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Synchronous extinction of North America’s Pleistocene mammals — PNAS

Following on from my recent post regarding the apparent lack of evidence for a Clovis comet, I want to address this recent paper by J. Tyler Faith, in which he and his fellow authors offer statistical evidence to suggest that the mammalian extinction event at the very end of the Pleistocene largely took place between 12 kya and 10 kya.

I had originally intended to review this paper alone, but after only a cursory amount of reading around, it soon became clear I needed to address some of the other research into this mysterious time, without recourse to an extraterrestrial impact to explain the sudden disappearance not only of large bodied mammals, but also significant numbers of birds at the same time, an anomaly that cannot be explained away simply by inferring overkill of mammals by human predators, hyperdisease or climate change as the main agents of destruction.

Rather than try to offer a specific answer to this massive extinction question of my own, I have instead attempted here to include consideration of a range of factors and circumstances over a longer period of time and from further afield (in Eurasia) to help explain the unique context in which the American extinctions took place. For reasons of necessitated brevity, I have in some cases only referred in passing to some papers, most of which are open access.  But suffice it to say, all these papers are worth reading in their entirety, if only to emphasize the idea that there is probably an intricate network of causal complexity that best accounts for the North American terminal extinction event, rather than a single, albeit spectacularly compelling cause that now as in the past,  is notable for its extremely elusive nature, and which in all likelihood, doesn’t actually exist.

This paper, by Faith et al, addresses the long-standing debate surrounding the timing of the megafaunal extinction event in the terminal Pleistocene, and in so doing trying to identify the contributory factors that led to the demise of 31 genera of fauna at that time. Here’s the abstract:

The late Pleistocene witnessed the extinction of 35 genera of North American mammals. The last appearance dates of 16 of these genera securely fall between 12,000 and 10,000 radiocarbon years ago (13,800–11,400 calendar years B.P.), although whether the absence of fossil occurrences for the remaining 19 genera from this time interval is the result of sampling error or temporally staggered extinctions is unclear. Analysis of the chronology of extinctions suggests that sampling error can explain the absence of terminal Pleistocene last appearance dates for the remaining 19 genera. The extinction chronology of North American Pleistocene mammals therefore can be characterized as a synchronous event that took place 12,000–10,000 radiocarbon years B.P. Results favor an extinction mechanism that is capable of wiping out up to 35 genera across a continent in a geologic instant.

In particular Faith et al set out to answer the question of whether it was an event drawn out over many millennia or one that involved simultaneous extinctions in the terminal Pleistocene  – with the implication that if this was a  staggered event there may be a gradual change in the environment that was the cause, whereas a for sudden event, one or more extraordinary explanations need to be sought.

Indeed, they conclude that their statistical models indicate the latter scenario, although other (undefined) scenarios were possible, with some extinctions – between 0 and 8 genera –  taking place prior to 12 kya, and that further the idea of comet strike or human hand are generally considered to be two of the most likely of an unknown number of as yet inconclusive theories that might have contributed to this simultaneous extinction event.

The authors propose that sampling error may account for the lack of information, concluding that two main causes might be responsible for the disappearance of these animals in the space of two thousand years, roughly equating to between 12 kya and 10 kya – an extraterrestrial impact, or overkill by humans. As noted elsewhere, the Last Apppearance Date for a species in the fossil record, doesn’t necessarily mean that every single animal had died by then, but that numbers may have been so reduced as to make them  ‘palaeontologically invisible’ but they were still in effect, extinct as a viable population – this invisibility is something that may turn out to be relevant to the question of credibly identifying a very early pre-Clovis human presence in the Americas, especially when addressing some of the claims of 50 kya, from sites such as Topper, and 40 kya from Mexico and Baja California.

However, as we have seen, and assuming the recently published data are correct, there is at present very little if any substantive geochemical evidence for a comet or asteroid strike. And as has been noted previously, it’s hard to imagine there were enough humans on the ground in Pleistocene America to effect such a widespread die off, regardless of whatever lithic technology they deployed – or even that they would deliberately have killed order of magnitudes more animals than they could eat or would have needed to process into manufactured artefacts, such as clothing from hides or utensils from teeth, ivory, sinews and gut etc.

