. J., North, G. R. 1 988. Abrupt climate change and extinction events in Earth history. Science 240: 996-1002

Davis, L. C. 1987. Late Pleistocene/Holo­cene environmental changes in the Central Plains of the United States: mammalian record. See Graham et al 1987, pp. 89-143

Diamond, J. M . 1984. Historic extinctions: a Rosetta stone for understanding pre­hi�toric extinctions. See M artin & Klein 1984, pp. 824--62

Dobzhansky, T., Ayala, F. 1., Stebbins, G. L., Valentine, 1. W. 1 977. Evolution. San Francisco: Freeman. 572 pp.

Emslie, S. D. 1987. Age and diet of fossil California condors in Grand Canyon, Arizona. Science 237: 768-70

Fejfar, O. 1983. Arvicolidae-Sukzession and biostratigraphie des Oberpliozans und Quartars in Europa. Schriftenreihe fuer Geologische Wissenschaften. Berlin 19/20: 6 H09

Fejfar, 0., Heinrich, W. D. 198 1 . Zur bio­stratigraphischen Untergleidering des Kon­tinentalen Quartars in Europa anhand von Arvicolidae (Rodentia: Mammalia). Eclogae Geol. Helv. 74: 997-1 006

Fisher, D. C. 1987. Mastodont procurement by paleo-Indians of the Great Lakes region: hunting or scavenging. In The Evolution of Human Hunting, ed. M . H. Nitecki, D. V. Nitecki, pp. 309--42 1 . New York: Plenum. 464 pp.

Foley, R. L. 1984. Late Pleistocene (Wood­fordian) vertebrates from the driftless area of southwestern Wisconsin, the Moscow fissure local fauna. Ill. State Mus. Rep. Invest. No. 39. 50 pp.

Frison, G. C. 1987. Prehistoric plains-moun­tain, large mammal, communal hunting strategies. In The Evolution of Human Hunting, ed. M . H. Nitecki, D. V. Nitecki, pp. 1 77-233. New York: Plenum. 464 pp.

Gingerich, P. D. 1983. Rates of evolution: effects of time and temporal scaling. Sci­ence 222: 1 59-61

Gingerich, P. D. 1 987. Evolution and the fossil record: patterns, rates and processes. Can. J. Zool. 65: 1053-60

GIS. 1987. Second Annual International Con-

PLEISTOCENE MAMMALS 435

ference on Geographic Information Sys­tems. Falls Church, Va: ASPRS. 756 pp. 2 vols.

Graham, R. W., Mead, J. 1. 1987. Environ­mental fluctuations and evolution of mammalian faunas during the last deglaci­ation. See Ruddiman & Wright 1 987, pp. 371--402

Graham, R. W., Semken, H. A. Jr., Graham, M. A., eds. 1 987. Late Quaternary Mam­malian Biogeography and Communities of the Great Plains and Prairies. Ill. State Mus. Sci. Pap., Vol. 22. 49 1 pp.

Grayson, D. K. 1977. Pleistocene avifaunas and the overkill hypothesis. Science 1 95: 691-93

Grayson, D. K. 1 98 1 . A mid-Holocene rec­ord for the heather vole, Phenacomys cf. intermedius, in the central Great Basin and its biogeographic significance. J. Mammal.

62: 1 1 5-2 1 Grayson, D. K. 1987a. An analysis of the

chronology oflate Pleistocene mammalian extinctions in North America. Quat. Res. 28: 28 1-89

Grayson, D. K. 1987b. The biogeographic history of small mammals in the Great Basin: observations on the last 20,000 years. J. Mammal. 68: 359-75

Grootes, P. M. 1 983. Radioactive isotopes in the Holocene. See Wright 1983, pp. 86-105

Guilday, J. E. 1 982. Dental variation in Mi­crotus xanthognathus, M. chrotorrhinus, M. pennsylvanicus (Rodentia: Mammalia). Ann. Carnegie Mus. 5 1 : 21 1-30

Guilday, J. E., Bender, M. S. 1 960. Late Pleistocene records for the yellow-cheeked vole Microtus xanthognathus (Leach). Ann. Carnegie Mus. 35: 3 1 5-30

Guilday, J. E., Hamilton, H. W., Anderson, E., Parmalee, P. W. 1978. The Baker Bluff cave deposit, Tennessee, and the late Pleistocene faunal gradient. Bull. Carnegie Mus. Nat. Hist. I I : 1-67

Guthrie, R. D. 1 984. Mosaics, allelochemics and nutrients. See Martin & Klein 1984, pp. 259-98

Hall, E. R. 1980. The Mammals of North America. New York: Wiley. 2nd ed. 2 vols.

