early americans and pleistocene mammals in north...
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Early Americans and Pleistocene Mammals in North America
Donald K. Grayson, Department of Anthropology, Campus Box 353010, University of Washington, Seattle, WA
98915
In: Environment, Origins, and Population. Handbook of North American Indians, Volume 3. D. H. Ubelaker, ed.,
Smithsonian Institution Press, Washington, D. C., in press.
Introduction
There are those who believe that the initial colonization of the earth’s lands by behaviorally modern people was
an inevitable cause of rapid biotic extinction. “Outside continental Africa and southeast Asia,” ecologist Paul S.
Martin (1967:114) has suggested, “massive extinction is unknown before the earliest known arrival of prehistoric
man” but once people arrive, vulnerable species “suddenly vanish from the fossil record” (1984:396-397). “Human
establishment on a virgin land mass,” Martin and Steadman (1999:47) argue, leads inexorably to “a geologically
rapid and historically monumental extinction event.”
This argument is an old one, first made well over a century ago in the wake of the recognition that people had
co-existed with such long-gone beasts as mammoth and woolly rhinoceros (Grayson 1983, 1984b). North America
has played an essential role in this history. Extinctions here were massive, removing some 35 genera of primarily
large mammals (of which six survive in other parts of the world), along with about 19 genera of birds. Complete by
about 10,500 14C years ago or soon thereafter, a number of these extinctions appear to coincide reasonably well with
the archaeological phenomenon known as Clovis. As a result, it has been tempting to some to attribute all the North
American extinctions to people.
North America has been key to the arguments concerning human-caused (“anthropogenic”) prehistoric
extinctions, but the debate over the causes of the losses that occurred here can be neither understood nor resolved
without taking a much broader look at our understanding of anthropogenic extinctions. Here, I explore some
questions whose simple words mask their complexity. Does our understanding of the past support the causal
equation of the initial human occupation of the world’s lands with extinctions? Were prehistoric small-scale human
societies avatars of Siva, as some maintain? What is the evidence that supports a human role for the massive
extinctions that swept away so many remarkable North American beasts toward the end of the Pleistocene?
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The North American Extinctions
An astonishing array of mammals was lost in North America as the Pleistocene drew to a close (Table 1). In
what follows, I provide a brief discussion of those mammals. I emphasize the genera that became extinct because it
can be very difficult to securely define species of some of the animals involved, though I do discuss a few well-
defined species that became extinct even though the genus lives on in North America. The geographic distributions
that I provide depend heavily on FAUNMAP Working Group (1994).
The Xenarthrans (Pampatheres, Glyptodonts, and Sloths)
As the Pleistocene came to an end, North America lost two genera of armored xenarthrans and four of giant
ground sloths. All were northern representatives of groups that were far more diverse in Central and South America.
The pampatheres were armadillo-like in that they were enclosed in a flexible armor of bony scutes but they
differed sufficiently from armadillos to be placed in their own family. Of the two genera of later Pleistocene
pampatheres, only one is securely known from North America. The northern pamapathere Holmesina
septentrionalis was some 2 m long and 1 m high, and has been found in sites ranging from Kansas and North
Carolina in the north to Texas and Florida in the south, with the greatest number of sites known from Florida. The
southern pampathere Pampatherium has been reported from only a single site in Texas, but this material is now
argued by some to be from Holmesina, even though no detailed reanalysis of it has appeared (James 1957; Edmund
1985; Edmund 1996; De Iuliis et al 2000).
Simpson’s glyptodont, Glypotherium floridanum, is known primarily from near-coastal localities in Texas,
Florida, and South Carolina. This animal had a turtle-like carapace, an armored tail and skull, massive limbs, and a
pelvic girdle that was fused to its shell. At some 3 m long and 1.5 m tall, it was roughly the shape and height of a
Volkswagen Beetle automobile, though it was about 1 m shorter than its metallic counterpart. From the settings in
which its remains have been found, Simpson’s glyptodont appears to have lived along lakes, streams, and marshes;
it may have been semiaquatic (Gillette and Ray 1981; Gillette and Whisler 1986).
The four ground sloths were, as a group, found from Florida in the southeast to Alaska in the northwest, but not
all of them were that widely distributed. The largest, Eremotherium laurillardi, combined the height of a giraffe
with the bulk of an elephant (Figure 1), and is known from South Carolina (and perhaps New Jersey) south through
Florida and west into Texas (Cartelle and De Iuliis 1995). The Shasta ground sloth Nothrotheriops shastensis,
which weighed about 150 kg, was the smallest of the North American sloths and is known only from western North
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America. Aspects of the ecology of this sloth are well-understood since its baseball-sized dung balls have been
found in dry caves of the American Southwest. Analyses of that material have revealed the remains of parasites,
DNA from the plants the animals ate, and bits and pieces of the plants themselves. As a result, we know that while
the sloth has been gone for some 10,000 years, the plants that it consumed remain common in the general region
(Hansen 1978; Schmidt et al. 1992; McDonald 1996; Poinar et al. 1998). Jefferson’s ground sloth, Megalonyx
jeffersoni was the most widespread of the North American ground sloths, distributed from Florida to Alaska (the far
northwestern specimens appear to date to the last interglacial: McDonald et al 2000). This was the first of the North
American sloths to be described, by Thomas Jefferson, who concluded from its massive claws (“megalonyx”) that it
was a carnivore, a conclusion that was soon corrected (Grayson 1984b). Harlan’s ground sloth Paramylodon
harlani (placed by some in the genus Glossotherium) was also widespread in North America, found from coast to
coast (McDonald 1995, 1996, 2003). Distinguished in part by the fact that it had small bones embedded in its skin,
this was the most abundant sloth at Rancho La Brea (Marcus and Berger 1984).
The Carnivores
The dhole (Cuon alpinus) is a pack-hunting, highly carnivorous member of the dog family that is widespread
(but dwindling) in Asia. During the Pleistocene, it was found from the Iberian Peninsula to Alaskan and the Yukon,
with a single additional site known from northern Mexico. Dire wolves (Canis dirus), on the other hand, were
broadly distributed in North America. These large canids (they are estimated to have weighed 50 kg) seem to have
been pack hunters capable of taking prey that weighed well in excess of 300 kg (Van Valkenburgh and Hertel 1998;
Van Valkenburgh and Sacco 2002). Dire wolves do not appear on Table 1 because the genus to which they belong
includes the extant wolves and coyotes (and domestic dogs).