The researchers used two simulations to assess the synchronous extinction theory, the first of which is referred to as the continental simulation, described thus:

In our first simulation, referred to as the continental simulation, each of the 1,955 stratigraphic occurrences are assigned randomly a pre- or post- 12,000 radiocarbon years B.P. date based on the observed relative frequency of terminal Pleistocene taxon dates in the fossil record (3.4% for the complete set of radiocarbon dates and 1.3% when excluding radiocarbon dates of intermediate reliability). For each of 10,000 iterations, the number of genera receiving a terminal Pleistocene taxon date is calculated. All of the 31 genera included in the analysis are assumed to survive to the terminal Pleistocene, and all occurrences are assumed to be equally likely to receive a terminal Pleistocene taxon date.

The continental simulation essentially estimates how many genera that we can expect to recover from the terminal Pleistocene if all of them had survived to that time, given the empirically derived probability of observing a terminal Pleistocene fossil occurrence. We ran two separate trials of the continental simulation, both including and excluding radiocarbon dates that received intermediate evaluation scores (30).

Next they describe their biogeographic simulation:

Our second simulation, referred to as the biogeographic simulation, recognizes that the extinct Pleistocene genera were not distributed uniformly across the continental United States and that some regions are more likely to provide a terminal Pleistocene taxon date than others. For example, the distribution of Hydrochoerus and Holmesina within the U.S. is limited to the southeast (28), an area that yields relatively few terminal Pleistocene taxon dates (Table S4).

Because of their biogeographic ranges, these taxa are less likely to have been recovered from terminal Pleistocene deposits if they had survived to that time. This issue is addressed in our biogeographic simulation, which recognizes seven physiographic zones within the continental United States (31) (Fig. 2). In this simulation, the stratigraphic occurrences of a given genus are assigned randomly to a physiographic zone based on its relative abundance in that region (Table S4). In turn, the probability that a simulated occurrence will be assigned a terminal Pleistocene taxon date is based on the relative frequency of terminal Pleistocene fossil occurrences known from that zone (Table S4). For each of 10,000 iterations, the number of taxa receiving a terminal Pleistocene date is calculated. The biogeographic simulation also explores the possibility of preterminal Pleistocene extinctions.

To do so, we prohibited between 0 and 15 randomly selected genera from receiving a terminal Pleistocene taxon date over 16 separate trials of 10,000 iterations. The biogeographic simulation estimates how many taxa that we can expect to recover from the terminal Pleistocene if anywhere from 16 to 31 genera had survived to that time. As with the continental simulation, we ran two trials of the biogeographic simulation, once using all of the terminal Pleistocene taxon dates and once excluding radiocarbon dates of intermediate reliability (30).

The authors conclude that all their models can be interpreted as being in accordance with an abrupt event at the end of the Pleistocene, although in contrast to the two previous papers, they contend that a cometary impact may have been one of four main options for consideration as a contributory factor – some of the others being overkill by humans, hyperdisease and climate change.

Obviously it’s very difficult to tell whether disease was a main cause, because there are no carcasses exhibiting pathology, or exactly what the nature of that disease may have been – did for example, the extinction of one or more species cause parasites to migrate to different hosts and expose them to new diseases to which they had no resistance, or was there something about the human incursion that caused it. I’ll discuss disease in the guise of avian influenza, later in this post.

It might seem surprising that all the extinctions could have taken place within a mere 2 thousand years, as opposed to a more staggered event where one population crash had a chain effect on others, with the extinctions beginning well before the terminal Pleistocene. In Europe for example there is good evidence to show that different fauna became extinct in different places and at different times, rather than all dying together in some cataclysm, and this too is a subject I’ll address again later.

If humans alone were to blame for the American extinctions, it seems reasonable to assume that their influence on the flora and fauna would have been felt well before the 12 kya-10 kya period envisaged by the research.

I should add here that I don’t have the necessary background in statistics to be able to judge the merit of various models in the context of others that may evince slightly (or starkly) different scenarios, and it may well be the case that others in the field have their own ideas as to whether those presented here tell the whole story, or whether other models can offer alternative options for research.

