Extinctions, scenarios, and assumptions: Changes in latest Pleistocene large herbivore abundance and distribution in western North America more

ARTICLE IN PRESS Quaternary International xxx (2009) 1–15 Contents lists available at ScienceDirect Quaternary International journal homepage: www.elsevier.com/locate/quaint Extinctions, scenarios, and assumptions: Changes in latest Pleistocene large herbivore abundance and distribution in western North America Eric Scott* San Bernardino County Museum, Division of Geological Sciences, 2024 Orange Tree Lane, Redlands, CA 92374, USA a r t i c l e i n f o Article history: Available online xxx a b s t r a c t Proposed explanations for the terminal Pleistocene large mammal extinction event in North America include climate warming and/or cooling, overhunting by early humans, disease, and bolide detonation or impact. A key assumption common to all these scenarios is that large mammals present in North America near the end of the Pleistocene were also present in similar abundance, with similar geographic distributions, during earlier, equally severe periods of climate change (e.g., w130 ka BP). This assumption is challenged here. An important difference in the latest Pleistocene was the profusion and geographic extent of the genus Bison, particularly in the American West. During the late Pleistocene, south of the glacial ice, the species Bison antiquus was more widely distributed and present in greater profusion than earlier species such as the larger B. latifrons. The increased abundance of these large, aggressive, herddwelling ruminants in the late Pleistocene constitutes a critical difference between this time period and earlier, similarly intense interglacials. Extinction scenarios for Pleistocene North America should avoid assuming a relatively static long-term faunal component, and account for the impacts of non-human immigrant species on natives, particularly when immigration events are close in time and space with climate changes. Ó 2009 Elsevier Ltd and INQUA. All rights reserved. 1. Introduction The widespread extinction of numerous genera of large mammals in North America at the end of the Pleistocene Epoch remains both unexplained and a highly contentious subject of investigation, despite decades of intensive study. In recent years, the long-term polarity established between two rival schools of thought – climate change versus overkill by human hunters – has seen the addition of several new scenarios, including ‘‘hyperdisease’’, bolide impact and/ or detonation, and various combinations of all of the above factors, as well as refinements of the major hypotheses themselves. Despite this level of effort, a consensus on the cause(s) of this extinction remains elusive. This study briefly reviews some of the major hypotheses and scenarios advanced to explain this extinction pulse. It will be shown that all of these proposals have one common feature: an almost exclusive focus on external forces, rather than consideration of the large mammals themselves as entities occurring in populations with frequencies that change through time. This study also proposes that changes in faunal diversity through time, particularly with respect to the immigration and subsequent expansion of bison (Bison) into North America in the latter part of the Pleistocene Epoch, have the potential to have changed the manner in which large mammal communities responded to climatic perturbations. 2. Background At the end of the Pleistocene Epoch, around 10 14C ka BP (Lourens et al., 2004), over 30 genera of mammals, primarily large [!44 kg (100 lbs)] mammals, went extinct in North America (Grayson, 2006, 2007; Koch and Barnosky, 2006). These extinctions appear to have been somewhat staggered in time (Grayson and Meltzer, 2003; Grayson, 2006, 2007), with nearly half (16 of 35) taking place between 12 14C ka and 10 14C ka BP (Grayson, 2006) at a time when numerous changes were transpiring on the continent. The Wisconsinan glaciation, having peaked at approximately 20– 18 14C ka BP (¼MIS 2), was waning, with generally rising temperatures accompanied by decreases in glacial cover, increases in available land area, and associated changes in vegetation (Pielou, 1991; Grayson, 1993; Mithen, 2003). Many of the extinctions also occurred near the time when humans are thought to have first Abbreviations: MNI, minimum number of individuals; MIS, marine isotope stage; Ma, million years; NALMA, North American Land Mammal Age; NISP, number of identified specimens * Tel.: þ1 909 307 2669x241; fax: þ1 909 307 0539. E-mail address: escott@sbcm.sbcounty.gov 1040-6182/$ – see front matter Ó 2009 Elsevier Ltd and INQUA. All rights reserved. doi:10.1016/j.quaint.2009.11.003 Please cite this article in press as: Scott, E., Extinctions, scenarios, and assumptions: Changes in latest Pleistocene large herbivore..., Quaternary International (2009), doi:10.1016/j.quaint.2009.11.003 ARTICLE IN PRESS 2 E. Scott / Quaternary International xxx (2009) 1–15 arrived in North America (Martin, 1967, 1984a,b, 2005; Mossiman and Martin, 1975; Alroy, 1999, 2001), although the timing of this arrival remains a controversial topic (Dixon, 1999; Dillehay, 2000; Adovasio, 2002; Scott, 2004; Gilbert et al., 2008). 2.1. Climate change The late Pleistocene was a period of frequent and dramatic changes in climate, with a general shift towards increasingly warm temperatures (Bond and Lotti, 1995), although this warming trend was interrupted by several short reversals, including the Younger Dryas stadial. The more severe of these late Pleistocene variations in climate would seem likely to have had dramatic impacts on a wide variety of continental ecosystems, with long-term changes in temperature, precipitation, and humidity leading to marked shifts in resource distribution and availability. Under such stressful conditions, it is not unreasonable to suppose that some organisms might be regionally extirpated, or become extinct entirely. Because the most recent glacial–interglacial transition numbers among the most severe climate oscillations of the Pleistocene, it could have so strained continental ecosystems that they essentially collapsed, at least in part, driving many native large mammals extinct as a result. Current references to climate change as the primary culprit in terminal Pleistocene extinctions are rarely simplistic constructs on the order of ‘‘it got hotter and the animals died out.’’ Rather, it is proposed that changes in climate at this time would have led to widespread changes in multiple environmental parameters that some animals, specifically, large mammals, could not effectively track. There are several permutations of this view that have received attention, either singly or collectively, including: general reduction and/or fragmentation of habitat (e.g., King and Saunders, 1984; Barnosky, 1986); reduced carrying capacity for herbivores due to shifts in the timing and length of annual growing seasons (e.g., Guthrie, 1984); disruption of established complex resource partitioning patterns by extremely rapid climate shifts (e.g., Graham and Lundelius,1984), and reduced reproductive success among mammals with longer gestation times due to increasingly pronounced changes in seasonality (Kiltie, 1984). However, attributing the extinction event to any one of the above climatic explanations would effectively require the terminal Pleistocene glacial–interglacial transition to have been significantly more severe, either in degree or in rapidity, than earlier such shifts (Koch and Barnosky, 2006). The climate changed numerous times throughout the late Pliocene and Pleistocene Epochs, and while there were small-scale extinctions associated with some of these ´ shifts (Kurten and Anderson, 1980), most of the animals inhabiting North America during this period survived. If changes in climate conditions and resulting new biological pressures were responsible for the powerful extinction pulse characterizing the end of the Pleistocene, it stands to reason that there must have been something distinctly different about these epoch-ending changes than similar, earlier changes. Here lies the primary challenge to climate change hypotheses. Based upon numerous studies of ice core data and other paleothermal proxies (Grootes et al.,1993; Alley and Clark,1999; Koch and Barnosky, 2006), the terminal Pleistocene glacial–interglacial transition does not appear to have been significantly more severe than several earlier such transitions occurring throughout the Pleistocene. It has been proposed that the short-term Younger Dryas stadial may have been a unique event, associated as it was with reduced ice sheets, increased atmospheric CO2, and more dramatic seasonal difference in insolation than during the preceding Wisconsinan glacial maximum (e.g., Meltzer, 2009). But the Younger Dryas was not a full glaciation, and so some critics have expressed incredulity at the idea that ‘‘Ice Age (!) megafauna . were fatally stressed by rapid cooling’’ (Fiedel and Haynes, 2004:125; emphasis in the original). Nor can the potential uniqueness of the Younger Dryas be reliably assessed without better quantification of the intensity of earlier stadials (Haynes, 2009). In any event, it remains to be demonstrated that climatic changes at the end of the Pleistocene were truly unique. If earlier climate shifts were approximately as severe as the last one, yet did not result in any extinctions of the intensity seen at the end of the Pleistocene, then – all other things being equal – climate change would appear to be insufficient to explain the terminal Pleistocene extinction pulse. Because of the perceived inadequacy of the climate-driven extinction model, other scenarios have been proposed to explain the die-off, including overkill by early human immigrants into North America (Martin, 1967, 1984a, b, 2005; Mossiman and Martin, 1975; Alroy, 1999, 2001; Fiedel and Haynes, 2004), hyperdisease (MacPhee and Marx, 1997), and – most recently – bolide detonation (Firestone et al., 2006, 2007). Each of these explanations has strengths and weaknesses, some more than others. But where these scenarios fail, the common fallback position is to further challenge the efficacy of the climate change model as a stand-alone hypothesis – most often by focusing on the lack of significant large mammal extinctions during penultimate and earlier Pleistocene glacial–interglacial transitions (e.g., Fiedel and Haynes, 2004; Martin, 2005). 