Since jaguars (Panthera onca) exist in North America as well, Table 1 also excludes the giant American lion,
Panthera leo (or Panthera atrox, depending on the author). Huge lions were not uncommon in North American
open environments during the Pleistocene. Weighing some 430 kg, they were the largest cat North America had to
offer (Anyonge 1993; Valkenburgh and Hertel 1998); their tracks, known from a Missouri cave, are significantly
broader than those of the African lion Panthera leo, which weighs about 180 kg (Graham et al. 1996). They were
not, however, the only huge member of the cat family to be found here. The famous saber-toothed cat Smilodon
fatalis weighed an estimated 390 kg and ranged from coast to coast (Figure 2). The less well-known scimitar cat
Homotherium serum (found from Alaska to Texas and Florida) weighed in at 190 kg, and the American “cheetah”
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Miracinonyx trumani, now known to be closely related to the cougar (Puma concolor), weighed an estimated 70 kg
(Van Valkenburgh, Grady, and Kurtén 1990; Rawn-Schatzinger 1992; Anyonge 1993; Turner 1996; Van
Valkenburgh and Hertel 1998; Barnett et al. 2005).
The largest late Pleistocene carnivore, however, was the giant short-faced bear Arctodus simus. Large males
averaged around 700 to 800 kg, and the biggest of them may have reached 1000 kg (Christiansen 1999). Found
from Pennsylvania to California, south into Mexico and north to Alaska and the Yukon (Richards et al. 1996), these
long-limbed bears were highly carnivorous, with the meat they consumed perhaps obtained largely by scavenging
(Bocherens et al. 1995; Matheus 1995, 2003). Some 40% of Arctodus simus sites in the United States are located in
caves and the individuals from those caves tend to be smaller than those found in open settings. This suggests that
the smaller female bears used caves as den sites, and that their remains represent animals that died while denning
(Schubert and Kaufmann 2003).
The second extinct genus of North American bear, Tremarctos, continues to exist today in the form of the
spectacled bear T. ornatus, which occupies the mountains of north-western South America. The North American
Pleistocene form, however, was more powerfully built and larger, with an estimated weight of 135 kg. Known from
Texas to South Carolina and Florida, it appears to have been omnivorous (Van Valkenburgh and Hertel 1998;
Sanders 2002).
Not all of the now-extinct North American Pleistocene carnivores were huge. The short-faced skunk, whose
later Pleistocene distribution is known to have included eastern North America, the Yukon and the Great Basin, was
roughly equivalent in size to a spotted skunk (Spilogale), the genus to which it appears to be most closely related
(Heaton 1985; Anderson 1996).
The Rodents and Lagomorphs
North America did not lose many rodents toward the end of the Pleistocene, but the ones that were lost are to be
mourned. The giant beaver Castoroides ohioensis ranged from New York to Florida in the east and to Alaska in the
far northwest but appears to have been most common in the Great Lakes region. They had incisors 2.5 cm across,
but these teeth had rounded and blunt tips, and this and other aspects of their morphology shows that they were not
dam builders, even though their preferred habitat appears to have been lakes, ponds, marshes and swamps
(Swinehart and Richards 2001; Parmalee and Graham 2002). Recent analyses suggest that they weighed some 75 kg
(Reynolds 2002), less giant than was once thought. The two North American capybaras—southeastern in
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distribution—are both related to the world’s largest living rodent, Hydrochaeris hydrochaeris of central and
northern South America, which weigh about 60 kg (Nowak 1999). The extinct Hydrochaeris holmesi was slightly
larger than this, but Neochoerus pinckneyi was perhaps half again larger.
Only one genus of North American lagomorph (the order that includes the rabbits, hares, and pikas) was lost
towards the end of the Pleistocene. This was Aztlanologus agilis, a small rabbit known from New Mexico and
Texas south into central Mexico; the few reasonably secure radiocarbon dates that are available for it suggests that it
may not have survived the last glacial maximum, some 18,000 years ago (Russell and Harris 1986; Jau-Mexia 2000;
Grayson 2001).
The Perissodactyls (odd-toed ungulates)
After evolving in the New World and crossing into the Old via the Bering Land Bridge, horses became extinct
in North America at the end of the Pleistocene; the wild horses of the west are recent introductions. To judge from
the number of sites in which they have been found, horses were especially common in western North America,
including the far north. The number of species represented by these horses is unknown and even experts routinely
identify them only to the genus level (Winans 1989; Morgan 2002). Tapirs, of at least two species, were also
reasonably common, known from Pennsylvania to California, with the largest number of sites found in eastern North
America (Graham 2003).
The Artiodactyls (even-toed ungulates)
North America lost thirteen genera of artiodactyls, from peccaries and camels to muskoxen and pronghorn, a
variety that is almost bewildering in scope. The long-nosed peccary Mylohyus nasutus apparently flourished in
wooded environments and was primarily eastern in distribution, though specimens are known from as far west as
western Texas. The flat-headed peccary Platygonus compressus, on the other hand, was distributed from coast-to-
coast and seemed to prefer more open settings; unlike Mylohyus, it also seems to have been gregarious, to judge
from the discoveries of “fossil herds” (Finch et al. 1972; Munson 1991).
Like horses, camels are New World natives but, unlike horses, some—the llamas (Lama and Vicugna)—survived
here. Toward the end of the Pleistocene, North America supported three members of the camel family. Camelops
hesternus looked something like a longer-legged, narrower-headed version of the dromedary (Camelus
dromedarius) and was very common in the western half of North America (Webb 1965). To judge from aspects of
the skull, this camel fed on a diet derived from both grazing and browsing (Dompierre and Churcher 1996), a view
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that matches food remains plucked from the teeth of individuals at Rancho La Brea (Akersten et al. 1988). The
large-headed llama Hemiauchenia macrocephala was widespread in the southern half of North America, with sites
known as far north as Iowa and Idaho. Also long-limbed, this camelid seems to have been a mixed feeder with a
preference for browse (Feranec 2003). The stout-legged llama Palaeolama mirifica has been reported from South
Carolina and Florida west to southern California (Webb and Stehli 1995), but most records are from the southeast.
As the common name suggests, it had relatively short and robust limbs, perhaps an evolutionary response to the
need to escape predators in closed to semiclosed habitats (Graham 1992). Analyses of Palaeolama mirifica
dentition, and of stable isotopes from that dentition, suggest that it was a browser (Webb and Stehli 1995; Meachen
and Hallmann 2002).
The large deer Navahoceros fricki had short, robust limbs and fairly simple antlers and was found from
southeastern California to the Plains. Detailed analyses of its skull suggest that its closest living relatives are
reindeer and caribou, which belong to the genus Rangifer (Kurtén 1975; Kurtén and Anderson 1980; Webb 1991,
2000). The “elk-moose” or “stag-moose” Cervalces scotti is far better known, with sites known from southern
Canada and the eastern and central United States; specimens that may or may not belong to the same species are
known from the Yukon and Alaska (Churcher and Pinson 1987; Churcher 1991). The contexts in which it has been
found suggest that this animal may have been similar to the moose (Alces alces) in habitat preference, to which it
was also similar in size.