Whilst writing this I noticed that Proc R.Soc B just published this paper,  The Role of Extinction in Large-Scale Diversity–Stability Relationships to see if any clues could be gleaned from the diversity-stability relationships over geologic time between reef builders and reef dwellers – as opposed to ecological time, which occurs more on the scale of millennia and their sub-divisions. Although of course the paper doesn’t offer any direct clues to the putatively sudden extinction event in the New World,  especially as it’s set in a marine environment, it does offer a look at the long-term cycle of diversity and extinction, which by default would appear to emphasise the extreme nature of the sudden ecological events during, and at the end of, the Pleistocene that is claimed to have taken place in only 2,000 years, the rough equivalent of a geologic nano-second. When an entire reef community in a single location dies, something has altered in the local ecology, and as we have seen in recent times, reefs separated in bio-geographical space are also dying off, implying that global changes, almost certainly the result of human activity, are affecting reefs as a whole.

In the case of the reefs it would appear that human activity has disrupted the complex environment needed to support reef life, and if we’re going to exclude extraterrestrial cometary action as a cause of New World extinctions, humankind’s recent arrival there n large numbers would appear once again to be a prime candidate. However I’m not at all sure that this was because of a directly attributable Overkill event, because I doubt there were enough humans around to embark on such a killing spree, but for now an obvious alternative remains as elusive as ever.

Moreover, as Firestone et al mention in their paper, there is an absence of kill sites for 33 of the extinct genera in the New World, including the camel and sloth – this would indicate that if humans were killing off these genera and their constituent species, they weren’t hunting for resources, but were killing for other reasons that may have involved some kind of sport. We might surmise that humans suddenly encountering vast numbers of animals they’d never seen before may have felt an uncontrollable urge to hunt them down for trophy values, for example, or keeping a score. Kill sites wouldn’t appear because the animals would most likely not have been butchered and processed in the same way as traditional prey species, or even at all, and thus the carcasses of sport prey would have been subject to other taphonomic and scavenging processes which obliterated all signs of the events surrounding their deaths.

This type of hunting would have a far more deleterious effect on the prey – a traditional hunting party would kill their prey and then process the carcass, leaving the surviving animals to continue their lives. But hunters interested only in killing wouldn’t need to stop once they’d killed a target animal, instead continuing the killing without pause. A well planned expedition might result in numbers of killed animals several orders of magnitude greater than casualties inflicted by hunters killing one or several animals, then ceasing the hunt in order to butcher the meat and glean skins, and other body products for later use.

Interesting too that these activities may have occurred precisely when the Younger Dryas was in phase, though weather the cooling climate itself motivated people to go out kill-hunting is again an open question. But it would seem clear that the YD would have adversely affected the resources available to the hunted prey, and the populations thus found themselves being squeezed by two different but equally deadly forces, and succumbed to a combination of the two. My point here being that it’s almost certain that there was no single contributory factor to the Pleistocene extinction, and that a mix of at least two and quite possibly more synchronous events and scenarios were to blame – but as we shall see, it’s unlikely that even a combination of these two factors was the main force behind the extinctions.

If we accept the paper’s findings in tandem with a pre-Clovis population model, we might ask why earlier humans didn’t attack the mega-fauna in the same way as the later Clovis people, but again care is needed – earlier extinctions could have taken place, which then might imply that pre-Clovis people too were engaged in hunting the faunal population for motives un-related to food and resource gathering, but it seems an unlikely scenario.

Moreover, if humans had for some bizarre reason been slaughtering vast numbers of herbivores, and not processed them for food, it might be expected that there would have been a population boom in scavenger species, such as vultures, hyaenas and rodents, but it would appear no such increase in their numbers at this time has been detected in the fossil record.

So although there is absolutely nothing in the archaeological or related records to support or even directly suggest such an overkill by wanton hunting, (such as caches of camel skulls or sloth tail-bones, both animals that went extinct, no relics of story-telling or mythology) maybe the lack of other credible clues offers circumstantial evidence of such overkill activities.