2.2. Overkill The overkill model proposes that the immigration of early humans into North America near the end of the Pleistocene would ¨ have spelled doom for the naıve native megafauna (Martin, 1967, 1973, 1984a,b, 2005; Alroy, 1999, 2001; Fiedel and Haynes, 2004). This scenario cites the ecological devastation wrought by humans following their arrival in new regions, particularly islands, and extrapolates these data to continental levels. It focuses on the key perceived difference between the terminal Pleistocene glacial– interglacial transition and earlier such transitions – the presence of humans – and invokes this as the deciding factor in the extinction event (Scott, 2004). The overkill scenario also purports to explain why large mammals were the primary animals to go extinct, as these would have been the preferred choice of immigrating hunters. The overkill scenario remains perhaps the most invigorating contribution to the Pleistocene extinction debate advanced to date. However, it may also be the most contentious, particularly because it comes with its own set of difficulties. Chief among these are the paucity of confirmed kill sites where fossil remains of Pleistocene megafauna are found in incontrovertible association with artifacts (but see Barnosky et al., 2004b), and the limited number of extinct taxa associated with artifacts (Grayson and Meltzer, 2003; but see Surovell and Waguespack, 2008). Additionally, many genera and species appear to have gone extinct before humans first appear in North America (Grayson, 2001; Grayson and Meltzer, 2003). Further, ´ ¨ the overkill hypothesis argues for a naıvete among the North American fauna that is neither demonstrated nor justifiable. Denizens of small islands frequently lack strong predation pressures and so enjoy a concomitant relaxing of defensive adaptations (witness flightlessness in many island birds as one example), rendering them vulnerable when non-native predators arrive (McFarlane, 1999). In contrast, the continental fauna of Pleistocene North America was replete with predators, and native mammalian herbivores would of necessity have developed suitable defensive mechanisms. Given this, the advent of a new predator in continental North America does not necessitate impacts as drastic as those affecting island avifaunas, irrespective of the efficiency of the predator (Grayson, 1991; Scott, 2004). Some of these concerns have been addressed by proponents of the overkill scenario, albeit with limited success. For example, Martin (1984a,b, 2005) refuted claims of insufficient evidence by Please cite this article in press as: Scott, E., Extinctions, scenarios, and assumptions: Changes in latest Pleistocene large herbivore..., Quaternary International (2009), doi:10.1016/j.quaint.2009.11.003 ARTICLE IN PRESS E. Scott / Quaternary International xxx (2009) 1–15 3 arguing for a ‘‘blitzkrieg’’ invasion – effectively, an influx of human hunters so sudden, swift, and powerful that it would have swept though North and South America leaving little evidence behind. Under the blitzkrieg model, ‘‘[s]uccess at finding much evidence of killing or processing of the extinct fauna is not predicted’’ (Martin, 1984b:397). However, recent studies (Adovasio et al., 1975, 1977, 1985, 1999; Dillehay, 1989, 1997, 2000; Adovasio, 2002; Gilbert et al., 2008) have suggested a presence of humans in North America far earlier than the Clovis hunters that are the focus of Martin’s (1984) scenario, although some of these studies do have detractors. Given the potential for an extended human presence, and in the absence of additional, pre-Clovis confirmed kill sites with megafaunal fossils in verifiable association with artifacts, it is increasingly clear that the first humans to enter North America near the ¨ end of the Pleistocene may not have been the uber-hunters envisioned in Martin’s (1984, 2005) blitzkrieg scenario. If this is the case, then a reformulation of the overkill hypothesis is in order. It is reasonable to suppose that any pre-Clovis entrants into North America south of the glacial ice, irrespective of their chosen ingress route, would likely have taken some advantage of the plentiful prey awaiting them. The record from Paisley 5 Mile Point Caves in Oregon – where human fecal matter dating to 12,300 14C BP is associated with a camelid astragalus and ‘‘likely butchered’’ bones (Gilbert et al., 2008) – supports such an interpretation. Yet if humans were present in North America for a thousand years or more prior to the advent of the Clovis technology, conducting even limited hunting of megafauna, then the relative dearth of kill sites on the continent becomes ´ ¨ even more glaring – and arguments for prey naıvete, even more obsolete (Scott, 2004). On the other hand, if it is proposed that Clovis hunters arrived south of the glacial ice after the initial immigrants, and that blitzkrieg commenced only then (Martin, 1984a,b:363), ‘‘[s]uch an explanation would propose a continental dispersion of Clovis hunters so rapid as to virtually preclude reasonable contact with established human populations, hunting prey that had likely lost ´ ¨ their naıvete hundreds or thousands of years earlier’’ (Scott, 2004:29). With respect to Martin’s (1984, 2005) blitzkrieg model predicting little success in finding much supporting evidence, Grayson and Meltzer (2003) rightly observed that any hypothesis predicting little or no evidence will be found to support it has essentially left the realm of empirical science altogether. These authors go farther than this, claiming that the only reason that the overkill hypothesis still continues to be considered valid is ‘‘not because of archaeologists and paleontologists who are expert in the area, but because it keeps getting repeated by those who are not’’ (Grayson and Meltzer, 2003:590). A less contentious interpretation is that overkill continues to be promoted because of the perceived inadequacy of the climate change hypothesis, specifically with respect to the lack of extensive extinctions associated with earlier, similar climate shifts. Indeed, in a rebuttal to Grayson and Meltzer (2003), Fiedel and Haynes (2004) spent considerable space, not providing new data to support overkill, but rather returning fire to challenge climate change as a valid hypothesis [despite Grayson and Meltzer (2003:591) having specifically stated that no climate change hypotheses had yet gained wide acceptance], with the primary rationale explicitly being the lack of extinctions during earlier climate shifts. ‘‘If climate change is to be taken seriously as the sole cause of extinction,’’ they wrote, ‘‘its advocates must show that the challenges posed to American fauna . were unique and more severe than in any past episodes. But . ice core records of past climates show that the Terminal Pleistocene changes were not unique in either their abruptness or severity’’ (Fiedel and Haynes, 2004:126). These authors conclude, ‘‘If Meltzer and Grayson have a compelling new climate change hypothesis to offer, it behooves them to set it out, in adequate detail, for scientific assessment . If they or anyone else ever does present a climate change hypothesis that purports to fit the rapidly expanding body of evidence, then we can all proceed to do real science’’ (Fiedel and Haynes, 2004:128). With these statements, Fiedel and Haynes (2004) clearly placed the burden of proof, not on proponents of overkill, but on advocates of climate change, in seeking a trigger for the extinction. 2.3. Hyperdisease Largely because both the climate change and overkill scenarios are perceived to be insufficient to explain the terminal Pleistocene extinction in North America, other hypotheses have been advanced. Chief among these is the hyperdisease scenario (MacPhee and Marx, 1997), which posits that humans, and possibly their dogs, entering North America at the end of the Pleistocene carried with them a disease or diseases to which the native fauna was not inured. The hypothesized hyperdisease(s) would have spread quickly through the indigenous fauna and caused a massive, rapid, widespread die-off. Large mammals with relatively low reproductive rates would be particularly vulnerable to such a hyperdisease, while smaller animals would have been less powerfully impacted (MacPhee and Marx, 1997). This scenario is particularly helpful in clarifying how the entry of humans into continental North America might have impacted the native fauna without requiring the immigrants to have been particularly proficient or prolific hunters or the prey to ¨ have been unreasonably naıve. Also explained would be the relative paucity of archaeological evidence of early human immigrants hunting megafauna (MacPhee and Marx, 1997). However, although the hyperdisease hypothesis is intriguing, it has yet to be supported by concrete data of any such hypervirulent disease actually existing. Further, while the hypothesis attempts to explain why large mammals might be preferentially impacted, it fails to account for the selectivity and severity of the extinctions (Alroy, 1999; Lyons et al., 2004). The imagined disease is proposed to have crossed multiple mammalian orders – a feat which has never been previously documented for any disease – and yet did not spread within some orders and even some genera. Dire wolves (Canis dirus) went extinct, for example, while grey or timber wolves (Canis lupus) and coyotes (Canis latrans) survived and even thrived in post-Pleistocene North America; similarly, the mountain goat Oreamnos harringtoni died out, but the closely related and morphologically similar O. americanus survived. In short, while the hyperdisease proposal is testable, to date it has garnered no empirical support. 2.4. Bolide impact/detonation Another recent contribution to the Pleistocene extinction debate is the proposal that detonation of a bolide over North America initiated the onset of the Younger Dryas, created the Carolina Bays, ignited continent-wide wildfires with consequent catastrophic reduction in biomass, and led directly to the extinction of the North American megafauna as well as the Clovis culture (Firestone et al., 2006, 2007). This new scenario, with its clear echoes of the theory that a massive asteroid impact caused the Cretaceous–Tertiary (KT) mass extinction (Alvarez et al., 1980), is based upon the purported identification of Pleistocene fossil bones containing embedded micrometeorite fragments (Firestone et al., 2006:54–66), as well as terminal Pleistocene sediments yielding magnetic grains with iridium, magnetic microspherules, charcoal, soot, carbon spherules, and nanodiamonds from multiple sites dating to w12.9 cal ka BP (Firestone et al., 2007; Kennett et al., 2009). This scenario has met with considerable popular acclaim, but in its current form it fails on a number of fronts (Dalton, 2007; Buchanan et al., 2008; Pinter and Ishman, 2008; Marlon et al., 2009; Surovell et al., 2009). Fossils containing embedded micrometeorite fragments, for example, initially implicitly assumed to date to the Please cite this article in press as: Scott, E., Extinctions, scenarios, and assumptions: Changes in latest Pleistocene large herbivore..., Quaternary International (2009), doi:10.1016/j.quaint.2009.11.003 ARTICLE IN PRESS 4 E. Scott / Quaternary International xxx (2009) 1–15 late Pleistocene (Firestone et al., 2006:56), were later determined to predate the end of the epoch by up to 20 ka BP or more (Dalton, 2007). Similarly, radiometric ages