There is only a single species of Antilocapridae in North America today, the pronghorn Antilocapra americana.
These speedy animals have two laterally compressed horn cores, each covered by a sheath with a forward-projecting
prong. During the late Pleistocene, this animal existed alongside three other genera of the same family, all of which
were characterized by the fact that they had four, rather than two, horns. The smallest of these was the aptly-named
diminutive pronghorn, Capromeryx minor, found from California to Texas. Some 50 cm tall at the shoulder,
estimates for the weight of this long-limbed antilocaprid range from about 10 to 15 kg (Scott 1983; Kurtén and
Anderson 1980). The other two extinct antilocaprids were closer to the pronghorn in size; Stockoceros is known
from late Pleistocene contexts in the Southwest and Texas, while Tetrameryx has been reported from Texas, New
Mexico, and southern Nevada.
Of the three genera of bovids that became extinct in North America towards the end of the Pleistocene, one, the
saiga Saiga tatarica, lives on in the arid steppes of Eurasia (Nowak 1999). During the later Pleistocene, however, it
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was to be found as far southwest as northern Spain (Agustí and Moyà-Solà 1992; Altuna 1996; Delpech 1999;
García and Arsuaga 2003) and as far northeast as Alaska, the Yukon, and the Northwest Territories (Harington and
Cinq-Mars 1995). The helmeted muskox Bootherium bombifroms is known from much of unglaciated North
America; only the far southeast and far southwest appear to have been without it. Compared to the extant muskox
Ovibos moschatus, the helmeted muskox had longer legs and stood taller, but was shorter from head to tail
(McDonald 1984; McDonald and Ray 1989) and had shorter pelage (known from preserved material from the far
north). Guthrie (1991) observes that the longer legs of the extinct form imply that they were likely far more mobile
than the species familiar to us today. The horn cores of male helmeted muskoxen fused in the midline, unlike those
of Ovibos, and it is this feature that provides the common name (Figures 3 and 4); those of the female differed so
considerably that the distinctions at one time helped to cause males and females being assigned to separate genera.
The shrub-ox Euceratherium collinum is characterized by horn cores that arise from near the rear edge of the frontal
bones, sweep up and back then out and forward, curling upwards near their tips. Their skulls and neck vertebrae
imply that, like mountain sheep (Ovis canadensis), they were head-butters. Kurtén and Anderson (1980) suggest
that the shrub-ox was a grazer that occupied lower hills, which coincides with Harris’ (1985) observation that sites
tend to be found in foothills or low mountainous terrain. The remains of this animal have been found from
California to Iowa and south into Mexico.
The mountain goat Oreamnos americanus is doing well in northwestern North America but Harrington’s
mountain goat Oreamnos harringtoni failed to survive the end of the Pleistocene. Sites that have provided the
remains of this animal are known from the central Great Basin south through the Colorado Plateau into northern
Mexico and as far east as southeastern New Mexico. The greatest amount of material, however, has come from sites
in the Grand Canyon region. Harrington’s mountain goat was about 30% smaller than its modern counterpart, had a
narrower face with thinner and smaller horns, had robust feet suitable for negotiating rough terrain, and had a mixed
diet, incorporating plants ranging from grasses to spruce (Picea sp.). It is even known that while the animal was
smaller than its modern counterpart, its dung was larger (perhaps due to the nature of its diet), and that it had white
hair (Mead 1983; Mead et al. 1986a, 1987; Mead and Lawler 1994; Jass et al. 2000).
The Proboscideans
Along with the saber-toothed cat, mammoths and the American mastodon are the iconic large mammals of the
North American Ice Age. The mastodon (Mammut americanum) was widespread in unglaciated North America,
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found from coast to coast and from Alaska into Mexico, but was particularly abundant in the woodlands and forests
of eastern North America (Graham 2001); in the Rocky Mountains, they have been found at elevations as high as
2981 m (Miller 1987). These “low, long, and stocky” (Saunders 1996:274) animals had shoulder heights of 2 m to
3 m and weighed an estimated 3000 kg. They also had distinctive cheek teeth with large cusps arranged in pairs to
form ridges that ran at right angles to the main axis of the tooth, appropriate to a browsing diet (Haynes 1991;
Saunders 1996; Hubbard et al. 2000), although at least some individuals incorporated significant amounts of grasses
into their meals (Gobetz and Bozarth 2001). The fact that their remains are frequently found as single individuals
has suggested to some that they were fairly solitary but others argue that they did form social groups (Haynes 1991;
Shoshani and Marchant 2001; Haynes and Klimowicz 2003). “Mastodont” is the more proper spelling of the
common name (Haynes 1991) but “mastodon” is in frequent use as well.
Disagreement continues over the number of species of full-sized mammoth that occupied North America during
the late Pleistocene, some experts recognizing three (Columbian mammoth Mammuthus columbi, Jefferson’s
mammoth Mammuthus jeffersoni, and woolly mammoth Mammuthus primigenius; see Graham 2001), while others
treat Columbian and Jefferson’s mammoth as belonging to M. columbi (Agenbroad 1984; Shoshoni and Tassy
1996), which is the approach I follow here. There is also a diminutive form, the pygmy mammoth M. exilis, known
from the Channel Islands off the coast of California. Derived from M. columbi, these animals had shoulder heights
that varied from about 1.2 m to 2.5 m (Roth 1990, 1996; Dudley 1999; Agenbroad 2001), compared to shoulder
heights of 3 m to 4 m for Mammuthus columbi males and 2.3 m – 2.5 m for M. primigenius females (Haynes 1991).
The famed woolly mammoth was the northern form, common in eastern Beringia and the upper midwest and
adjacent Canada and, to a lesser extent, the northeastern United States. The Columbian mammoth (Figure 5) was
found coast to coast, but was the common form west of the Mississippi River (Agenbroad 1984; Graham 2001),
known from elevations as high as 2940 m (Gillette and Madsen 1992). Mammoth cheek teeth were flat and high-
crowned, each composed of a series of enamel-bordered plates running at right angles to the main axis of the tooth,
forming an efficient grinding device. The analysis of dried Columbian mammoth dung in Southwestern caves
shows that their diet in this area was dominated by grasses, sedges, and reeds, although they ate woody plants as
well. A similar combination of browse and graze is represented in the remains of well-preserved Mammuthus
primigenius innards from Siberia (Mead et al. 1986b; Haynes 1991; Agenbroad and Mead 1996).