Harris, J. M ., White, T. D. 1979. Evolution of the Plio-Pleistocene African suidae. Trans. Am. Phi/os. Soc. 69(2): 1-128

Haynes, C. V. 1 984. Stratigraphy and late Pleistocene extinction in the United States. See Martin & Klein 1 984, pp. 345-S3

Hibbard, C. W. 1 944. Stratigraphy and ver­tebrate paleontology of the Pleistocene deposits of southwestern Kansas. Geot. Soc. Am. Bull. 55: 707-54

Hibbard, C. W. 1 960. An interpretation of Pliocene and Pleistocene climates in North

Ann

u. R

ev. E

arth

Pla

net.

Sci.

1989

.17:

413-

436.

Dow

nloa

ded

from

ww

w.a

nnua

lrev

iew

s.or

gby

Uni

vers

idad

e de

Bra

silia

on

04/2

8/14

. For

per

sona

l use

onl

y.

436 WEBB & BARNOSKY

America. Ann. Rep. Mich. Acad. Sci. 62: 5-30

Hibbard, C. W., Zakrzewski, R. J. 1 967. Phyletic trends in the late Cenozoic mi­crotine, Ophiomys gen. nov., from Idaho. Univ. Mich. Mus. Paleontol. Contrib. 2 1 ( 1 2): 255-71

Hibbard, C. W., Zakrzewski, R. J. 1 972. A new species of micro tine from the late Pliocene of Kansas. J. Mammal. 53: 834-

. 39 Holman, J. A., Grady, F. 1 987. Herpeto­

fauna of New Trout Cave. Natl. Geogr. Res. 3: 305-17

Hughes, A: L. 1988. The quagga case: molec­ular evolution of an extinct species. Trends Ecol. Evol. 3(5): 95-96

Huxley, T. H. 1986. Man's Place in Nature. New York: Appleton. 328 pp.

Jones, C. A., Choate, J. R., Genoways, H . H. 1984. Phylogeny and paleobioge­ography of short-tailed shrews (genus Blarina). In Contributions in Quaternary Vertebrate Paleontology: A Volume in Me­morial to John E. Guilday. Carnegie Mus. Nat. Hist. Spec. Publ. ed. H. H. Genoways, M. R. Dawson. 8: 56-148

Kennedy, S. K., Lin, W.-H. 1 986. FRACT­a FORTRAN subroutine to calculate the variables necessary to detennine the frac­tal dimension of closed forms. Comput. Geosci. 1 2: 705-1 2

King, J. L . , Jukes, T . H. 1 969. Non-Dar­winian evolution. Science 1 64: 788-98

King, J. E., Saunders, J. F. 1984. Environ­mental insularity and the extinction of the American mastodont. See Martin & Klein 1984, pp. 3 1 5-39

Kurten, B. 1 965. The Pleistocene Felidae of Florida. Bull. Fla. State Mus. 9: 2 1 5-73

Kurten, B. 1968. Pleistocene Mammals of Europe. London: Weidenfeld & Nichol­son. 3 1 7 pp.

Kurten, B. 1 985. Turnover in Pleistocene faunas. Acta Zool. Fenn. 1 70: 1 63--66

Kurten, B. , Anderson, E. 1 980. Pleistocene Mammals of North America. New York: Columbia Univ. Press. 442 pp.

Kutzbach, J. E. 1 987. Model simulations of the climatic patterns during the deglaci­ations of North America. See Ruddiman & Wright 1987, pp. 425-46

Kutzbach, J. E., Wright, H. E. Jr. 1 985. Simulation of the climate of 1 8,000 yr B.P.; results for the North American/ North Atlantic/European sector. Quat. Sci. Rev. 4: 147-87

Langley, C. H., Fitch, W. M. 1974. An exam­ination of the constancy of the rate of molecular evolution. J. Mol. Evo!. 3: 161-77

Lindsay, E. H ., Johnson, N. M., Opdyke, N. D., Butler, R. F. 1 987. Mammalian

chronology and the magnetic polarity time scale. See Woodburne 1987, pp. 267-84

Lowenstein, J. M. 1986. Molecular phylo­genetics. Ann. Rev. Earth Planet. Sci. 14: 71-83

Lundelius, E. L. Jr. 1 983. Climatic impli­cations of late Pleistocene and Holocene faunal associations in Australia. Alche­ringa 7: 1 25-49

Lundelius, E. L. Jr., Graham, R. W., Anderson, E., Guilday, J., Holman, J. A. et al. 1 983. Terrestrial vertebrate faunas. See Wright 1 983, pp. 3 1 1-53

Lundelius, E. L. Jr., Downs, T., Lindsay, E. H., Semken, H. A., Zakrzewski, R. A. et al. 1 987. The North American Quaternary sequence. See Woodburne 1987, pp. 2 1 1-35

Lyell, C. 1 863. The Geological Evidences of the Antiquity of Man. London: Murray. 287 pp.