acquired from the Carolina Bays (e.g., Whitehead, 1981) do not in many cases correspond with the date of the proposed impact event; if some of these bays originated due to terrestrial processes, there is little justification for proposing that others of them must have been formed by extraterrestrial forces. Nor is there evidence of continent-wide wildfires at any time during terminal Pleistocene deglaciation (Marlon et al., 2009). The proposed ‘‘inferred population decline’’ and elimination of the Clovis culture (Firestone et al., 2007:16,021) is also not supported by the available evidence; a recent review of w1500 North American archaeological sites revealed no major Paleoindian population bottlenecks from the suggested time of the impact (Buchanan et al., 2008). Other data advanced as evidence to support the impact/ detonation scenario – iridium, magnetic minerals, microspherules, carbon, and nanodiamonds – are subject to differing interpretations as to their nature and origin, and may be explained in many cases by purely terrestrial and/or non-catastrophic factors (Pinter and Ishman, 2008; Kerr, 2008, 2009). This is especially important because collectively these materials would have had to derive from either multiple impactors of differing composition, or else from a veritable ‘‘Frankenstein monster’’ of a bolide with no known analogue (Pinter and Ishman, 2008:37). An independent examination of seven localities, including two of those discussed by Firestone et al. (2007), for magnetic minerals and microspherules failed to reproduce the results of the original study (Surovell et al., 2009). Finally, the survival of some megafauna (Mammut) well into the Younger Dryas stadial (Woodman and Athfield, 2009) shows that, if there was an extraterrestrial impact event, it was not immediately responsible for at least some extinctions. From the perspective of the end-Pleistocene extinction debate, perhaps the most challenging aspect of the bolide detonation scenario – other than the fact that much of the supporting data (micrometeorite-riddled fossils, continent-wide wildfires, cultural bottlenecks, impact debris, etc.) have not been corroborated, replicated or otherwise confirmed – is its lack of specificity. The scenario assumes that, if such an impact and/or detonation took place, then the extinction of the North American megafauna must necessarily have resulted – the classic cum hoc fallacy in a terminal Pleistocene setting. The apparent structure of the argument – (a) large impacts cause mass extinctions (e.g., the KT event); (b) there is evidence of an extraterrestrial event of some sort; (c) there is an extinction event around the same time as the presumed extraterrestrial event; (d) the event and the extinction must be related; therefore; (e) the event must have caused the extinction – fails to establish causality, assuming the conclusion rather than demonstrating it. Further, no attempt is made to explain why large mammals would be more heavily impacted than smaller forms, or why some large mammals went extinct and others did not. In this respect, all of the other major hypotheses advanced to explain the extinction (climate change, overkill, hyperdisease) are more precise, explicit, and testable in their predictions and assumptions. Nor can convincing analogies be made with the earlier KT impact, associated as that event it was with a global mass extinction involving large and small animals and plants across the biological spectrum (Dingus and Rowe, 1998). In fact, the reasons underlying the extinction event, as described by Firestone et al. (2006, 2007), are muddled at best, with bolideinduced dramatic cooling at the onset of the Younger Dryas apparently combined with ‘‘insufficient food resources, overhunting, climate change, disease, flooding, and other effects, all triggered or amplified by the Y[ounger] D[ryas] event’’ (Firestone et al., 2007:16,021). Such an all-encompassing ‘‘explanation’’ serves little useful purpose, because its very expansiveness effectively neuters its explanatory power; any other causal mechanism could be easily subsumed into the scenario, simply by amending it to read ‘‘as enhanced or augmented by the effects of the bolide impact’’. In its present form, then, the bolide impact/detonation scenario provides little utility in elucidating the terminal Pleistocene extinction event in North America, although it is perhaps premature to reject it outright at this early stage. In the future, should proponents of this scenario produce data that can be corroborated and/or replicated independently, they still must go beyond merely demonstrating whether or not an extraterrestrial event took place, and construct a plausible, testable framework for the extinction event that relates the actual physical manifestation of such an impact to the actual animals that were affected. 2.5. Distinguishing among alternatives As can be seen from this brief review, none of the various scenarios advanced to explain the terminal Pleistocene extinction event in North America have proven satisfactory. Each explanation has challenges. However, there are key differences among the difficulties faced by the various scenarios. For example, the hyperdisease and bolide detonation explanations are both comparatively new proposals, and so the relative lack of supporting evidence faced by each (Lyons et al., 2004; Koch and Barnosky, 2006; Pinter and Ishman, 2008; Marlon et al., 2009; Surovell et al., 2009; Woodman and Athfield, 2009) may only be a matter of their novelty. In contrast, the overkill scenario has failed to garner convincing supporting evidence for more than four decades, and this suggests that such evidence may not be forthcoming. Computer simulations (Alroy, 2001) have shown that such a scenario is not impossible, as some critics claimed, but the results of such simulations ‘‘are highly sensitive to implicit assumptions concerning the degree of prey naivety to human hunters’’ (Brook and Bowman, 2002:14,626) and may not accurately reflect the full continental distribution of either mammals or their presumed human hunters (Scott, 2004). It is noted that much of the data presented as rejecting the overkill scenario is open to interpretation, and what overkill naysayers find convincing is usually quickly dismissed by overkill proponents. As one example, data indicating that some genera and species went extinct before humans first appeared in North America, which suggests that the extinction was not driven by human agents (e.g., Grayson, 2001), is rejected out of hand as nothing more than negative evidence by others (e.g., Haynes, 2007). This allows for a wide variety of interpretations, with some authors challenging whether overkill even warrants consideration as science (e.g., Grayson and Meltzer, 2003) while others interpret the scenario to have ‘‘become so resilient that it can survive a succession of criticisms’’ (Hallam and Wignall, 1997:241). Resolution of the argument therefore seems increasingly impeded by the polarization of these two camps. Nor have recent attempts to salvage overkill as part of a larger explanatory construct proven entirely effective, or necessary. For ´ example, Nogues-Bravo et al. (2008) provided a seemingly viable mechanism for explaining the extinction of the woolly mammoth, Mammuthus primigenius, in Eurasia, invoking a combination of range reduction due to climate change and human hunting pressures. However, it remains to be determined whether or not similar combinations of environmental and anthropogenic factors were responsible for the extinction of other megafauna, on other continents. Such a hypothetical one-two punch has been proposed for North American megafauna (e.g., Barnosky et al., 2004b; Koch and Barnosky, 2006), but support for this scenario derives, not from the strength of overkill per se, but rather from the perceived weakness of climate change as a sole agent of extinction. Such an explanation also has little utility in explaining extinctions in regions like continental southern California, where – despite decades of work and numerous late Pleistocene localities (Jefferson, 1991; Scott and Please cite this article in press as: Scott, E., Extinctions, scenarios, and assumptions: Changes in latest Pleistocene large herbivore..., Quaternary International (2009), doi:10.1016/j.quaint.2009.11.003 ARTICLE IN PRESS E. Scott / Quaternary International xxx (2009) 1–15 5 Cox, 2008) – no confirmed associations of Pleistocene megafauna with early human hunters have yet been demonstrated (Scott, 2004). The climate change hypothesis differs from all of the above scenarios because it does not suffer from a lack of evidence, but rather from a glut of data – much of which would appear to negate the hypothesis. As noted previously, the changes in climate at the end of the Pleistocene have not been documented to have been substantially more severe or more prolonged than earlier glacial– interglacial transitions. Further, no major extinction pulses are recognized in the fossil record from any earlier glacial–interglacial transitions similar in intensity to the terminal Pleistocene transition. Primarily for these reasons, climate change is routinely dismissed at the sole agent behind the terminal Pleistocene extinction event in North America. The difficulty is not a lack of supporting data: it is an abundance of data that rejects the hypothesis as currently framed. Paul Martin expressed this point effectively: ‘‘The heart of the argument for me is that late-Quaternary climatic change, while impressive, is essentially no different from what we see in many, many swings from cold-dry to warmwet and dusty to dust-free climates in the past 700,000 years or so. Unless oceanographers, ice-core stratigraphers, and climatologists find some unique event, the classic approach to explaining Quaternary extinctions by some physical means is inoperable’’ (Martin, 2005:116–117). side of this equation: the composition of the large mammal community itself. 