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A third genus of proboscidean, Cuvieronius, is at times listed in bestiaries compiled for latest Pleistocene North
America. However, although these animals, known as gomphotheres, survived into the latest Pleistocene in South
America (found, for instance, at the Monte Verde archaeological site: see Dillehay 1997), there is no evidence for
their survival this late in North America (Morgan and Seymour 1997; E. L. Lundelius, Jr., personal communication
2003).
The Overkill Argument
The most visible explanation of the North American extinctions maintains that all were due to human hunting,
either directly (the herbivores) or indirectly (the carnivores). The current version of this hypothesis began to be
developed by Paul Martin more than 40 years ago (Martin 1958). It was in 1967, however, that Martin put this
argument on the intellectual map in a forceful way (Martin 1967), and he has continued to develop it ever since
(e.g., Martin 1973, 1984; Martin and Steadman 1999). Even though the presentation of the hypothesis has become
increasingly detailed as empirical data have accumulated, the core assertions have remained much the same
(Grayson 2001; Grayson and Meltzer 2002, 2003):
1) Archaeological and paleontological research has demonstrated that prehistoric human colonization of
islands was followed by often-massive vertebrate extinctions;
2) The archaeological phenomenon known as Clovis, marked by distinctive projectile points and well-dated
to about 11,000 radiocarbon years ago, is extremely likely to have been created by the first people to have
entered North America south of glacial ice, and certainly represents the first people known to have hunted
large mammals in this area;
3) Clovis people preyed on a diverse variety of now-extinct mammals; and,
4) The late Pleistocene North American mammal extinctions occurred at or near 11,000 years ago.
Therefore, Martin concludes,
5) Clovis peoples caused the extinction of North America’s Pleistocene mammalian fauna, that “large
mammals disappeared not because they lost their food supply but because they became one” (1963:70).
In the remainder of this paper, I review these core assertions.
Island Colonization is Followed by Extinction
From the very beginning, Martin has relied heavily on comparing extinctions on islands to those on continents.
It is this comparison that has allowed him to base a significant part of his argument on the chronological correlation
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between human colonization and extinction. Were it not for islands, that correlation would have to stand largely on
the New World case alone and the argument would be far less compelling. Because island extinctions are at the
heart of his arguments about North America, they must be touched on here.
The evidence that extinction inexorably followed the prehistoric human colonization of the world’s islands is
incontrovertible (see the review in Grayson 2001). The first well-documented case was provided by New Zealand.
Just prior to permanent human settlement around 900 14C years ago, New Zealand supported 10 species of moas,
large, flightless birds that ranged in weight from about 20 kg to over 200 kg. A few hundred years after human
colonization, all were extinct (Anderson 1989; Worthy 1999; Worthy and Holdaway 2002; on the number of
species of moas, see also Bunce et al. 2003 and Huynen et al. 2003). A role for human predation in moa extinction is
clearly indicated by the fact that there are some 300 archaeological sites in New Zealand that have been interpreted
as related to moa hunting (Anderson 1989), and over 100 sites documented to contain moa remains (Worthy 1999).
When he first incorporated New Zealand into his argument, Martin (1967) mentioned in passing that moas were
not the only birds to have become extinct after people colonized these islands, but did not note what those other
species were. Today, we know that several dozen species of much smaller vertebrates became extinct on the main
New Zealand islands after the establishment of a permanent human presence (Holdaway 1989, 1999; Towns and
Daugherty 1994; Worthy and Holdaway 2002). Many years ago, Fleming (1962:117; see also Fleming 1953)
suggested that this array of losses was best explained by “the profound ecological changes brought about by the
arrival of man with fire, rats, and dogs.” This argument has now carried the day. The extinctions here seem to have
had multiple causes, all related to human activities.
The human colonizers of these islands brought with them Pacific rats (Rattus exulans) and domestic dogs.
Indeed, Pacific rats may have been present in New Zealand some 2000 14C years ago, suggesting ephemeral human
visits long before the onset of permanent human settlement (Holdaway 1996; Worthy and Holdaway 2002, but see
also Anderson 2000 and Wilmshurst and Higham 2004). While Holdaway (1999) thinks it unlikely that either rats
or dogs played an important role in reducing moa numbers, others have disagreed (Fleming 1969; Anderson and
McGlone 1992; Towns and Daugherty 1994; Worthy 1999). No matter what their impact on moas, however, it is
widely agreed that Pacific rats played a significant role in driving the loss of lizards, frogs, wrens, and other small
animals on New Zealand (Holdaway and Worthy 1994; Towns and Daugherty 1994; Holdaway 1999). Even though
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it is not possible to document that the introduction of these rodents led to the extinction of any particular organism,
their impact on the New Zealand biota was certainly substantial.
The permanent human colonization of New Zealand was also quickly followed by massive, fire-caused
deforestation. Within a few hundred years of human settlement, almost all of the lowland forest of both North and
South Islands had been destroyed by fire, with higher, wetter sites affected as well (McGlone 1983, 1989; Horrocks
and Ogden 1998, 2000; McGlone and Wilmshurst 1999; Wilmshurst, Eden, and Froggatt 1999). No one doubts that
this habitat disruption also played a role in driving prehistoric extinctions in New Zealand.
In short, the wave of extinctions that followed the human colonization of New Zealand was a result of the
multiple impacts associated with this event: Massive burning, the introduction of non-human competitors and
predators, and direct predation by people themselves (Duncan et al. 2002). It was the overwhelming sum of these
processes that led to the losses. In no case can we decipher which cause or causes led to the extinction of any given
animal.
In fact, prehistoric human colonization of all of Oceania’s islands appears to have been followed by vertebrate
extinction (see the review in Grayson 2001). Just as in New Zealand, there is widespread agreement that prehistoric
extinctions on these other islands were due to “prehistoric human activities, including habitat alteration, direct
predation, and predation from introduced vertebrates” (Steadman 1999:319). Martin himself explicitly recognized
the multiple causes of island extinctions in 1984. “While they did not necessarily hunt all the small mammals or
island birds and other animals to extinction,” he noted, “the side effects of the arrival of prehistoric colonizers were
severe and involved fire, habitat destruction, and the introduction of an alien fauna” (Martin 1984:396).
Martin excluded moas from the set of island-dwelling animals that he considered prone to extinction from causes
unrelated to direct human predation. Indeed, the presumption that the cause or causes of extinction must scale to the
size of the organism is common among those who work with prehistoric island faunas. Steadman, Pregill, and
Olson (1984:4450), for instance, concluded that “small species of lizards, snakes, birds, and bats” must have been
lost on Antigua for reasons other than human predation because they are not “large edible vertebrates.” However,
while small size can offer some degree of protection against human predation (Madsen and Schmitt 1998; Grayson
and Cannon 1999), it does not follow that the extinction of larger forms must perforce be due to human predation
alone, as Martin assumes (Martin 1984; Martin and Steadman 1999:32).
