Hey there every peoples!
The Pleistocene extinction has rightly been dubbed by some as the Olympics of paleontology. Except that the actual Olympics can decide a winner. For decades scientists have been embroiled in a raging debate about what killed off North America’s megafauna at the end of the Pleistocene epoch over 11,000 years ago. Just so you know scientists define megafauna as animals weighing 100 pounds or more (though when you look at the beasts of the ice age, 100 pounds is puny. I say you have to weigh at least 600 pounds to be considered megafauna).There are currently three competing hypotheses as to why such beasts as mammoths, mastodons, horses, camels, ground sloths, sabertooth cats, giant jaguars, and dire wolves vanished from the face of the earth … Well, technically there are four but that hyperdisease hypothesis is so ridiculous I don’t know why it keeps getting brought up in discussions on this subject. The first hypothesis posits that rapid and unstable climate change at the end of the Pleistocene wreaked havoc on the animal’s habitat, driving them to extinction. Critics state that the animals survived similar climate events in the past, bringing up the question “Why did these mega mammals survive so many climate fluctuations only to succumb to this one?” Others claim that overhunting by the continent’s first humans is to blame. Based on kill sites such as Dent, Colorado; Kimmswick, Missouri; and Colby, Wyoming, some paleontologists think that animals who never encountered humans would have made for easy targets and as humans spread across the continent they left a trail of exterminated beasts in their wake. But this hypothesis has problems of its own, namely that early humans lacked the means to cause such widespread damage to animal populations and evidence is mounting that hunting megafauna wasn’t as common as previously thought. A third and more recent hypothesis is that a comet entered the atmosphere and exploded somewhere over Canada, triggering a repeat of the Cretaceous extinction. But… Paleoindians didn’t decline at the time this comet supposedly hit. And such an event would have triggered extinctions across the globe but the fossil record shows that different continent’s megafauna died at different times. Many suggest that it may have been a combination of factors (quite likely, as things in nature are never simple), but others cling to single cause scenarios and the debate continues.
Enter another contender. He be Eric Scott, curator of paleontology at the San Bernardino County Museum in Redlands, California (you may remember him from earlier posts, like the Lake Manix post). He posits a different hypothesis. In a paper published a couple years ago (only recently obtained because he sent me a copy) he points out that the climate, overkill, comet, and even hyperdisease ideas all have something in common: they all assume that the fauna itself remained the same and that only external forces could have driven the extinction. But animals are never static. They are always changing, constantly in flux based on a wide variety of factors. And because of this, Eric has put forward a hypothesis unlike all the others: bison may have had a hand in the Pleistocene extinction.
How is that possible? Well that’s what this paper is hoping to find out (this is my first time blogging about a technical paper, so all I request is a little slack. If you would like a copy of the paper, just email me and I’ll send you one). Scott begins the paper by discussing the various scenarios I explained above. Then he moves on to when bison entered midcontinent North America. Just when bison entered the continental United States (midcontinent North America) has not been established with any certainty. Since the appearance of bison heralds the beginning of the Ranchelabrean NALMA (North American Land Mammal Age), determining just when they came onto the scene is important. A bison ankle bone found in the 240k to 220k year old Ten Mile Hill Beds in South Carolina was used to set the beginning of the Rancholabrean but this interpretation has not been widely accepted. Scott goes on to explain why this time frame may be a little more solid:
Bell et al. (2004) defined the beginning of the Rancholabrean by
the first unequivocal appearance of Bison in North America south of
55N. A minimum arrival time of 160 kawas 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 60 ka and 72 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 for bison have been proposed but none have withstood rigorous examination. For example a supposed Bison fossil from Lake Manix in the Mojave Desert was thought to be 290k years old. This fossil later turned out to be the sacral vertebra of a camel. Perhaps the most freakish outlier would be alleged horn core fragments of bison from the Macasphalt Shell Pit and Inlgis 1A sites in Florida. The pit is late Pliocene in age, around 2.6 million to 2.2 million years old while Inglis 1A is between 2.2 million and 1.8 million years old. If these finds were confirmed to be bison, it would force a redefinition of the Rancholabrean NALMA. But these fossils suffer from dubious identification (horn core fragments, mind you) and hail from uncertain stratigraphic positions. Scott further explains that given the lack of Bison from other Irvingtonian sites, these Florida specimens, if genuine, would represent “an early immigration pulse for the genus that eventually perished.”
So using the time frame of 240k to 220k years ago, the next challenge is to figure out the abundance of bison through time. This is far easier said than done because fossils sites can be biased. Furthermore, there were at least 2 species of bison in the midcontinent North America: Bison latifrons, a large long-horned species and Bison antiquus, a species larger than it’s descendant but still smaller than B. latifrons. But most importantly, many sites containing bison cannot be reliably dated either because they extend beyond the reach of carbon dating or lack datable materials. But these are nonetheless obstacles, not road blocks. One method Scott discusses to assess the 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.” Sites on the southern Great Plains hint that bison had become more numerous at the end of the Pleistocene: Bison were abundant in localities less than 20k years old. But in sites before that time, bison are outnumbered by horses, camels, and mammoths. But these finds were not quantified, and only 16 of the sites in the survey had dates considered to have been obtained from reliable materials.