3.1. Bison and the Rancholabrean North American Land Mammal Age In this respect, it is important to observe that there is one major difference between late Pleistocene large mammal communities and earlier faunas. This is the presence and abundance of species of the genus Bison in later Pleistocene large mammal faunas throughout North America. Bison, a Eurasian immigrant, first entered North America in the latter part of the Pleistocene. This event is sufficiently apparent and widespread in the fossil record that Bison is the index taxon for the Rancholabrean North American Land Mammal Age (NALMA) (Savage, 1951), and the earliest unambiguous appearance of the genus south of 55 N defines the beginning of this age (Bell et al., 2004). The timing of the arrival of Bison in the coterminous United States has not been determined with any certainty, and so the beginning of the Rancholabrean NALMA has not been reliably established. Early estimates (e.g., Guthrie, 1970; Schultz and Hille´ rud, 1977; Kurten and Anderson, 1980) estimated that Rancholabrean faunas might have appeared as early as 600 ka. However, more recent studies suggest dates younger than w250 ka (Lundelius et al., 1987; Sanders, 2002; Bell et al., 2004; Scott and Cox, 2008; Sanders et al., 2009). Sanders (2002) reported an astragalus assigned to Bison from the Ten Mile Hill Beds, South Carolina; fossils from this locality date between approximately 240 ka and 220 ka. This record of Bison was employed to date the beginning of the Rancholabrean NALMA by Sanders (2002) and Sanders et al. (2009), but this interpretation has not been widely accepted to date. Bell et al. (2004) defined the beginning of the Rancholabrean by the first unequivocal appearance of Bison in North America south of 55 N. A minimum arrival time of 160 ka was provided by the record of the genus from Jones Spring, Missouri (after Haynes, 1985; Saunders, 1988). Further, because fossils of Bison are common from the American Falls Formation in Idaho, which dates between 210 Æ 60 ka and 72 Æ 14 ka (Scott et al., 1982), Bell et al. (2004) employed this time span to bracket the beginning of the Rancholabrean NALMA. The date of approximately 240 ka for the beginning of this age proposed by Sanders (2002) and Sanders et al. (2009) does fall within the confidence interval provided by the older date for the fauna from American Falls (Scott and Cox, 2008). Older dates than these for the first appearance of Bison south of 55 N have been proposed, but in most cases these have not withstood careful scrutiny. For example, a published record of early ´ Bison from the Terapa locality in Sonora, Mexico, originally interpreted to be between 570 ka and 310 ka (Mead et al., 2006), has more recently been determined to date to roughly the middle of the most recent glaciation, the Wisconsinan (J. Mead, pers. comm., 2009). A tooth assigned to Bison sp. cf. B. antiquus from the San Pedro region of the Los Angeles Basin in southern California, proposed to have been recovered from sediments dating to w330 ka, lacks firm stratigraphic placement and may equally likely have been recovered from other, younger sediments in this geologically complex region (Scott and Cox, 2008). Fossils identified as records of early Bison from Pleistocene Lake Manix in the Mojave Desert of California, thought to date to approximately 290 ka (Jefferson, 1968, 1987) were subsequently determined to be Camelops (Scott and Cox, 2008). Another early record of Bison from North America is more intriguing. McDonald and Morgan (1999, 2004) discussed fossils of Bison from latest Pliocene–early Pleistocene vertebrate faunas in Florida, based upon horn core fragments from the Macasphalt Shell Pit (w2.6–2.2 Ma) and the Inglis 1A (w2.2–1.8 Ma) localities. If 3. Discussion Close examination of all of the scenarios discussed above reveals that they all share a subtle but important underlying assumption: that the North American late Pleistocene megafaunal community did not differ substantively in its biotic composition throughout much of the epoch. This assumption is frequently explicit, if unrecognized. For example, Fiedel and Haynes (2004:125) stated, ‘‘[T]o make any case for [colder] climate as the killer, one would now have to show that the . megafauna, having survived for 2 million years in climates often much colder than the present, were fatally stressed’’ (emphasis added). Similarly, Martin (2005:167) observed, ‘‘[T]he change (or changes) [in climate] must, alone or in combination with other factors, have been unique in the Quaternary. A change closely resembling others that the megafauna had repeatedly survived . is not a good candidate in the search for explanations of extinction’’ (emphasis added). Another recent example (Koch and Barnosky, 2006:224) made this statement: ‘‘[M]ost environmental hypotheses require that climate or ecosystem structure at the time of extinction was unusual relative to conditions earlier in the Pleistocene. Therefore it is important to look beyond the Last Glacial Maximum . recognizing that Pleistocene faunas survived many earlier events only to succumb during the most recent glacial– interglacial transition’’ (emphasis added). Given the apparent pervasiveness of this assumption, it is reasonable to examine its validity. If Pleistocene large mammal communities can be shown to have changed their composition significantly through time, then – depending upon the timing and nature of such changes – any arguments that these animals survived numerous climate shifts as some sort of a cohesive entity (‘‘the megafauna’’) is called into question. More specifically, if the suite of large mammals inhabiting North America at the end of the Pleistocene can be shown to have differed substantively in composition from earlier such communities, then a factor of exceptionality is introduced. In effect, the terminal Pleistocene extinction event in North America might indeed have been unique in the Quaternary – not because of any unusual severity or duration of the climate conditions, but rather because of the relative uniqueness of the other Please cite this article in press as: Scott, E., Extinctions, scenarios, and assumptions: Changes in latest Pleistocene large herbivore..., Quaternary International (2009), doi:10.1016/j.quaint.2009.11.003 ARTICLE IN PRESS 6 E. Scott / Quaternary International xxx (2009) 1–15 confirmed, the Florida records would document the earliest presence of Bison in North America and force a redefinition of the Rancholabrean NALMA. However, Bell et al. (2004:285) expressed reservations about these fossils, including them in a discussion of ‘‘purported records [that] suffer from dubious identification, unclear stratigraphic position, or unsatisfactory chronologic control’’. Further, given the absence of fossils of Bison from numerous Irvingtonian NALMA localities (see Lundelius et al., 1987; Barnosky et al., 2004a; Bell et al., 2004), the record of ?Pliocene Bison from Florida would likely document an early immigration pulse for the genus that eventually perished (Scott and Cox, 2008). Clearly, Bison is a relative latecomer to Pleistocene North America. This is an important point as it relates to terminal Pleistocene extinctions. Using the date of between 240 ka and 220 ka proposed by Sanders (2002) and Sanders et al. (2009) for the beginning of the Rancholabrean NALMA, it can be seen that Bison was present in North America during only three major glacial– interglacial transitions: those between MIS 7b and 7a, 6 and 5e, and the terminal Pleistocene transition between MIS 2 and 1 (Fig. 1). 3.2. Abundance of North American Bison in time and space The late appearance of Bison in North America is one facet of this review. Another, more critical point is the increasing relative abundance of bison through geologic time in Pleistocene North America. For the purposes of this study, species to be discussed include Bison latifrons Harlan, 1825, Bison priscus Bojanus, 1827, Bison antiquus Leidy, 1852, Bison occidentalis Lucas, 1898, and Bison bison Linnaeus, 1758. It is recognized that this taxonomic scheme does not concur with other, prior systematic interpretations (e.g., McDonald, 1981; Shapiro et al., 2004) primarily because it accords species-level distinction to morphological groups that actually may not have been different species. For example, McDonald (1981) proposed that some Pleistocene Bison groups were subspecies rather than species, and preferred the trinomials Bison antiquus antiquus and B. antiquus occidentalis for later Pleistocene and early Holocene forms. Similarly in approach but differently in content, Wilson (1996) employed the trinomials B. bison antiquus and B. bison occidentalis for these same animals. These interpretations assume that these animals were all interfertile – and, in the case of Wilson’s (1996) study, also interfertile with living bison subspecies (B. bison bison and B. bison athabascae Rhoads, 1897). Because the present review primarily discusses extinct species, among which interfertility has not been demonstrated, subspecies designations are not used. Shapiro et al. (2004) proposed through DNA analysis that B. antiquus, B. occidentalis, and living B. bison all formed a clade that separated from Beringian Bison between 83 ka and 64 ka, and had a most recent common ancestor between 22 ka and 15 ka. However, Shapiro et al. (2004) did not argue for strict conspecificity among these taxa.1 Wilson et al. (2008) noted that based upon mtDNA data, living B. bison evolved from B. antiquus through B. occidentalis, making the latter species essentially a chronomorph. Because this evolution apparently took place in midcontinent North America, while the holotype of B. occidentalis is from Alaska, Wilson et al. (2008) expressed concern regarding whether the species name B. occidentalis was applicable to midcontinent fossils normally referred to that taxon. These midcontinent fossils are therefore referred to hereafter using quotes, as Bison ‘‘occidentalis’’. As noted by McDonald (1981), early species of Bison in North America south of the glacial ice, including B. latifrons, were not quite as geographically widespread as later species such as B. antiquus and B. ‘‘occidentalis’’. Bison antiquus was mapped farther north and south than B. latifrons, while B. ‘‘occidentalis’’ reportedly had a much more northerly extent (but see Wilson et al., 2008). In addition to the broad geographic extent of later Pleistocene species of Bison, there also appears to be an increase in their local and relative abundance across much of the continent as the epoch waned. B. latifrons was inferred to have been more widespread geographically than B. antiquus during the Illinoian glacial stage (¼MIS 6), because of the relatively large extent during this time of woodland and forest habitats preferred by B. latifrons (McDonald, 1981:236, 242). However, B. latifrons was also inferred to have been a more solitary and ‘‘unsociable’’ species that did not form herds (McDonald, 1981:200); this is supported by the fossil record for this species, which consists primarily of single finds [American Falls, Idaho being the main exception, although this assemblage is a time-averaged sample (Pinsof, 1992)]. In contrast, B. antiquus was proposed to have been present during MIS 6, in smaller overall numbers, but with higher population densities (McDonald, 1981:242), because the species was considered to have formed socially-cohesive groups (McDonald, 1981:205). This interpretation is also supported by fossil evidence, since remains of B. antiquus are often indicative of herds or herding behavior (e.g., Bement, 1999; Jefferson, 2001). During the Sangamonian interglacial (¼MIS 5), both B. latifrons and B. antiquus were interpreted to have extended their respective ranges. But during the subsequent Wisconsinan glaciation (¼MIS 4–2), B. latifrons suffered a dramatic reduction in numbers and in distribution, becoming extinct or nearly so prior to the end of this Fig. 1. Marine isotope record. This plot is a composite of oxygen isotope records of benthic foraminifera obtained from deep sea cores. These data reflect global ice volume, and so are proxies for global temperatures during the later Pleistocene. The solid black line represents the approximate time of the arrival of Bison in North America south of 55 N and the beginning of the Rancholabrean North American Land Mammal Age. Isotope data after Porter (1989), incorporating data from Shackleton and Pisias (1985) and Martinson et al. (1987); timing of Bison from Sanders (2002), Bell et al. (2004), Scott and Cox (2008), and Sanders et al. (2009). 