The carbon-rich black mat, above which no Clovis tools or extinct mammals appear still needs to be defined more clearly from a causal perspective – were people deliberately setting fire to the landscape, trying to destroy it as well as the mega-fauna? Was it even caused by burning at all?  This  fully accessible paper, Younger Dryas “Black Mats” and the Rancholabrean Termination in North America, by C. Vance Haynes Jr. in 2008, explores various aspects of its formation, whilst noting that dating for the top and bottom of the profile needs to be further defined. He attributes the presence of this carbon-rich layer to soil interactions with groundwater levels, rather then a landscape set alight from on high, or humans on the ground.

On the subject of the carbon rich layer, I was interested by an observation in another paper, Carbon Starvation in Glacial Trees Recovered from the La Brea Tar Pits, Southern California, (fully accessible), notes this curious fact:

The climate of the late Pleistocene involved a series of pronounced glacial/interglacial cycles, with glacial periods characterized by low temperatures and reduced atmospheric CO2 concentration ([CO2]) (1). During the last glacial period, minimum [CO2] occurred between 18 and 15 thousand years (kyr) B.P. (radiocarbon age) at values of 180–220 ppm, and modeling efforts suggest that such glacial values were among the lowest that occurred during the evolution of higher land plants (2)

Although I’m not sure how directly this affected events in the ensuing millennia, or if there was any input at all into the later faunal extinctions, the fact that one unusual carbon-based event was followed by another equally anomalous event as indicated by the black mat, would at the very least indicate that there were additional environmental stresses within the Pleistocene era that might warrant further investigation.

We have too the curious phenomenon of the nano-diamonds, cited by many to have been a direct result of cometary or similar impacts, and for those who have access, Shock-Synthesized Hexagonal Diamonds in Younger Dryas Boundary Sediments by Kennet et al should shed some light on their presence, despite the fact that other impact markers, commonly associated with them, such as Platinum Group Elements, have not been found so far. This is indeed a mystery in its own right, because there is nothing obvious to explain their presence from a terrestrial perspective, and it remains to be seen what part if any, they play in the overall story.

Although this study by Faith and his team goes into some detail regarding those mammalian genera that became extinct, and offers tentative explanations for their demise, it would have been even more interesting to see some discussion on those genera of large mammals (excluding humans) that did survive, and more importantly why they didn’t succumb to whatever disaster befell what had been their contemporary mega fauna.

On that note, it’s time not only to consider which mammalian species survived the terminal Pleistocene but also to look at the significant avifaunal – or bird – extinction event that appears to have strong correlations with the mammalian event. As Donald Grayson noted in 1977, in ‘Pleistocene Avifaunas and the Overkill Hypothesis(PDF):

During the North American Pleistocene, 22 genera of birds are known to have become extinct (Table 1 ) . In order to determine the proportion of these genera which became extinct at the end of the Pleistocene, it is necessary to turn to paleontological sites which contain avifaunas, demonstrably deposited between about 13,000 and 10,000 B.P….

…These dates indicate that the end of the North American Pleistocene saw the extinction of at least ten genera o f birds. That is, o f the 22 genera of North American birds known to have become extinct during the Pleistocene, 45 percent became extinct at the end of this epoch. This figure is nearly identical to that derived for mammals, and does not support the overkill hypothesis.

I had some difficulty tracking down much in the way of accessible papers that discuss the bird extinctions from a broad perspective, but this one, Avian influenza as the Cause of Late Pleistocene Mammalian Megafaunal Extinction in the Americas‘ by  (PDF – full) does provide some intriguing insights, not only on avian extinction, but also into the large mammals that survived the Pleistocene. Here’s the abstract:

We attribute Late Pleistocene mammalian extinctions in the Americas to an avian influenza virus originating in Southeast Asia. We hypothesize that an antigenic shift produced a viral strain that was asymptomatic in waterfowl but highly pathogenic to mammals though not necessarily contagious among them. Migratory American waterfowl sharing breeding grounds with Asian waterfowl transported the virus throughout the Americas. Mammals inhabiting regions with shallow bodies of water contaminated by infected migrating waterfowl were most vulnerable.