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While I have emphasized Oceania here, other islands show the same pattern, including the West Indies,
Madagscar, and those in the Mediterranean (see the review in Grayson 2001). The vulnerability of such faunas to
the human arrival is no surprise. As Steadman (1989a:178) has observed, island birds are vulnerable because they
have relatively small population sizes, are confined to well-delineated areas of land that may undergo rapid
environmental change, and have likely lost, and in some cases have clearly lost, the mechanisms needed to cope
successfully with introduced predators, pathogens, and competitors. To these factors, we can add that the isolated
nature of oceanic islands means that there is no ready source of conspecific individuals to replenish dwindling
populations, one of the fundamental precepts of island biogeographic theory (MacArthur and Wilson 1967;
MacArthur 1972; Rosenzweig, 1995). While we may not be able to pinpoint the exact cause of extinction of any
animal on any island, we can conclude with Steadman et al. (1991:126) that “animals on oceanic islands tend to be
more vulnerable to extinction or extirpation than their continental counterparts” and with Paulay (1994:134) that
island faunas are “among the most vulnerable in the world.” It is this fact that makes their post-colonization
extinction records so pronounced.
If it is agreed that island biotas are far more vulnerable to extinction than are continental ones and that
prehistoric post-colonization extinctions were due to a complex set of interacting causes—as even the most ardent
overkill theorists agree (e.g., Steadman 1989a, 1997a; Martin and Steadman 1999)—then the relevance of island
extinctions to continental ones is unclear since the overkill hypothesis stipulates that continental extinctions were
driven by human predation alone. As a result, the fact that island extinctions came later than the continental ones—
perhaps the key argument in Martin’s position—becomes irrelevant. No one denies that the island extinctions were
anthropogenic and no one denies that islands were generally colonized later than mainlands. Given the known
differences in the vulnerability of island and continental faunas and given the stark contrast in causes that have been
invoked to explain the extinctions in these two very different contexts, there is no reason to assume that they are
meaningfully comparable.
North America was Colonized by Clovis Peoples
No one involved in the search for an explanation of the North American extinctions questions that they were
over, or nearly so, by about 10,500 14C years ago and that a number of them occurred after 12,000 14C years ago.
Thus, it is intriguing that the earliest well-dated North American cultural complex, Clovis, dates to between about
11,500 and 10,800 14C years ago (see the review in Meltzer 2004). Clovis is marked not just by a distinctive fluted
13
projectile point, but also by the fact that Clovis artifacts have been found tightly associated with mammoth in a
dozen North American sites and with mastodon in two (Grayson and Meltzer 2002).
Martin (1967, 1984; Martin and Steadman 1999) combines Clovis with the apparent magnitude and chronology
of the North American extinctions, observes that the prehistoric human colonization of islands was routinely
followed by extinction, and concludes that the Clovis colonization of North America caused extinction here as well.
This argument has intuitive appeal. We know that predation by human hunters can cause local extinction in
continental settings (e.g., Bodmer, Eisenberg, and Redford 1997), and Winterhalder and Lu (1997) have shown that
some versions of the overkill hypothesis are conceptually plausible. However, most scholars deeply versed in the
late Pleistocene archaeology and paleontology of North America reject Martin’s argument. There are many reasons
for this but two loom largest, and have done so for some time.
These reasons do not include apparent conflicts between Martin’s firm acceptance of Clovis as representing the
earliest human colonization of the New World and developing evidence that it does not. Referring to the Monte
Verde site (Dillehay 1997), located some 16,000 km south of the Bering Land Bridge, Martin and Steadman
(1999:34) observed that the overkill model “loses credibility . . . if humans in Chile lived in houses covered with
skins of gomphotheres over 1000 years before proboscideans became extinct in North America.” They called for
independent verification of the claim that Monte Verde is 12,500 14C years old, but this verification had already
been provided by an international team of scholars (Meltzer et al. 1997; Meltzer 1997). The reason for this apparent
lapse may relate to the fact that Martin had rejected Monte Verde out-of-hand, since he found the search for pre-
Clovis sites in the New World as “something less than serious science, akin to the ever popular search for ‘Big Foot’
or the ‘Loch Ness Monster’ ” (Martin 1999:278).
Nonetheless, overkill adherents have been carefully dealing with the possibility of pre-Clovis Americans for
decades. In 1967, Martin noted that “the possibility that Homo sapiens spread into the Americas long before the
late-glacial by no means eliminates the hypothesis of overkill” (Martin 1967:101), and, in 1984, that “whether or not
prehistoric people were in the Americas earlier, 11,000 B.P. is the time of unmistakable appearance of Paleo-Indian
hunters using distinctive projectile points” (Martin 1984:363). As a result, it is hard to see that a dozen Monte
Verdes, with a tool kit poor in projectile points and a diet apparently rich in plant foods (Dillehay 1997), could
possibly make a difference (see the discussion in Grayson 1984a). Only the discovery of Clovis-like hunting
implements thousands of years before the extinctions had ended has not been covered, and this is a discovery no one
14
expects—and even this would allow slow overkill. As a result, the chronology of the human colonization of the
Americas has become largely irrelevant to the overkill position.
Clovis Peoples Hunted the Now-Extinct Herbivores
The overkill hypothesis requires that early Americans hunted a diverse variety of now-extinct mammals in
substantial numbers. As archaeologists have long observed (e.g., Hester 1967), this in turn would seem to require
that we have some evidence of that hunting, just as we do in New Zealand for moas. However, such evidence exists
only for mammoth (12 sites) and, to a much smaller extent, for mastodon (2 sites: see Grayson and Meltzer 2002).
Other assessments do not come up with many more such sites (Haynes 2002). There is no secure evidence that
people hunted, or even scavenged, any of the other mammals involved.
This lack of evidence does not seem to result from some sampling fluke (Grayson 1991). Figure 6 shows the
number of stratigraphically distinct occurrences of extinct late Pleistocene mammals in a recent compilation of the
late Quaternary vertebrate faunas of the United States (FAUNMAP Working Group 1994). It does not take deep
insight to observe that while horses (Equus), camels (Camelops), and Harlan’s muskox (Bootherium) are extremely
well-represented in the late Pleistocene paleontological record of North America, there are no sites suggesting that
they were hunted (see the detailed discussion in Grayson and Meltzer 2002). Nor, as I mentioned, are such sites
known for any of the other animals on the list except for mammoth and mastodon. Mammoth account for 18.8% of
the late Pleistocene occurrences of extinct non-carnivores on the FAUNMAP database, but for 85.7% of the sites
showing that these animals were targeted by human hunters. Mammoth and mastodon together account for 30.3%
of the extinct non-carnivore occurrences, but for all of the sites related to the hunting of now-extinct late Pleistocene
mammals.