1 Only two specimens referred to B. antiquus by Shapiro et al. (2004) (extraction samples BS569 and BS570) yielded successful DNA amplification results; these specimens were metapodials, which implies that the key character used by most paleontologists to distinguish among extinct bison – horn core morphology – was not applied in this instance. Further, one of the metapodials in question (BS569) yielded an uncorrected radiocarbon date of only 3600 Æ 70 BP, which is more recent than the generally accepted age range of B. antiquus. Please cite this article in press as: Scott, E., Extinctions, scenarios, and assumptions: Changes in latest Pleistocene large herbivore..., Quaternary International (2009), doi:10.1016/j.quaint.2009.11.003 ARTICLE IN PRESS E. Scott / Quaternary International xxx (2009) 1–15 7 period, while B. antiquus – a herd species with higher population densities than B. latifrons – expanded to become ‘‘the most widely distributed of all North American bison’’ (McDonald, 1981:82), reaching its highest geographic distribution in the late Wisconsin. Determining to what extent bison increased numerically through the latter part of the Pleistocene is a challenge. Many localities yielding fossils of bison cannot be placed in a temporal framework, either because they are older than the maximum extent of radiocarbon dating, or because they are from localities that lack datable materials. One approach used to estimate past abundance of Bison has been simply to graph what radiocarbon dates are available. This method has obvious limitations, since it relies in part on negative evidence – yet the absence of radiocarbon dated remains from a given time bracket does not necessarily indicate absence of animals. Many faunas in the southwestern US, for example, have yielded abundant remains of late Pleistocene megafauna (Jefferson, 1991; Jefferson et al., 2004; Scott and Cox, 2008), but lack organic materials suitable for dating; species present in these faunas would therefore not be included in graphs of radiocarbon dated occurrences. McDonald (1981:244) also elucidated several additional potential biases in this approach: ‘‘(1) the greater likelihood of bone preservation in archeologic than paleontologic sites because of the greater initial quantity of bones . and greater potential for preservation; (2) the selective killing of certain species by human hunters, which amplifies the representation of these game species . while deemphasizing the representation of nongame species; and (3) the differential dating of faunal remains – archeologists use dating more frequently than paleontologists, certain genera appear to be of more interest than others to both archeologists and paleontologists, certain periods of the prehistoric period have been more consistently dated than others, and more radiocarbon dates are available from some regions of North America than others.’’ Despite these difficulties, this approach has been used to assess bison abundance through time in midcontinent North America. McDonald (1981) employed this method for bison and other Pleistocene megafauna from localities dating from 30 14C ka BP to historic times (Fig. 2A). The resulting distribution showed few available dates for any megafauna prior to 16 14C ka BP, after which several mammals other than bison – most notably Mammut (mastodon) and Equus (horse) – were represented by an increasing number of radiocarbon samples. This trend changed abruptly around 12 14C ka BP, when dated samples of Bison quadrupled in number compared to the previous interval, while other megafauna showed a less dramatic increase in sample abundance. Between 12 14C ka and 8 14C ka BP, the abundance of bison remained more or less constant, while that of the other megafauna decreased precipitously (Fig. 2A). The pattern exhibited suggests that, by this measure, bison were increasing in abundance in North America prior to the reduction in numbers of other megafauna. Guthrie (2006) incorporated radiocarbon dates on Bison from Alaska and the Yukon Territory provided by Shapiro et al. (2004), comparing them with like dates obtained from Equus, Mammuthus (mammoth), Cervus (wapiti), Alces (moose), and evidence of early humans from the region. The presented data showed that Bison was rare prior to 13 14C ka BP, at which time the genus enjoyed a small but distinct increase in relative abundance (Fig. 2B). Mammuthus was moderately abundant, and Equus was somewhat less so, from 18 14C ka to 12 14C ka BP, after which the former was recorded at only three sites and the latter disappeared altogether (Fig. 2B). Over that same time span, Cervus was absent until 13 14C ka BP, after which its recorded abundance exploded, with w50 dated occurrences between 13 14C ka and 12 14C ka BP, and then abruptly truncated again, with only 7–8 dated records in each of the subsequent Fig. 2. Bar graph depicting the number of radiocarbon dated samples of Bison, in thousand-year increments, from Pleistocene and Holocene North America. A: data from McDonald (1981); B: data from Guthrie (2006). See text for discussion. millennia (Fig. 2B). Alces and evidence of early humans first appeared around 12.5 14C ka BP, and both become moderately abundant thereafter (Guthrie, 2006:207). The graphing of radiocarbon dates employed by McDonald (1981) and Guthrie (2006) indicates that, by this measure, bison were increasing in abundance in North America prior to the reduction in numbers of other megafauna (Fig. 2). Previous suggestions that bison expanded in numbers and extent only after the extinction of the other megafauna – or even that North American Pleistocene bison died out and were replaced by Bison bison (e.g., Guthrie, 1970; McHugh, 1972; Flannery, 2001) – are not supported by these data. Another approach to estimate past abundance of Bison, utilized by Shapiro et al. (2004), applied Bayesian phylogenetic analyses and coalescent theory to mtDNA control region sequences obtained from 191 samples2 of extinct and extant Bison from Canada, Siberia, China, Alaska, and Wyoming. This analysis did not include any specimens assigned to Bison latifrons; fossils of this species were sampled, but did not yield mtDNA. The analysis ‘‘strongly supported a boom–bust demographic model . in which an 2 442 specimens were sampled; mtDNA was successfully amplified and sequenced from 352 of these. Less than 2/3 of the total 685-bp target sequence could be amplified from 24 specimens, and so these were excluded from phylogenetic analyses (Shapiro et al., 2004, supplemental data). Of the remaining sample, 191 specimens were associated with finite radiocarbon dates (Shapiro et al., 2004, Table 1). Please cite this article in press as: Scott, E., Extinctions, scenarios, and assumptions: Changes in latest Pleistocene large herbivore..., Quaternary International (2009), doi:10.1016/j.quaint.2009.11.003 ARTICLE IN PRESS 8 E. Scott / Quaternary International xxx (2009) 1–15 exponential expansion of the bison population was followed by a rapid decline’’ (Shapiro et al., 2004:1563–1564). The transition was inferred to have taken place at roughly 37 ka. A subsequent study (Drummond et al., 2005) introduced a Bayesian skyline plot to test the same sample of mtDNA control region sequences. This analysis showed a somewhat different demographic model; here, Bison exhibited a sharp increase in abundance between 80 ka and 75 ka, then a sharp decline around 25 ka and a reduced rebound in population size after 10 ka. Both distributions indicate that, although terminal Pleistocene Bison in Beringia was reduced in numbers relative to proposed population peaks, it was nevertheless more abundant at the end of the epoch than at any graphed point earlier than w110 ka – including the glacial–interglacial transition between MIS 6 and 5e. The data presented by Shapiro et al. (2004) and Drummond et al. (2005) are almost exclusively restricted to Beringia for Pleistocene fossils, and to midcontinent North America for Holocene specimens. The southernmost Pleistocene fossils yielding both mtDNA and finite radiocarbon dates for this study were two specimens obtained from Natural Trap Cave in northern Wyoming. The Pleistocene population curve proposed from these molecular data (Shapiro et al., 2004; Drummond et al., 2005) therefore likely does not represent geographic areas for which mtDNA was not obtained, including the majority of the midcontinent region. Observed differences between the Beringian ‘‘boom–bust’’ curve and the clearly dissimilar pattern inferred from radiocarbon dated samples from eastern Beringia and midcontinent North America (McDonald, 1981) require further scrutiny. These disparities can easily and correctly be attributed at least in part to differences in methodology; this interpretation is supported by the similar lack of congruence between the Beringian molecular curve and that based on radiocarbon dates from Alaska and the Yukon reported by Guthrie (2006). However, this does not mean that the radiocarbon data present no useful data – particularly with respect to the relative abundance through time of Bison compared to other taxa from the region. In any event, it is probably premature to assume that bison in midcontinent North America followed the same trajectory in abundance through time as Beringian bison, although it is noted that the midcontinent animals must also have undergone a population bottleneck at some point in the Pleistocene, due to the lack of genetic diversity evident in the sample (A. Cooper, pers. comm., 2009). With this in mind, it is noted that the overall sample considered by Shapiro et al. (2004) can be separated into two subsets, north and south of the glacial ice, that exhibit somewhat different population curves through time than each other and than the overall curve (A. Cooper, pers. comm., 2009). It is therefore not unreasonable to expect that different populations of Bison in different geographic regions of North America would exhibit population curves through time that differed substantively from the overall North American curves presented by Shapiro et al. (2004) and Drummond et al. (2005). Further research in this area would be welcome. Another way to assess past abundance of Bison is to review the relative abundance of fossil remains of this genus from paleontological localities where