A logistic regression of estimated level of water contamination, mass, and reproductive rate against extinction status for 383 species of mammals living in North America at the end of the Pleistocene supports this model. Some characteristics of the pathogen proposed for this model closely resemble reported characteristics of currently circulating strains of avian virus, suggesting an effective first line of defense against all avian viruses would be protection of sources of drinking water.

The authors remark on the types of large-bodies mammals that survived, many of which were forest-dwellers in the guise of elk, moose and white tailed deer, whilst the survival of plains dwellers such as modern bison and pronghorns might have survived at higher elevations in the Rocky Mountains, well away from the watering holes on the plains, which the authors suspect may have been harbouring the avian influenza.

The observation that forest dwelling large mammals survived is particularly telling when considering claims that a cometary or other impact event caused massive burning of the landscape which killed off so much of the fauna – if such fires had been so widespread, it seems reasonable to assume that those forests would have been destroyed in the inferno, killing the indigenous fauna, mammalian and avian alike, in the process. The fact that forests appear largely to have survived intact would then appear to argue strongly against any fireball event or subsequent wild-fires.

Observations in the abstract of a paper, Dietary Controls on Extinction Versus Survival Among Avian Megafauna in the Late Pleistocene by J. Alden Lackey et al, explain further the relationship between inland avian extinctions and corresponding coastal survival, as we see:

The late Pleistocene extinction decimated terrestrial megafaunal communities in North America, but did not affect marine mammal populations. In coastal regions, marine megafauna may have provided a buffer that allowed some large predators or scavengers, such as California condors (Gymnogyps californianus), to survive into the Holocene. To track the influence of marine resources on avifaunas we analyzed the carbon, nitrogen, and hydrogen isotope composition of collagen from late Pleistocene vultures and raptors, including species that survived the extinction (condor, bald eagle, golden eagle) and extinct species (teratorn, black vulture). At the Rancho La Brea and McKittrick tar pits of southern California, isotope values for extinct teratorns (Teratornis merriami, n = 10) and black vultures (Coragyps occidentalis, n = 8) show that they fed entirely in a terrestrial C3 ecosystem.

In contrast, La Brea condors cluster into two groups, one with a terrestrial diet (n = 4), and the other with a strong marine influence (n = 5). At localities in the American southwest, Texas, and Florida, where condors became extinct, they have isotope values indicating entirely terrestrial diets (n = 10). Our results suggest that dependence upon terrestrial megafaunal carrion as a food source led to the extinction of inland California condor populations and coastal populations of teratorns and black vultures at the Pleistocene-Holocene boundary, whereas use of marine foods allowed coastal condor populations to survive.

In my opinion, this distinction between birds that preyed on plains dwelling fauna and those that lived in a more littoral environment does indeed point strongly towards the possibility that the inland water resources could have been key in transmitting disease, or an as yet unknown toxic element into the mammalian populations, whose avian predators and scavengers suddenly faced a dramatic shortfall in available food, as well as getting ill themselves, whilst those mammals living far from putatively contaminated water were much less vulnerable to the deleterious effects of that water.

To consider the North American extinctions in their global perspective, I want next to refer to another fully accessible 2007  paper, ‘Deciphering North American Pleistocene Extinctions’ authored by Donald Grayson to whom I referred above. Here’s part of his introduction:

…I will argue that the debate is not likely to be solved unless we take to heart, and act on, one of the prime lessons taught us by historic biogeography during the past 50 years: that understanding the histories of assemblages of species requires that the history of each of those species be analyzed on its own. The benefits of this approach are clearly shown by contrasts between our understanding of the North American losses and the roughly comparable extinctions that took place in Eurasia. The former remain unexplained, but there is only muted debate over the causes of the latter. Recent advances in understanding Eurasian extinctions provides a research guide for extracting ourselves from the explanatory morass that now characterizes the North American situation.

The point he makes throughout the paper is that although large extinction events are often regarded as discrete occurrences, not everything becomes simultaneously extinct, as we see for example in the case of Late Pleistocene/Upper Palaeolithic Europe, where there is much greater evidence of staggered extinctions as well as migrations which saw displaced populations surviving at locations far removed from original habitats – as has been the case in some American examples. Unlike America, Eurasia witnessed one (particularly regrettable) extinction, that of the Neanderthals, another  mystery whose specific causes and termination date are still the subject of lively debate.