Martin is well aware of this, and has long argued that the extinctions occurred so quickly that associations with
people are not to be expected (Mosimann and Martin 1975; Martin 1984; Martin and Steadman 1999). This,
however, could not account for the fact that convincing associations are not simply rare, but are available only for
mammoth and mastodon (Grayson 1984c). Indeed, to support the claim that archaeological associations with extinct
mammals are expected to be rare, Martin and Steadman (1999:26) note that it is only on New Zealand and “some
islands in the Polynesian heartland” that such associations have been documented.
However, the remains of now-extinct birds and other organisms have been found in archaeological contexts on
island after island in Oceania. The anthropogenic nature of these extinctions, and the possible role that direct human
15
predation may have played in causing them, has been accepted because of this evidence, not in spite of its absence.
It is for this reason that Steadman (1989b:539) gave the name Megapodius alimentum to a new species of large,
extinct megapode from archaeological deposits in Lifuka, “the name alimentum [referring] to the presumed eating of
this species by the early Tongans who deposited their bones at Tongoleleka.” On the other side of the world,
Steadman, Pregill, and Olson (1984:4450) strengthened their argument for human-driven extinctions in Antigua by
observing “bones that are charred or broken in a manner indicative of human consumption.” Statements of this sort
are common in the island literature (e.g., Olson and James 1982:634; Kirch et al. 1995:56; Rolett 1998:104). Any
appeal to the better preservation afforded by the relative recency of these sites fails in the face of the rich faunas
found in Pleistocene archaeological sites ranging from France (e.g., Audouze and Enloe 1997; Costamagno 1999,
Grayson and Delpech 2003) to Tasmania (e.g., Cosgrove 1999).
Perhaps, as Owen-Smith (1987; see also Owen-Smith 1999) has argued, it was the cascading ecological impact
of the human-caused demise of the largest of the herbivores that led to the extinction of all the others. If this were
the case, mammoth and mastodon must have become extinct, or at least significantly diminished in number, before
the others. However, the available radiocarbon chronology suggests that the proboscideans were among the last to
go (Grayson 1991; R. W. Graham, personal communication 2003; the third megaherbivore noted by Owen-Smith,
the ground sloth Eremotherium, was confined to the southeast and cannot as yet be shown to have survived beyond
about 38,000 14C years ago in North America [Thulman and Webb 2001]).
The archaeology of the Americas thus seems oddly irrelevant to argument about overkill. It does not matter that
there is strong evidence that Clovis peoples were not the first to set foot south of glacial ice. It does not matter that
there is no evidence that of the full suite of extinct mammals at issue, only mammoth and mastodon can be shown to
have been hunted. This is the case even though, in other parts of the world, those who argue for overkill take the
archaeology as critical to their arguments.
The Extinctions Occurred During Clovis Times
No matter what their position on overkill, nearly all scientists accept that the North American late Pleistocene
extinctions occurred around 11,000 radiocarbon years ago. Nonetheless, we are far from demonstrating that this was
the case (Grayson 1987, 1989, 1991). Of the 35 or so genera involved, only 16 can now be shown to have survived
beyond 12,000 14C years ago and thus into Clovis times (Table 2). That is, we cannot yet demonstrate that some 19
genera survived long enough to have been hunted by Clovis people.
16
In western Europe, blessed with numerous well-stratified and carefully excavated paleontological and
archaeological sites, late Ice Age extinctions were staggered in time and space, closely reflecting the climate and
glacial history of this region (Delpech 1999; Stuart 1999). There is no comparable chronology for North America.
For 40 years, it has been assumed that since some genera can be shown to have become extinct here around 11,000
14C years ago, all genera became extinct at that time. This enormous assumption has framed the nature of the debate
since 1967 (Grayson 1991), even though there is no compelling evidence that North American late Pleistocene
extinctions were not just as—or more—staggered in time and space as were the European ones.
Of course, the more abundant a genus was on the latest Pleistocene landscape, the greater the chance that we
would have obtained a terminal Pleistocene date for it. In fact, of the 16 most common extinct genera in the
FAUNMAP data base, 14 have been dated to between 12,000 and 10,000 14C years ago (compare Figure 6 and
Table 2); of the remaining 19 genera, only two have been so dated. This is distinctly different from the situation for
kill sites, which do not scale to the quantitative structure of the late Pleistocene record. The radiocarbon evidence
for terminal Pleistocene extinctions does scale to this record, providing significant presumptive support for those
who argue that all extinctions occurred at this time. It does not, however, demonstrate that the extinctions were
essentially synchronous. This poses a problem not just for the overkill hypothesis, but for climate-based alternatives
as well. Before we can adequately explain the extinctions, we need to know when they occurred.
But even if it turns out that the great majority of these extinctions did occur at the very end of the Pleistocene,
this would not make North America unique. In Ireland, the latest radiocarbon date for the giant deer (or “Irish elk”)
Megaloceros giganteus falls at 10,610 years ago (Stuart et al. 2004; the animal appears to have survived into the
early Holocene on the Isle of Man: Gonzalez et al. 2000); the latest date for reindeer (Rangifer tarandus) here falls
at 10,250 years ago (Woodman, McCarthy, and Monaghan 1997). The same slice of time that saw the arrival of
Clovis in North America saw the disappearance of reindeer, mammoth, saiga, and giant deer from southwestern
France (Delpech 1999). In the southern Jura and northern French Alps, reindeer disappeared shortly after 12,000
years ago (Bridault et al. 2000). In the Taimyr Peninsula of northern Siberia, mammoth disappeared from the
mainland shortly after 10,000 years ago (they persisted well into the Holocene on Wrangel island: see MacPhee et
al. 2002). Human hunters cannot account for these Eurasian extinctions. People had hunted reindeer for tens of
thousands of years in France (Grayson and Delpech 2002, 2003), yet the animals persevered until the end of the
Pleistocene. There were no Clovis hunters in northern Siberia and no one can blame hunting for the Irish extinctions
17
because there were no people here at that time. While all of this was happening, Harrington’s mountain goat and the
Shasta ground sloth disappeared from the American Southwest (Martin et al. 1985; Mead et al. 1986a; Mead et al.