multiple individuals of multiple taxa are preserved. For example, in the southern Great Plains, Wyckoff and Dalquest (1997) noted that bison were numerically less abundant than other herbivores (horses, camels, and mammoths) in faunas older than w20 ka, but became relatively more numerous near the Pleistocene–Holocene transition. These data were unfortunately not quantified with numbers of specimens or estimates of MNI across all taxa. Nor were these data unequivocal; of 67 ‘‘non-archaeological’’ sites from the southern Great Basin where remains of Bison had been previously recorded, both as isolated finds and as components of local faunas, radiometric dates were available for only 31 sites (46% of the available sample) (Wyckoff and Dalquest, 1997). Further, only 16 of these dates were considered to have been acquired from reliable materials. Nevertheless, the inference drawn by Wyckoff and Dalquest (1997), although subject to further scrutiny, remains suggestive. Moving from the Great Plains to the Great Basin, the record becomes less clear, primarily because many faunas from this region are cave assemblages formed through accumulation of small skeletal elements by raptors, rodents, and carnivorans (Heaton, 1990; Hockett and Dillingham, 2004). In these contexts, fossils of large mammals can often be underrepresented due to constraints imposed by the size of their bones. For this same reason, sampling in caves can be skewed towards smaller genera or species of large mammals; Heaton (1990), for example, observed that remains of mammoths, sloths, bison, and ‘‘oxen’’ [sic] are rare in all late Pleistocene Great Basin assemblages, although smaller large mammals such as horses, camels, llamas, and bighorn sheep can often be found in abundance. Nevertheless, remains of Bison appear to be infrequent in this region. The genus is absent at late Pleistocene Crystal Ball Cave, and rare in assemblages from latest Pleistocene/earliest Holocene Hidden Cave and Danger Cave; at Mineral Hill Cave, the genus is represented by multiple elements, but is not as abundant as small horses, camels, llamas, pronghorn or bighorn sheep (Hockett and Dillingham, 2004). Jefferson et al. (2004) reported records of Bison from 20 of 96 Pleistocene localities in Nevada (exclusive of the Las Vegas Valley; see below), but many of these records remain to be verified. The relative paucity of Bison also extends to open (as opposed to cave) sites; for example, the peak-Wisconsin Rye Patch Reservoir fauna contains remains of multiple individuals of Equus and Camelops, but only a few individuals of Bison (Firby et al., 1987). An exception to the observed scarcity of Bison in the Great Basin is the Las Vegas Valley, including the Tule Springs locality, an open site where numerous late Pleistocene fossils are preserved (Simpson, 1933; Mawby, 1967; Springer et al., 2006, 2009a). Here, in the southern Great Basin/eastern Mojave Desert, remains of Bison are relatively abundant, more so than has previously been recognized. Bison was originally reported from the region by Simpson (1933) from a single locality in sediments later named the Las Vegas Formation by Longwell et al. (1965). Fossils identified included two skulls, several teeth, and other bone fragments. The skulls represented a relatively long-horned species, which Simpson (1933) assigned to Bison aff. occidentalis. A subsequent study (Skinner and Kaisen, 1947) assigned these fossils to B. alleni Marsh, 1877 (¼B. latifrons following McDonald, 1981), while McDonald (1981:225) interpreted one of the skulls to be a probable hybrid between B. latifrons and B. antiquus. Based upon the geologic descriptions of Simpson (1933), these fossils are likely from one of the older strata of this formation, unit B2 of Haynes (1967) (K. Springer, pers. comm., 2009); this unit has recently been dated to between 144 ka and 89 ka (Page et al., 2005:3). Subsequent records of Bison from the Las Vegas Formation, reported by Mawby (1967), were also from this older unit. Here, two individuals – a male and a female – were represented by partial skeletons at one locality, while one other locality yielded isolated teeth. Mawby assigned these remains to Bison sp., and emphasized that no remains of Bison were known from younger strata in the Las Vegas Formation. McDonald (1981) referred Mawby’s fossils to Bison antiquus antiquus; Scott and Cox (2008) preferred an assignment to Bison sp. cf. B. antiquus, based upon measurements of postcranial remains. Although the latter study did not include analysis of the fossils reported by Simpson (1933), it nevertheless appears that two species of bison, one longhorned and the other shorter-horned, are represented in the older unit B2 of the Las Vegas Formation. De Narvaez (1995) reported on remains of Bison sp. from the Las Vegas Formation at the Gilcrease Ranch site in the western Las Vegas Valley. Scott and Cox (2008) tentatively inferred that these fossils, which comprised roughly 17% of the large mammal fauna from the site based upon NISP, might derive from unit E1 of the Please cite this article in press as: Scott, E., Extinctions, scenarios, and assumptions: Changes in latest Pleistocene large herbivore..., Quaternary International (2009), doi:10.1016/j.quaint.2009.11.003 ARTICLE IN PRESS E. Scott / Quaternary International xxx (2009) 1–15 9 formation; this unit dates to between w15 ka and w7.5 ka BP. The first definitive record of Bison from unit E1 of the Las Vegas Formation was that of Springer et al. (2006), who reported bison fossils in association with a radiocarbon date of 14,780 Æ 40 BP; these remains were later assigned to Bison sp. cf. B. antiquus (Springer et al., 2009a). Coupled with the records provided by Simpson (1933) and Mawby (1967), this indicates that Bison was present at least intermittently in the southern Great Basin from as early as Æ144 ka through to the end of the Pleistocene. Additionally, it has been suggested that individuals of Bison may have been more abundant relative to other large mammal taxa in unit E1 than in earlier members of the Las Vegas Formation, although presently the evidence is weak. Jefferson (1992) estimated that Bison comprised 12% of the large mammal fauna from unit B2 of the formation, based upon NISP, while Scott and Cox (2008) suggested that the genus comprised 17% of the large mammal fauna from the younger unit E1 of the formation using NISP data from de Narvaez (1995). Scott and Cox (2008) also indicated that the percentage reported by Jefferson (1992) almost certainly overestimated the true relative abundance of Bison in the overall sample from unit B2, since two partial bison skeletons – one of them nearly complete – were included in the count (after Mawby, 1967). Other large mammal taxa in the sample were represented largely by isolated elements, and so the relative abundance of Bison reported by Jefferson (1992) was interpreted to be skewed. In any event, the suggested difference in abundance between the samples from the older unit B2 and the younger E1 is not well supported. The observed difference is not great, the sample sizes are too small for statistical rigor, and NISP is often not the best tool for interpreting relative abundance. It is hoped that the much more expansive sample of fossils from the Las Vegas Formation currently under study (Springer et al., 2006, 2009a) will provide more detail with respect to these questions. Elsewhere in the Mojave Desert, fossil remains of Bison are rare. Scott and Cox (2008) reviewed published ‘‘Rancholabrean’’ localities from throughout this region, and determined that most of the localities were not truly Rancholabrean because they lacked the index taxon, Bison. Further, some localities where remains of Bison had been reported were erroneous, based on misidentified elements. The earliest confirmed appearance of Bison in the Mojave Desert is that discussed above from the Las Vegas Formation between 144 ka and 89 ka. As observed by Scott and Cox (2008), all other dated records of Bison from the Mojave Desert are geologically young. A partial dentary of Bison sp. cf. B. antiquus, recorded from unit D of the Manix Formation at Pleistocene Lake Manix in the central Mojave Desert (Jefferson, 1968, 1987), is the only confirmed record of the genus from the site (Scott and Cox, 2008); unit D of the Manix Formation dates to between 35 ka and 19 ka BP (Jefferson, 1987). Multiple fossils of Bison sp. cf. B. antiquus are recorded from China Lake, in the northwestern Mojave Desert (Fortsch, 1978), in association with mammoth remains dated to roughly 18.6 ka BP (Davis and Panlaqui, 1978). At China Lake, Bison is less common than Camelops or Equus, but more common than other large mammal taxa. At Dove Spring Wash in the western Mojave Desert, a partial skeleton of Bison sp. cf. B. antiquus was directly dated to 16,860 Æ 1175 BP; additional remains of Bison were also recovered from an older sedimentary unit in this area, which was dated to 19,190 Æ 410 BP (Scott et al., 2001). This assemblage is too small for significant determinations of relative abundance. The youngest record of Bison in the Mojave Desert is that from unit E1 of the Las Vegas Formation, associated with a date of 14,780 Æ 40 BP (Springer et al., 2006); the actual relative abundance of the genus in the assemblage is an open question at present. In the Mojave Desert, then, there appear to have been two species of Bison present between 144 ka and 89 ka – one with long horns resembling B. latifrons, the other smaller, similar in size to B. antiquus. In later to latest Pleistocene localities, however, only the latter type is recorded from the fossil record of the Mojave Desert. This smaller, shorter-horned bison also appears to have been more common in the latest Pleistocene than earlier in the epoch in this region (Scott and Cox, 2008), although clearly more fossils and localities are necessary to confirm or refute this interpretation. Towards the Pacific coast, a similar picture emerges. In the Los Angeles basin, Miller (1971) described fossils from several localities; only a few of these sites yielded relatively whole faunas as opposed to isolated remains. Of the faunas described, that from the Costeau Pit locality, which dates to !40 ka BP (Miller, 1971; Jefferson, 1991), was the largest. In this fauna, horses (Equus) were the most abundant herbivores, comprising roughly 48% of the large herbivore sample, based upon MNI (Miller, 1971). Fossils of two species of Bison, B. latifrons and B. antiquus, were also present in this fauna; respectively, these species made up 10% (NISP ¼ 134, MNI ¼ 5) and 2% (NISP ¼ 4, MNI ¼ 1) of the large herbivore sample (Miller, 1971). This fauna was proposed to compare favorably in age with the Jinglebob and Cragin Quarry faunas of Kansas, among others, and it was thought probable that the Costeau material dated to the Sangamon interglacial or the early Wisconsin glacial (i.e., MIS 5a or 4). The percentages from Costeau Pit contrast