Madigan, M. E., Sturdevant, J. E., Roberts, C. A. 1988. Using GIS to evaluate map data. Geotimes 33(4): 14- 1 5

Maglio, V . J. 1 973. Origin and evolution of the Elephantidae. Trans. Am. Phi/os. Soc. 63(3): 1-149

Marshall, L. G., Butler, R. F., Drake, R. E., Curtis, G. H. , Tedford, R. H. 1979. Calibration of the Great American inter­change. Science 204: 272-79

Martin, L. D. 1 979. The biostratigraphy of arvicoline rodents in North America. Trans. Nebr. Acad. Sci. 7: 9 1-100

Martin, L. D. 1984. Phyletic trends and evo­lutionary rates. In Contributions in Qua­ternary Vertebrate Paleontology: A Volume in Memorial to John E. Guilday. Carnegie Mus. Nat. Hist. Spec. Publ. , ed. H. H. Genoways, M . R. Dawson, 8: 526-38

Martin, P. S., Klein, R. G., cds. 1984. Qua­ternary Extinctions: A Prehistoric Revo­lution, Tucson: Univ. Ariz. Press. 892 pp.

Martin, R. A. 1 984. Thc evolution of cotton rat body mass. In Contributions in Qua­ternary Vertebrate Paleontology: A Volume in Memorial to John E. Guilday. Carnegie Mus. Nat. Hist. Spec. Publ., ed. H. H . Genoways, M . R. Dawson, 8 : 1 79-83

McDonald, H. G. 1985. The Shasta Ground Sloth Nothrotheriops shastensis (Xenar­thra, Magatheriidae) in the middle Pleis­tocene of Florida. In The Evolution and Ecology of Armadillos, Sloths, and Ver­milinguas, ed. G. Montgomery, pp. 95-104. Washington, DC: Smithson. Inst. 45 1 pp.

Mead, J. 1., Meltzer, D. J., eds. 1 985. Environments and Extinctions: Man in Late Glacial North America. Orono, Maine: Cent. Study Early Man. 209 pp.

Meltzer, D. J., Mead, J. 1. 1985. Dating late Pleistocene extinctions: theoretical issues,

Ann

u. R

ev. E

arth

Pla

net.

Sci.

1989

.17:

413-

436.

Dow

nloa

ded

from

ww

w.a

nnua

lrev

iew

s.or

gby

Uni

vers

idad

e de

Bra

silia

on

04/2

8/14

. For

per

sona

l use

onl

y.

analytical bias, and substantive results. See Mead & Meltzer 1985, pp. 1 45-73

Nei, M . 1 97 1 . Interspecific gene differences and evolutionary time estimated from electrophoretic data on protein identity. Am. Nat. 105: 385-98

Nelson, R. S., Semken, H. A. Jr. 1970. Paleo­ecological and stratigraphic significance of the muskrat in Pleistocene deposits. Geol. Soc. Am. Bull. 8 1 : 3733-38

Owen, R. 1 846. A History of British Fossil Mammals and Birds. London: Van Voorst. 468 pp.

Patterson, B. D. 1 984. Mammalian extinc­tion and biogeography in the southern Rocky Mountains. In Extinctions, ed. M. H . Nitecki, pp. 247-93. Chicago: Univ. Chicago Press. 354 pp.

Plotnick, R. E. 1 986. A fractal model for the distribution of stratigraphic hiatuses. J. Geol. 94: 885-90.

Porter, S. c., ed. 1 983. Late Quaternary Environments of the United States, Vo!' 1 , The Late Pleistocene. Minneapolis: Univ. Minn. Press. 407 pp.

Prothero, D. R. 1985. North American mammalian diversity and Eocene-Oligo­cene extinctions. Paleobiology 1 1 : 389-405

Reed, C. W. 1 970. Extinction of mammalian megafauna in the Old World late Qua­ternary. BioScience 20: 284-88

Rensberger, J. M., Barnosky, A. D., Spencer, P. K. 1 984. Geology and paleon­tology of a Pleistocene-to-Holocene loess succession, Benton County, Washington. Archaeol. Hist. Servo East. Wash. Univ. Rep. A rchaeol. Hist. No. 100-39. 105 pp.