2003), caribou (North American reindeer) retreated from their late Pleistocene ranges in the American midwest and
southeast (McDonald et al. 1996), and mammoth and mastodon (among others) were lost from the American
landscape. Genetic data suggest that even cheetahs in Africa and cougars in North America may have undergone
severe population declines as the Pleistocene ended (Menotti-Raymond & O’Brien 1993; Culver et al. 2000).
Some other Matters
These are not the only problems with the North American version of the overkill hypothesis. The focus on large
mammals that marks the debate over the North American extinctions may lead an unwary reader into thinking that
other organisms were unaffected by whatever it was that caused those extinctions. This was far from the case. The
North American—and Eurasian—latest Pleistocene was also a time of dramatic alterations in the ranges of many
small mammals (FAUNMAP Working Group 1996; Stafford et al. 1999). At least one species of plant became
extinct at this time, the spruce Picea critchfeldii (Jackson and Weng 1999; Jackson and Overpeck 2000). These
changes are attributed without controversy to terminal Pleistocene climate fluctuations and attendant massive
reorganization of biotic communities. They were also at least broadly coeval with the vertebrate extinctions at issue
here.
Some 19 genera of birds also appear to have become extinct during the later Pleistocene, and while nine of these
were either predators or scavengers whose extinction might well have been driven by the loss of large mammals, the
others ranged from storks and flamingoes to shelducks and jays (Emslie 1998; Van Valkenburgh and Hertel 1998).
While Martin and Steaman (1984, contra Grayson 1977) suggested that many of these remaining genera might
actually pertain to forms that still exist in North America, and that many of the others were dependent on the large
mammals that became extinct, the analyses needed to support these assertions have not been provided.
What Exactly is the Overkill Argument?
This suite of problems leads overkill adherents into odd logical crannies. Archaeological evidence for human
predation on moas becomes support for overkill in New Zealand at the same time as the lack of comparable
evidence becomes support for overkill in North America. The small rabbit Aztlanolagus was “large enough” to have
been terminated by human predation during Clovis times (Martin and Steadman 1999:34), even though there is no
evidence that this animal survived the last glacial maximum some 20,000 years ago (Russell and Harris 1986;
18
Grayson 2001). Overkill might be falsified by evidence for pre-Clovis peoples in North America, but the search for
such evidence is “less than serious science” (Martin 1999:278) and it would not matter if earlier peoples were here.
Islands provide an integral part of the argument for North American overkill, yet the processes operative on islands
are recognized as distinctly different from those that pertain to continents. It is no surprise that the overkill
hypothesis has won support from so few specialists in the North American empirical records involved.
Over the years, the North American version of the overkill argument has led to the generation of significant new
information on a wide variety of important topics. We have, for instance, learned a tremendous amount about
ground sloths—from the chronology of their extinction to their diets and parasites (Martin, Thompson, and Long
1985; Schmidt, Duszynski, and Martin 1992; Long, Martin, and Lagiglia 1998; Poinar et al. 1998)—as a direct
result of Martin’s efforts. Steadman’s research on islands has stemmed directly from Martin’s concerns and his
research has been important in multiple realms (e.g., Steadman 1995, 1997b). Martin’s work has helped to alter in
fundamental ways our understanding of the relationships of small-scale societies to their biotic contexts. It even led
to the rebirth of the hypothesis that massive extinctions may be disease-driven (MacPhee and Marx 1997; Ferigolo
1999).
However, while the initial presentation of the overkill hypothesis was good and productive science, the current
logical status of the argument is a matter for concern. The argument has been so severely patched over the years
that there would seem to be no way that it can actually be tested. It is also an argument with striking chronological
connections to the environmental movement, having been introduced in its current form in 1967, a time when human
impacts on the biosphere were coming to be of heightened concern. Temporally wedged between Silent Spring
(Carson 1962) and The Population Bomb (Ehrlich 1968), Martin’s “Prehistoric Overkill” (Martin 1967) caught the
environmental wave that helped launch the Environmental Defense Fund in 1967 (Taylor 1989), the U.S. National
Environmental Policy Act in 1969, and Earth Day in 1970 (Baden 1980). Indeed, favorable discussions of overkill
by ecologists are routinely presented as parts of arguments meant to foster general concern for the future of our
planet (e.g., Diamond 1992; Western 2001; Murray 2003). One may applaud the intent, but it is hard to avoid the
fact that that overkill’s continued popularity in this context appears more closely related to the environmental
movement than to any supporting evidence (Grayson and Meltzer 2003).
The overkill position would thus seem to survive in spite of the lack of support it receives from the
archaeological and paleontological records of North America. The support it does receive stems primarily from
19
comparing islands to continents and from the environmental concerns of the descendants of the Europeans who
colonized this region long after the arrival of the people implicated by the argument. While the overkill notion as
applied to North America may be intuitively pleasing and conceptually convenient, there is no reason to believe that
the early peoples of North America did what the argument says they did.
What Did Cause the Extinctions?
We do not know what caused the extinction of some 35 genera of North American mammals toward the end of
the Pleistocene, although all indications are that these losses were driven by massive climate change. A number of
climate-based hypotheses have been forwarded to account for these losses (e.g., Graham and Lundelius 1984;
Guthrie, 1984), but none have gained widespread acceptance, since none connect particular climate variables with
particular organisms in powerful ways. The most popular explanation is perhaps that posited by Graham and
Lundelius (1984), which attributes the extinctions to changes in seasonal swings of temperature as the Pleistocene
came to an end. However, this explanation does not seem to account for the fact that extinctions also occurred in
areas where this mechanism was not likely in play (Grayson 1991), and also appears to be at odds with vegetational
histories available for eastern North America (Williams et al. 2001).
This does not mean that there has been no progress made in recent years. For instance, Johnson (2002) has
shown that it is more likely to have been reproductive rate, rather than body size per se (e.g., Lessa et al. 1997), that
mediated mammal extinctions in North America and elsewhere toward the end of the Pleistocene, and similar
observations have been made for birds (Duncan et al. 2002). Guthrie (2003) has shown that Alaskan horses declined
significantly in size prior to their extinction in that region, now dated to ca. 12,500 14C yr BP, and finds this
morphological change consistent with climatic causation. Further afield, Sánchez et al. (2004) have shown that
gomphothere diets became more specialized prior to their extinction in South America, and suggest that this dietary
specialization led to their climatically-induced extinction at the end of the Pleistocene. Fisher (1996) has laid out an
ambitious research design meant to unravel the causes of extinction of North American mammoths and mastodons.
Detailed biogeographic histories of mammals have established that particular mammal species respond to
climate change in their own ways (Graham 1985, 1992; FAUNMAP Working Group, 1996; see also Lyons 2003).