sharply with those in evidence from the famed Rancho La Brea ‘‘tar pits’’ in Los Angeles, which are geologically younger, dating between w38 ka and w12 ka BP (¼MIS 2) (Marcus and Berger, 1984; Spencer et al., 2003). Here, fossils of Bison comprise the bulk (w43%) of the large herbivore sample, based upon MNI (data compiled from Stock, 1925; Marcus, 1960; Scott, 1991, 2001; Stock and Harris, 1992; Harris, 2001; Jefferson, 2001; Shaw, 2001; Farrell and Shaw, 2009). Two species, B. latifrons and B. antiquus, are present here as well, but B. latifrons is represented by only a few elements, while B. antiquus is represented by several thousand fossils. Further, Bison is the most common large herbivore in every major locality or ‘‘pit’’ sampled from the site. The dramatic reduction in population size inferred for North American midcontinent Bison based upon the limited genetic diversity noted by Shapiro et al. (2004) and Drummond et al. (2005) is nowhere in evidence at Rancho La Brea. Of course, the Rancho La Brea fossil deposits are taphonomically biased in many ways (Shaw and Quinn, 1986; Stock and Harris, 1992; Spencer et al., 2003), so the relative abundance observed in the large herbivore sample from this site might be called into question as an artifact of sampling. However, recent studies of a large sample of late Pleistocene vertebrates and invertebrates from the Diamond Valley Lake locality outside of Hemet in southwestern Riverside County, California (Springer et al., 2009b; this issue) have determined that similar relative abundance is exhibited by large herbivores from that region, as well. The most common large mammal represented in the Diamond Valley Lake local fauna, which dates from 60 ka to w11 ka BP (¼MIS 3 and 2), is Bison, followed by Equus and Camelops (Springer et al., this volume). Further, two species of Bison – a longhorned form assigned to B. latifrons, as well as the smaller B. antiquus – are present in the fauna, although only the latter species is common. Finally, fossils of B. latifrons in the fauna are only present in the older sediments in this region (Springer et al., this issue). The fact that the Diamond Valley Lake local fauna, which was recovered from an open-environment setting rather than from asphalt seeps, exhibit a similar representation among its large herbivores to that shown at Rancho La Brea is significant. The taphonomic factors operating at these two sites are very different, so the congruity of representation of Bison at these sites indicates that the observed distribution likely reflects the actual relative abundance of these large mammals in the living population. The fact that both the Diamond Valley Lake and the Rancho La Brea local faunas show a strong preponderance of Bison antiquus in MIS 3 and Please cite this article in press as: Scott, E., Extinctions, scenarios, and assumptions: Changes in latest Pleistocene large herbivore..., Quaternary International (2009), doi:10.1016/j.quaint.2009.11.003 ARTICLE IN PRESS 10 E. Scott / Quaternary International xxx (2009) 1–15 2, while the Costeau Pit fauna (considered to date to MIS 4) has abundant B. latifrons but very limited B. antiquus, indicates that B. antiquus increased in abundance with the onset and subsequent waning of the Wisconsin glaciation in southwestern North America. 3.3. Ecology of Bison and implications Living bison are large [300–1000 kg (Nowak, 1991)], aggressive, herd-dwelling ungulates (McHugh, 1972; Lott, 2002) that can consume up to 14 kg of forage and 30–40 l water per individual, per day, although these numbers vary with body weight. Bison are ruminants, which enables them to maximize the nutritive value of even low-quality forage (Janis, 1976; Van Soest, 1994). Bison are considered ‘‘keystone species’’ [species that have the predominant impact on ecosystem structure and function relative to other species (Mills et al., 1993)] in extant grassland ecosystems, since their behavior helps enhance vegetative diversity and maintain open grasslands that might otherwise undergo succession to woodland or forest (Knapp et al., 1999). Pertinent activities in this respect include grazing, dung and urine deposition, trampling, and wallowing (Knapp et al., 1999; Truett et al., 2001). With the possible exception of wallowing, which is difficult to establish for Pleistocene populations, all of these activities would have been more pronounced in individuals of Pleistocene Bison, which were larger than modern bison and consequently would have weighed more, consumed more, and presumably urinated and defecated more. Although Pleistocene bison are often considered to have been primarily grazers, like their modern counterparts (Nowak, 1991), this is not obligatory. Akersten et al. (1988), for example, examined plant fragments preserved in teeth of Bison antiquus from Rancho La Brea, and determined that only 13.4% of the material consisted of monocotyledonous plants – a sharp contrast with the w75% of monocotyledons recovered from dental boluses in the teeth of living bison. These data suggest that Pleistocene Bison at Rancho La Brea, and presumably elsewhere in southern California and the southwest, had diets that incorporated a much higher percentage of browse than those of living bison, although this may reflect opportunistic feeding. This in turn suggests that feeding habits of Pleistocene Bison may, at least in some geographic areas, have impacted the feeding habits of other browsing mammals, as well as grazers. These data are enhanced by recent isotopic studies that suggest the diet of Pleistocene bison at Rancho La Brea was dominated by plants using the C3 photosynthetic pathway (trees, shrubs, and cool-growing-season grasses) (Coltrain et al., 2004; Feranec et al., 2009), although small percentages of C4 plants (tropical, warmseason grasses and sedges) also appear to have been ‘‘regularly’’ consumed (Feranec et al., 2009:158). Other large herbivores from Rancho La Brea, in contrast, appear to have preferred only C3 plants (Coltrain et al., 2004), although Equus reportedly ingested very small amounts of C4 vegetation (Feranec et al., 2009). Thus at this locality, these large mammals were seasonally competing for similar foodstuffs, with only bison differing somewhat by also regularly incorporating C4 grasses. Nor is such competition unique to Rancho La Brea; across the continent, in Florida, late Pleistocene Mammuthus, Equus, and Bison consumed essentially the same diets as one another – although here, C4 plants formed a higher proportion of the diets of each animal, and these diets varied at different geographic locales across the state (Feranec, 2004). Based upon these data, it is clear that bison and other North American megafauna were competing for available resources. Given this apparent relative similarity in resource consumption, at least within select geographic regions, it is reasonable to propose that shifts in resource abundance and distribution due to changing climatic factors associated with the waning of the Wisconsinan glaciation would tend to increase competition for those resources. Reversals associated with the onset of the Younger Dryas stadial would likely have a similar effect, essentially keeping animal adjustments to resource shifts in a state of flux. Responses to such increased competition would be many and varied, depending upon a host of ecological factors, but could very reasonably be distilled down to some variant of one of three alternatives: adapt, move, or die. Where adapting or moving away were the preferred options, however, the widespread abundance and population density of Bison in North America at the end of the Pleistocene ensured that the response would of necessity have been essentially a unique event. Large mammals across the continent were faced with an altered community structure, and earlier responses to climate shifts – selecting different forage, reducing body size, or simply moving to a different area – would have required alteration or abandonment depending upon whether, and to what degree, bison were also competing for those same resources or regions. Further, communities where bison were rare or absent could also be impacted, as large mammals displaced by bison elsewhere moved in. Here, too, competition for resources would likely have increased as a result. The level of competition hypothesized here for late Pleistocene large mammal faunas does not need to have been dramatic. In this regard, note that Fiedel and Haynes (2004:127) observed with respect to the overkill scenario that ‘‘[v]arious simulations show that a very small increase in predation loss due to humans (less than 5%) can wreak havoc upon animal populations.’’ It is perhaps equally likely, although it remains to be demonstrated, that population losses of this small percentage could also accrue from climate-driven reduction of resources and consequent increases in competition for remaining food and water. Owen-Smith (1987, 1988, 1999) argued that the terminal Pleistocene elimination of megaherbivores (!1000 kg), particularly mammoths, might have been pivotal in driving the extinctions of other large mammalian species, since he interpreted mammoths to have likely been keystone species in Pleistocene ecosystems. Although this hypothesis fails to explain the apparent extinction of some Pleistocene large mammal species prior to the advent of human hunters or the extinction of mammoths (Grayson, 1991; Scott, 2004), it does encourage inquiry as to when and how such a mammothbased ecosystem – if one accepts that mammoths were in fact keystone organisms – might have been replaced by one in which bison became the keystone animals. Owen-Smith (1987, 1988, 1999) proposed that human hunters were responsible for exterminating the mammoths and thereby destroying the ecosystem. As an alternate hypothesis, it is possible that the relatively recent immigration, broadening distribution, and increasing density of another megaherbivore, Bison, coupled with changes in climate, would have led to competition of a kind that the mammoths (and other megafauna) had never before experienced. In this respect, it is suggestive to recall the Pleistocene population curves from radiocarbon dates documented by McDonald (1981) and Guthrie (2006), which showed bison increasing in abundance after w13 14C ka BP, prior to the reduction in abundance of other herbivores such as mammoth and horse. [It is acknowledged that the molecular data for bison in Beringia actually show a continuing population reduction (Shapiro et al., 2004), while midcontinent populations of Bison may have been going through a genetic bottleneck of their own (A. Cooper, pers. comm., 2009), at this time; however, lacking similarly-derived molecular data pertinent to estimating the relative abundance of other large mammals at this time, these molecular data do not obtain to the question at hand.] This remains an important point to consider: based upon the radiocarbon record, bison were expanding their numbers and geographic extent while the other megafauna were still abundant and widespread. Given finite resources, competition among these organisms must have resulted. Please cite this article in press as: Scott, E., Extinctions, scenarios, and assumptions: Changes in latest Pleistocene large herbivore..., Quaternary International (2009), doi:10.1016/j.quaint.2009.11.003 ARTICLE IN PRESS E. Scott / Quaternary International xxx (2009) 1–15 11 4. Conclusions Giving voice to a question asked by several scientists over many years of study, Fiedel (2009:30) asked ‘‘if Holocene warming was so disastrous for megafauna, why wasn’t there a wave of extinction around 125 ka in the last interglacial?’’ The nature of this query assumes that similar changes in climate should have similar results – an assumption that presupposes little or no change in the taxonomic composition and ecological requirements of species in earlier and later Pleistocene faunas, or in the responses of these species to changes in environmental conditions. The present review rejects this assumption, demonstrating that the composition of the late Pleistocene large mammal faunas inhabiting several regions of North America, including the southern Great Plains, the southern Great Basin, the Mojave Desert, and inland and coastal southern California, were in no way static, but instead were actively evolving through time – and so, would have responded to environmental fluctuations through time in unique ways. Essentially, the terminal Pleistocene in North America was biotically unique – and since extinction often derives from biotic responses to climate change, this uniqueness is pertinent. In particular, the nature, presence, and relative abundance of Bison were all changing at the end of the epoch. Entering continental North America south of the glacial ice at approximately 240 ka or shortly thereafter, Bison spread out across the continent, moving into new environments for which they were nevertheless ‘‘notably preadapted’’ (Guthrie, 1980:67). Long-horned forms such as B. latifrons were widespread up to the onset of the Wisconsinan glaciation; these massive, relatively solitary animals do not appear to have formed large herds, but were nevertheless large enough and widespread enough to have impacted native ecosystems. Subsequently, the smaller but more densely populous B. antiquus became the most widely dispersed species of bison in the epoch. Preferring more open habitats than B. latifrons, and traveling in herds, B. antiquus successfully colonized most of the continent, and in many areas became the most abundant large herbivore. The true abundance of these animals in Pleistocene North America remains a subject warranting detailed investigation, with molecular evidence and the fossil record presently yielding discordant answers. However, both of these approaches indicate that Bison was more numerous during terminal Pleistocene climate shifts than during any earlier, similarly intense climate change. These data force a reconsideration of the mechanisms underlying terminal Pleistocene extinctions in North America. As discussed, the reasons that climate change and consequent biological responses have been often discounted as the sole cause of these extinctions are: (1) the latest Pleistocene climate fluctuations were no more severe than those occurring several times earlier in the epoch; and (2) the Pleistocene megafauna had survived all earlier such climatic shifts. Recognizing that ‘‘the megafauna’’ was not a static construct, but rather an actively changing amalgam of organisms exploring diverse evolutionary trajectories, as demonstrated herein, effectively negates this interpretation. The hypothesis presented here differs subtly from one advanced by Guthrie (1980) to explain why bison might have survived the endPleistocene extinction. Guthrie (1980) proposed that bison in North America had a competitive advantage over other megafauna as continental biotic communities reorganized due to terminal Pleistocene changes in climate. In that study, bison survived while other megafauna perished, but climate change and resulting environmental responses apparently caused the extinction (Guthrie, 1980). Herein, the proposal is that competition from bison, in combination with climate-caused changes in resource abundance and distribution, directly drove the megafaunal extinction event. Barnosky et al. (2004a) proposed that climatic warming primarily affects animals in lower trophic levels and of smaller body size, which contrasts with the pattern observed in latest Pleistocene North America. The present study proposes that the North American pattern is the result of climate change coupled with a robust and consequential immigration event: the immigration of Bison into North America circa 240 ka. This event would have demanded dramatic biotic reorganizations within the upper trophic levels of biological communities across much of the continent; the ongoing evolution, expansion, and increased density of the genus through the remainder of the Pleistocene would have ensured that these reorganizations remained in flux. Nor were these reorganizations uniform across the continent, but rather would have differed depending upon various geographic constraints. Because of these facts, any inferences that megafaunal response to climate changes at the end of the Pleistocene would have resembled most or all earlier such responses are unwarranted. Biologically, the terminal Pleistocene was a unique time in North America – faunas were not simply linear continuations of earlier faunas, but exhibited different taxonomic compositions and ecological interactions. Consequently, the responses of large mammal communities to environmental changes were also unique. Martin (2005) established three criteria that in his opinion must be met for any climate-based explanation of the terminal Pleistocene extinction to be plausible: ‘‘First, the evidence must show that there in fact was significant climate change around the various times of the extinctions in the various places they occurred. Second, the change (or changes) must, alone or in combination with other factors [emphasis added], have been unique in the Quaternary. A change closely resembling others that the megafauna had repeatedly survived . is not a good candidate in the search for explanations of extinction. Third, the change must have been one capable of striking large terrestrial mammals while sparing most other terrestrial animals, as well as plants and marine life’’ (Martin, 2005:167). It is proposed here that increasing levels of competition for dwindling resources brought about by the late immigration and subsequent widespread dispersal of Bison throughout much of continental North America, coupled with terminal Pleistocene changes in climate, may answer each of Martin’s (2005) criteria. Climate change at this critical time has been reliably documented (e.g., Porter, 1989; Bond and Lotti, 1995), while the advent, distribution, and increase in abundance through time of Bison have been reported elsewhere (e.g., McDonald, 1981; Guthrie, 2006); these data are corroborated and enhanced herein. The combined impacts of these events would be most likely to primarily affect large mammals, although this remains to be explored more fully. If climate change and resultant biological pressures can be reestablished as a viable hypothesis for explaining terminal Pleistocene North American extinctions, as proposed here, it remains to be determined the actual mechanics of the event: whether by loss or fragmentation of habitat, reduced carrying capacity for herbivores due to phenologic shifts, disruption of established resource partitioning, or some combination of these factors. The point to be emphasized is that, irrespective of which of these means is the primary force at work, the widespread presence and abundance of a large, aggressive animal such as Bison (herd-forming in B. antiquus and later species) would have had a substantial impact in any of these eventualities. Further, this impact would be effectively unique, and could not be automatically inferred to have been similar or identical to effects during earlier changes in climate, since bison themselves were actively evolving during the latest Pleistocene. Of course, other explanations for the terminal Pleistocene extinction event in North America – overkill, hyperdisease, and bolide impact/detonation – continue to remain in play, to the extent that they can be tested and falsified. But based upon the present Please cite this article in press as: Scott, E., Extinctions, scenarios, and assumptions: Changes in latest Pleistocene large herbivore..., Quaternary International (2009), doi:10.1016/j.quaint.2009.11.003 ARTICLE IN PRESS 12 E. Scott / Quaternary International xxx (2009) 1–15 study, these scenarios can no longer fall back on any perceived weakness in the climate change hypothesis in order to garner support where their own limitations are exposed. Each of these proposals must stand or fall on its own merits. As always, more work needs to be performed in order to evaluate the merits of this analysis. It should be clear from this review that many late Pleistocene mammalian faunas lack sufficient specimen abundance to provide statistical rigor, or else these data have not yet been published. Many faunas remain to be fully documented, with accurate radiometric dates, details on numbers of individuals and relative abundance of species, and careful accounting for taphonomic factors among faunas deposited in differing circumstances (e.g., open versus cave deposits) where possible. These important data can also be supplemented with information gleaned from stable isotope analyses, to better understand how late Pleistocene large mammals interacted and competed during environmental fluctuations. These data are critical for enhancing our interpretations of the ecologic needs and constraints, both biotic and abiotic, of individual species. Paleoenvironmental data from plant and pollen remains and other contextual information will also be pertinent to refining our hypotheses, by providing baseline data from which to determine available food niches, compare feeding preferences, and assess areas of potential competition and/or conflict in feeding among the large herbivores. But it is essential that we move away from standing arguments based on old dichotomies seeking affirmation more from perceived weaknesses in alternative scenarios than from real data. This study is intended as a step in this direction. Grayson (2007) has argued that, in order to more fully understand the terminal Pleistocene extinction, it is necessary to better grasp the histories of assemblages of species by deciphering the history of each individual species within such assemblages. Clearly, the evolutionary history of Bison in late Pleistocene North America is a textbook example of this approach. 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