Repenning, C. A. 1 980. Faunal exchanges between Siberia and North America. Can. J. Anthropol. 1 ( 1): 37-44

Repenning, C. A. 1984. Quaternary rodent biochronology and its correlation with cli­matic and magnetic stratigraphies. In Cor­relation of Quaternary Chronologies, ed. W. C. Mahoney, pp. 105- 1 8. Norwich, Eng!.: Geo Books

Repenning, C. A. 1 987. Biochronology of the microtine rodents of the United States. See Woodburne 1987, pp. 236-66

Rhodes, R. S. 11 . 1984. Paleoecology and regional paleoclimatic implications of the Farrndalian Craigmile and Woodfordian Waubonsie mammalian local faunas, southwestern Iowa. Ill. State Mus. Rep. Invest. 40: 1-51

Ruddiman, W. F., Wright, H. E. Jr., eds. 1 987. North American and Adjacent Oceans During the Last Deglaciation. The Geology of North America, Vol. K-3. Boulder, Colo: Geol. Soc. Am. 501 pp. + maps

Savage, D. E., Russell, D. E. 1 983. Mam-

PLEISTOCENE MAMMALS 437

malian Paleofaunas of the World. London: Addison-Wesley. 432 pp.

Savin, S. M . , Douglas, R. G. 1985. Sea level, climate and the Central American land bridge. See Stehli & Webb 1985, pp. 303-24

Schmidt-Kittler, N. 1986. Evaluation of occlusal patterns of hypsodont rodent dentitions by shape parameters. Neues Jahrb. Geol. Palaeontol. Abh. 1 73 : 75-98

Shackleton, N. J . , Opdyke, N. D. 1976. Oxy­gen isotope and paleomagnetic stra­tigraphy of equatorial Pacific core V28-239, late Pliocene to latest Pleistocene. In Investigations of Late Quaternary Paleo­oceanography and Paleoclimatology. Geol. Soc. Am. Mem., ed. R. M. Cline, J. D. Hays, 1 45: 449-64

Shackleton, N. J., Opdyke, N. D. 1977. Oxy­gen isotope and paleomagnetic evidence for early Northern Hemisphere glaciation. Nature 270: 2 1 6-19

Shipman, P., Fisher, D. C., Rose, J. J. 1 984. Mastodon butchery: microscopic evidence of carcass processing and bone tool use. Paleobiology 10: 358--65

Spaulding, W. G., Leopold, E. B., Van Devender, T. R. 1983. Late Wisconsin paleoecology of the American southwest. See Porter 1 983, pp. 259-93

Stanley, S. M. 1 986. Anatomy of a regional mass extinction: Plio-Pleistocene deci­mation of the western Atlantic bivalve fauna. Palaios 1 : 1 7-36

Steadman, D. W., Martin, P. S. 1 984. Extinc­tion of birds in the late Pleistocene of North America. See Martin & Klein 1 984, pp. 466-77

Steadman, D. W., Miller, N. G. 1 987. Cali­fornia condor associated with spruce-pine woodland in the late Pleistocene of New York. Quat. Res. 28 : 4 1 5-26

Stehli, F. G., Webb, S. D., eds. 1 985. The Great American Biotic Interchange. New York: Plenum. 532 pp.

Stuart, A. J. 1982. Pleistocene Vertebrates in (he British Isles. New York: Longman. 2 1 2 pp.

Tedford, R. H., Skinner, M. F., Fields, R. W., Rensberger, J. M., Whistler, D. P., et al. 1987. Faunal succession and bio­chronology of the Arikareean through Hemphillian interval (late Oligocene through earliest Pliocene epochs) in North America. See Woodburne 1987, pp. 1 53-2 1 0

Van Devender, T . R . , Spaulding, W . G . 1 979. Development o f vegetation and cli­mate in the southwestern United States. Science 204: 701-10

Van Valen, L. 1 974. Two modes of evolution. Nature 252: 298-300

Velichko, A. A., ed. 1983. Late Quaternary

Ann

u. R

ev. E

arth

Pla

net.

Sci.

1989

.17:

413-

436.

Dow

nloa

ded

from

ww

w.a

nnua

lrev

iew

s.or

gby

Uni

vers

idad

e de

Bra

silia

on

04/2

8/14

. For

per

sona

l use

onl

y.