He continues in the same vein in another paper, Holocene Underkill, where island populations and extinctions further illustrate the complex reasons surrounding why some populations become extinct, others seem to miraculously recover, and others proliferate in the face of ostensibly deadly contact with humans.

Despite the rather meandering nature of this post, and the many points I haven’t yet addressed, I hope at least to have conveyed in broad outline some of the many avenues of research that directly affect our perception of the North American Pleistocene extinctions from both a global and local perspective. Clearly, I could have written at much greater length as each of the supplementary papers referenced here warrant a great deal more comment than offered just now, and moreover, I haven’t covered the initial paper in quite the detail I’d first envisaged, which is of course what set me off on this trail in the first place. But I hope at the very least to have placed it in a relevant context with these other papers, which in my opinion, together do a great job in what John Hawks would hopefully refer to as “eschewing reductionism”.

I have no doubt that we will return to these discussions many times in the future, but for now all that remains is for me to do is thank J. Tyler Faith for forwarding me a copy of his paper, as well as Todd Surovell’s paper ‘An Independent Evaluation of the Younger Dryas Extraterrestrial Impact Hypothesis’, which as extant and eagle-eyed readers will notice, I still haven’t finished writing up either – stay tuned.

References: Synchronous extinction of North America’s Pleistocene mammals by J. Tyler Faith and Todd A. Surovell, Published online before print November 23, 2009, doi: 10.1073/pnas.0908153106    PNAS  December 8, 2009   vol. 106  no. 49  20641-20645

Evidence for an Extraterrestrial Impact 12,900 Years Ago that Contributed to the Megafaunal Extinctions and the Younger Dryas Cooling , Firestone et al, Published online before print September 27, 2007, doi: 10.1073/pnas.0706977104    PNAS  October 9, 2007   vol. 104  no. 41  16016-16021 Open Access

Dietary Controls on Extinction Versus Survival Among Avian Megafauna in the Late Pleistocene (Abstract) by Kena Fox-Dobbs et al, Geology; August 2006; v. 34; no. 8; p. 685-688; DOI: 10.1130/G22571.1 © 2006 Geological Society of America

Younger Dryas “Black Mats” and the Rancholabrean Termination in North America (Full) by C. Vance Haynes Jr., Published online before print April 24, 2008, doi: 10.1073/pnas.0800560105    PNAS  May 6, 2008   vol. 105  no. 18  6520-6525

Deciphering North American Pleistocene Extinctions – by Donald K. Grayson, Journal of Anthropological Research, in press (2007 Distinguished Lecture)

Holocene Underkill (Extract) by Donald K. Grayson, Published online before print March 11, 2008, doi: 10.1073/pnas.0801272105    PNAS  March 18, 2008   vol. 105  no. 11  4077-4078

Pleistocene Avifaunas and the Overkill Hypothesis (PDF – full) by Donald K. Grayson, Science 18 February 1977: Vol. 195. no. 4279, pp. 691 – 693 DOI: 10.1126/science.195.4279.691

Shock-synthesized Hexagonal Diamonds in Younger Dryas Boundary Sediments (Abstract) by Douglas J. Kennett et al, Published online before print July 20, 2009, doi: 10.1073/pnas.0906374106    PNAS  August 4, 2009   vol. 106  no. 31  12623-12628

Avian Influenza as the Cause of Late Pleistocene Mammalian Megafaunal Extinction in the Americas (PDF – full) by J. Alden Lackey et al, 2006

The Role of Extinction in Large-Scale Diversity–Stability Relationships by Carl Simpson and Wolfgang Kiessling, Published online before print December 9, 2009, doi: 10.1098/rspb.2009.2062 – Open Access

Carbon Starvation in Glacial Trees Recovered from the La Brea Tar Pits, Southern California (Full) by Joy K. Ward et al, Published online before print January 10, 2005, doi: 10.1073/pnas.0408315102    PNAS  January 18, 2005   vol. 102  no. 3  690-694

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