Luckily bison ranged over the whole of midcontinent North America. But even this can be troublesome. For example, in the southwest, it is difficult to assess the abundance of bison because many faunas are represented by small fragmentary found in caves where they were accumulated by rodents, raptors, and carnivorans. The exception to this is the Las Vegas Valley, which includes Tule Springs, a site rich in ice age fossils (I’d like to congratulate Eric Scott and the other geology staff at the San Bernardino County Museum for somehow accumulating $1.4 million to survey and excavate fossils at Tule Springs). Bison fossils are relatively abundant here, and seem to span from 144k years ago to the end of the Pleistocene. Bison appear to make up a larger part of the fauna in younger strata, but the data is weak (hopefully as more work is conducted in the area the answer will become more clear). As for the rest of the Mojave Desert, bison is rare. But while these fossils are rare, and represent two species of Bison, they are suggestive: the smaller shorter horned species is more common in later to latest Pleistocene localities. Scott notes “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.”
We get the same from fossil sites on the Pacific coast. Only a few localities consisted of whole faunas rather than isolated animal remains. The largest is the Costau Pit. Based on similarities with a couple of quarries in Kansas, the Costau Pit is thought to be at least 40k years old and likely older. Of the fossils recovered, horses are the dominant component of the fauna, making up 48% of large mammals (while the two species of Bison make up only 10%). This could hardly be anymore different from the younger (38k to 12k years ago) La Brea Tar Pits, where Bison antiquus comprises 43% of the large mammals found. But given the bias of Rancho La Brea this sample may be null and void in determining the abundance of bison. Not to worry though; another site can provide a clue. The site is called Diamond Valley Lake, near the town of Hemet. When the reservoir now known as Diamond Valley Lake was being constructed, thousands of fossils from the late Pleistocene (69k to 11k years ago) were found. And the most common large mammal was, you guessed it, bison! Take it away Eric:
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 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.
So now that we have explored how to determine when Bison came to midcontinent America and how we might figure out when they became more abundant, one more piece of the puzzle remains: how does this suggest bison may have been a factor in the Pleistocene extinction? First off, the obvious: modern bison are bloody big animals, often standing 6 feet at the shoulder and weighing up to 2000 pounds. Now imagine one of those but bigger. A modern bison individual can consume around 28 pounds of food and 8 to 10 gallons of water a day, and since its ancestors were bigger they conceivably would have consumed more. And when you take into consideration that bison form large herds, you can easily see how these animals can have an effect on an ecosystem. I’m sure you’re asking “But Doug, bison are grazers which means they eat grass. Wouldn’t that mean that just mammoths, horses, and that one grazing sloth would have died out while browsers like mastodons, camels, those other sloths, and giant peccaries would have survived?” Oh nice try but no prize. Based on plant fragments recovered from the teeth of Rancho La Brea specimens, bison in the Los Angeles Basin (and presumably throughout southern California and the Southwest) weren’t munching on as much grass as their descendants. This is backed up by isotope studies of said Rancho La Brea specimens that showed that the majority of their diet consisted of browse such as trees, shrubs, and cool-growing-season grasses, plus some of the usual grass. And this isn’t unique to Rancho La Brea: mammoths, horses, and bison over in Florida were all eating the same things as each other. Now it is becoming clear. Bison in the latest Pleistocene were competing for resources with the other large animals. As stated above, since these animals formed herds, they would have required a lot of food, water, and space to sustain them. Plus all those big animals (and Eric mentioned this in his paper) would have been urinating and defecating all over the place (to use the technical terms). As bison became more numerous, so did their piles of crap. Westward expansion would have taken so much longer if pioneers had to slog through the mess these things were leaving behind. Competition may have put a strain on the ecosystem, but it may have created a delicate balance. However, once you introduce climate change which could have strained resources, competition would have become much more intense. With dwindling resources, bison had a competitive edge that made them the last meagbeast standing with the others being lost to the annals of time.
But this doesn’t mean that Bison are evil. They have no control over what they do. They are just another species trying to survive. And as the paper suggests, they were merely a factor in a combination of events that wiped out North America’s megafauna. Eric Scott’s paper, like any radical new idea, is far from conclusive. It has put forward a new hypothesis based on current evidence. Remember that word: current. Scientists simply don’t come up with an idea and stick with it (that’s religions department). They constantly search for new evidence to test whether their hypothesis was right or wrong. Eric’s Doom Bison Hypothesis seems to explain some of the thing observed in the fossils record, but even he admits throughout his paper that several aspects of his data need expansion and improvement. It’s science in action.
God damn that was an epic post! How did I do? I hope I didn’t butcher Eric’s paper too badly. If I did, he could probably beat the crap out of me on the next field trip. And I imagine Kathleen would be more than willing to hold me down while he works my gut! I’m just joking (at least I hope I am)!
Till next time!