438 WEBB & BARNOSKY

Environments of the Soviet Union (Engl. version, ed. H. E. Wright Jr., C. W. Bar­nosky) . Minneapolis: Univ. Minn. Press. 327 pp.

Vereshchagin, N. K., Baryshnikov, G. F. 1984. Quaternary mammalian extinctions in northern Eurasia. See Martin & Klein 1984, pp. 483-51 6

Vermeij, G. J . 1 985. Evolution and Escal­ation: An Ecological History of Life. Princeton, NJ: Princeton Univ. Press. 527 pp.

Vrba, E. S. 1 984. Evolutionary pattern and process in the sister group Alcelaphini­Aepycerotini (Mammalia: Bovidae). In Living Fossils, ed. N. Eldredge, S. M. Stan­ley, pp. 62-79. New York: Springer-Ver­lag

Walker, A. 1984. Extinction in Hominid . evolution. In Extinctions, ed. M. H.

Nitecki, pp. 1 19-52. Chicago: Univ. Chicago Press. 354 pp.

Webb, S. D. 1 969. Extinction-origination equilibria in late Cenozoic land mammals of North America. Evolution 23: 688-702

Webb, S. D. 1 977. A history of savanna ver­tebrates in the New World. Part I: North America. Ann. Rev. Ecol. Syst. 8: 355-80

Webb, S. D. 1 983. A new species of Pedio­meryx from the late Miocene of Florida, with comments on Dromomerycid rumi­nant evolution. J. Mammal. 64: 261-76

Webb, S. D. 1984a. On two kinds of rapid faunal turnover. In Catastrophes and Earth History: The New Uniformitarian­ism, ed. W. A. Berggren, J. A. Van Couv­ering, pp. 417-36. Princeton, NJ: Prince­ton Univ. Press. 464 pp.

Webb, S. D. 1984b. Ten million years of mammal extinctions in North America. See Martin & Klein 1984, pp. 189-210

Webb, S. D. 1 985a. Main pathways ofmam­malian diversification in North America. See Stehli & Webb 1 985, pp. 201-1 7

Webb, S. D. 1985b. Late Cenozoic mammal

dispersals between the Americas. See Stehli & Webb 1 985, pp. 357-86

Webb, S. D., Hulbert, R. C. Jr. 1 986. Sys­tematics and evolution of Pseudhipparion (Mammalia, Equidae) from the late Neo­gene of the Gulf Coastal Plain and the Great Plains. In Vertebrates, Phylogeny and Philosophy, Contrib. Geol., Univ. Wyo. Spec. Pap. No. 3, ed. K. Flanagan, J. Lillegraven, pp. 237-72

Webb, T. III, Bartlein, P. J., Kutzbach, J. E. 1987. Climatic change in eastern North America during the past 1 8 ,000 years: comparisons of pollen data with model results. See Ruddiman & Wright 1987, pp. 447-<l2

West, R. H . 1983. The antiquity of man in America. See Porter 1983, pp. 364-82

Wetzel, R. M . , Dubas, R. E., Martin, R. L., Myers, P. 1 975. Catagonus, an "extinct" peccary, alive in Paraguay. Science 1 89: 379-8 1

Wolfe, J. A. 1 985. Distribution of major vegetational types during the Tertiary. In The Carbon Cycle and A tmospheric CO2: Natural Variations Archaean to Present. Geophys. Monogr., 32: 357-74. Washing­ton DC: Am. Geophys . Union

Wolpoff, M . H. 1984. Evolution in Homo erectus: the question of stasis. Paleo­biology 1 0(4): 389-406

Woodburne, M. 0., ed. 1987. Cenozoic Mammals of North America: Geochron­ology and Biostratigraphy. Berkeley/Los Angeles: Univ. Calif. Press. 336 pp.

Wright, H. E. Jr. , ed. 1983. Late Quaternary Environments of the United States, Vol. 2. The Holocene. Minneapolis: Univ. Minn. Press. 277 pp.

Wright, H. E. Jr. 1987. Synthesis: land south of the icc sheets. See Ruddiman & Wright, pp. 479-88

Zakrzewski, R. J. 1 974. Fossil Ondatrini from western North America. J. Mammal. 55(2): 284-92

Ann

u. R

ev. E

arth

Pla

net.

Sci.

1989

.17:

413-

436.

Dow

nloa

ded

from

ww

w.a

nnua

lrev

iew

s.or

gby

Uni

vers

idad

e de

Bra

silia

on

04/2

8/14

. For

per

sona

l use

onl

y.