This in turn suggests that an adequate explanation of the extinctions will have to be built one species at a time, just
as the studies cited above have begun to do and has already been done in other parts of the world (e.g., Delpech,
1999). It is clearly time to shed both community-based and overkill approaches to these extinctions and begin what
20
is likely to be the arduous task of finally solving one of the longest-standing questions in American vertebrate
paleontology and archaeology.
Acknowledgments
I have learned much from Paul Martin and much because of him, and have never failed to enjoy his company and
conversation. Sincere thanks to Angela E. Close, Michael D. Cannon, Barbara E. Grayson, Ernie L. Lundelius,
James F. O’Connell, and Eric A. Smith for insightful comments on versions of this manuscript, and to Bax R.
Barton, Russell W. Graham, Ernie L. Lundelius, Jr., and Blaine W. Schubert for significant help along the way.
This paper is dedicated to the memory of Elaine Anderson, who excelled both as a paleontologist and as a human
being.
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Table I. The extinct late Pleistocene mammals of North America.
Order Family Genus Common Name
Xenarthra Pampatheriidae Pampatherium1 Southern Pampathere
Holmesina Northern Pampathere
Glyptodontidae Glyptotherium Simpson's Glyptodont
Megalonychidae Megalonyx Jefferson's Ground Sloth
Megatheriidae Eremotherium Laurillard’s Ground Sloth
Nothrotheriops Shasta Ground Sloth
Mylodontidae Paramylodon2 Harlan's Ground Sloth
Carnivora Mustelidae Brachyprotoma Short-faced Skunk
Canidae Cuon3 Dhole
Ursidae Tremarctos3 Florida Cave Bear
Arctodus Giant Short-faced Bear
Felidae Smilodon Sabertooth Cat
Homotherium Scimitar Cat
Miracinonyx American Cheetah
Rodentia Castoridae Castoroides Giant Beaver
Hydrochaeridae Hydrochaerus3 Holmes's Capybara
Neochoerus Pinckney's Capybara
Lagomorpha Leporidae Aztlanolagus Aztlan Rabbit
Perissodactyla Equidae Equus3 Horses
Tapiridae Tapirus3 Tapirs
Artiodactyla Tayassuidae Mylohyus Long-nosed Peccary
Platygonus Flat-headed Peccary
Camelidae Camelops Yesterday's Camel
Hemiauchenia Large-headed Llama
2 Harlan’s ground sloth continues to be referred to as both Glossotherium (e.g., Yates and Lundelius 2001) and
Paramylodon (e.g., McDonald, 1995).
45
Palaeolama Stout-legged Llama
Cervidae Navahoceros Mountain Deer
Cervalces Stag-Moose
Antilocapridae Capromeryx Diminutive Pronghorn
Tetrameryx Shuler's Pronghorn
Stockoceros Pronghorns
Bovidae Saiga3 Saiga
Euceratherium Shrub Ox
Bootherium Harlan's Musk Ox
Proboscidea Mammutidae Mammut American Mastodon
Elephantidae Mammuthus Mammoths
1 See text for the discussion of the evidence for this genus in North America.
3 Genus survives outside of North America
Arctodus Sheridan Cave Tankersley 1997, Bills and McDonald 1998 , Tankersley et al. 2001
Table 2. Extinct Late Pleistocene Mammalian Genera with Radiocarbon Dates < 12,000 C-14 yr BP and Illustrative Sites
Platygonus Sheridan Cave, OH Tankersley 1997, Bills and McDonald 1998, Tankersley et al. 2001
Nothrotheriops Muav Caves, AZ Long and Martin 1974, Mead and Agenbroad 1992
Megalonyx Little River Rapids, FL Muniz 1998, Webb, Hemmings, and Muniz 1998
Palaeolama Woody Long, MO Graham 1992, Pers. Comm., Grayson 1991
Genus Illustrative Site References
Bootherium Wally’s Beach, AB Kooyman et al. 2001
Camelops Casper, WY Frison 2000
Castoroides Dutchess Quarry Caves, NY Steadman et al. 1997
Cervalces Kendallville, IN Farlow and McClain 1996
Equus Rancho La Brea, CA Marcus and Berger 1984
Euceratherium Falcon Hill, NV Dancie and Jerrems 2001
Mammut Pleasant Lake, MI Fisher 1984
Mammuthus Dent, CO Stafford et al. 1991
Mylohyus Sheriden Cave, OH Redmond and Tankersley 2005
Smilodon Rancho La Brea, CA Marcus and Berger 1984
Tapirus Lehner, AZ Haury et al. 1959, Haynes 1992
Figure 1. The extinct ground sloth Eremotherium (photograph courtesy of the Smithsonian Institution).
Figure 2. The extinct sabertooth Smilodon fatalis (photograph courtesy of the Smithsonian Institution).
Figure 3. The skull of a male helmeted muskox, Bootherium bombifroms, viewed from the rear; note how the horn
cores meet in the midline of the skull (photograph from McDonald and Ray 1981, courtesy of the Smithsonian
Institution).
Figure 4. The skull of a male muskox Ovibos moschatus viewed from the rear; compare the shape of the horncores
with those of the male Bootherium bombifroms illustrated in Figure 3 (photograph from McDonald and Ray
1981, courtesy of the Smithsonian Institution).
Figure 5. The skull of Mammuthus columbi under excavation in the Black Rock Desert, northern Nevada
(photograph by D. K. Grayson).
Figure 6. The number of occurrences of extinct late Pleistocene mammalian genera in the United States (after
Grayson and Meltzer 2002; data from FAUNMAP Working Group 1994)
Figure 1. The extinct ground sloth Eremotherium (photograph courtesy of the Smithsonian Institution).
Figure 2. The extinct sabertooth Smilodon fatalis (photograph courtesy of the Smithsonian Institution).
Figure 3. The skull of a male helmeted muskox, Bootherium bombifroms, viewed from the rear; note how the
horn cores meet in the midline of the skull (photograph from McDonald and Ray 1981, courtesy of the
Smithsonian Institution).
Figure 4. The skull of a male muskox Ovibos moschatus viewed from the rear; compare the shape of the
horncores with those of the male Bootherium bombifroms illustrated in Figure 3 (photograph from McDonald
and Ray 1981, courtesy of the Smithsonian Institution).
Figure 5. The skull of Mammuthus columbi under excavation in the Black Rock Desert, northern Nevada
(photograph by D. K. Grayson).
Figure 6. The number of stratigraphically distinct occurrences of extinct late Pleistocene mammalian
genera from sites in the United States (after Grayson and Meltzer 2002; data from FAUNMAP Working
Group 1994)