Did bison play a role in horses dying out in North America?

Ice Age horse specialist Eric Scott argues that climate change and the vegetation changes that resulted cannot fully explain the extinction of horses and other large mammals from North America at the end of the Pleistocene epoch. He believes increased competition from prospering bison may have played a part.

Paleontologist Eric Scott, a specialist in Ice Age horses, has long pondered the extinction of these mammals from the Americas.

So his excitement was understandable over the recent discovery of a skull and lower jaw of an extinct horse species not far from his home patch.

The curator of paleontology for the San Bernardino County Museum in Redlands, California, says the discovery of the remains in neighbouring Nevada, in the Tule Springs area north of Las Vegas, adds another piece to the puzzle as scientists continue to unravel the mysteries around the mass extinction of large mammals in North America.

Horses are not uncommon in the Tule Springs fossil record, but the quality and size of these fossil finds enabled scientists to identify the species as Equus scotti, a large horse common in much of western North America during the Pleistocene Epoch – the geological period roughly spanning 12,000 to 2.5 million years ago.

The site has been dated to nearly 12,000 years in age, making the fossils among the youngest records of Equus scotti anywhere in North America. The new discovery is forcing scientists to revise their understanding of horse evolution and extinction at the end of the Pleistocene epoch.

Scott’s studies have revealed that three species of horse lived in the area of Tule Springs and the upper Las Vegas Wash during the Pleistocene, but finds were not sufficiently complete to make firm species identifications possible.

“There have been some species names suggested here and there, but nothing really concrete,” observes Scott. “It’s a long-running joke with our team in Vegas that every time a new site is excavated, I plead for them to find a horse I can name!” Hence his excitement over the Equus scotti find.

With identifiable horse fossils so scarce, paleontologists have long been forced to make inferences about what horse species were present in and around Tule Springs.

Since the world-famous Rancho La Brea “tar pits” in southern California are relatively close by, other scientists have proposed that the large horse from that site was also present elsewhere throughout the Mojave Desert and the southwest.

“We now think that’s erroneous,” Scott explains. “Our new horse from Vegas has a different anatomy from the big La Brea horse. It looks more like large Ice Age horses from northern Nevada that were living at about the same time.”

The difference is critical.

Equus scotti was a highly successful species throughout much of western North America for most of the Ice Ages during the Pleistocene, including southern California and Nevada.

The new discovery shows that these horses survived in southern Nevada right up until the end of the Pleistocene – a fact not before known. But in southern California, the species was replaced in the later Ice Ages by an anatomically distinct form.

“It means we had as many as four horse species living in the American southwest at the end of the Ice Ages. Compared to horses today, that’s quite a lot of species.”

While the Equus scotti discovery adds to scientific knowledge about horse distribution in North America, it cannot provide answers to a tantalizing question that experts have been pondering since the first discoveries of extinct horse species on the continent.

Why did horses and other large land mammals, such as woolly mammoths, camels, dire wolves, woolly rhinos, saber-toothed cats, and giant ground sloths, die out around the last glacial period, which began about 110,000 years ago and ended about 12,500 years ago? The glaciation was at its peak about 18,000 years ago.

Some 70 per cent of North American large mammals became extinct between 20,000 and 10,000 years ago.

The widespread extinction event remains both unexplained and a contentious subject, despite decades of intensive study.

It is a question that intrigues Scott, who has explored some interesting hypotheses in trying to cast more light on possible answers.

A changing climate and the resulting vegetation changes are the reasons most commonly cited by scientists for the cause. Other scenarios include overhunting by early humans, disease, the explosion or impact of a large meteorite, or some combination of all these factors.

It is not altogether simple, however, and consensus is proving elusive.

As Scott points out, even if the weight of evidence favors climate and vegetation changes, it does not explain why the large mammals managed to survive similar and equally dramatic climate changes in the past.

What made the crucial difference that wiped out so many species in the comparatively recent past?

Scott argues that the difference was the rise and abundance of bison.

He says the evidence reveals an important difference during the emergence from the last Ice Age –  the profusion and spread of  bison, particularly in the American West.

Scott says the increased abundance of bison – large, aggressive, herd-dwelling ruminants – in the late Pleistocene constitutes a critical difference between this time period and earlier, similarly intense interglacials.

He notes that all major hypotheses advanced to explain the extinctions have one common feature: an almost exclusive focus on external forces, rather than consideration of the large mammals themselves, and changes in their populations and makeup.

He believes the immigration and expansion of bison numbers in North America in the last part of the Pleistocene had the potential to change the way in which large mammal communities were able to respond to climate changes.

At the end of the Pleistocene, around 10,000 to 14,000 years ago, over 30 genera of primarily large mammals died out in North America. Many of the extinctions occurred around the time when humans are thought to have first arrived in North America.

There is little doubt, says Scott, that the late Pleistocene involved frequent and dramatic climate changes, with a general shift towards warmer temperatures, although this warming trend was interrupted by several short reversals.

These had dramatic affects on continental ecosystems, with long-term changes in temperature, rain, snowfall, and humidity leading to marked shifts in feed and water distribution and availability.

It is understandable, he says, such changes would have place some large mammals under pressure, and may have even directly resulted in their extinction.

However, attributing the extinctions to only climate change would effectively require the last Pleistocene climate change to have been much more severe or faster than earlier shifts. This is simply not the case, he says.

The climate changed many times during the late Pliocene and Pleistocene Epochs. While there were small-scale extinctions associated with some of these shifts, most of the animals living in North America during this period survived.

Scott observes, in a paper he wrote for the journal, Quaternary International: “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.”

The problem with blaming climate change alone is that the last Pleistocene warming does not appear to have been significantly more severe than several earlier such transitions during the epoch.

It remains to be shown that climate changes at the end of the Pleistocene were truly unique, he says.

If earlier climate shifts were equally severe yet did not result in 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.

Scott says the other scenarios proffered to explain the die-off, including overkill by early human immigrants into North America, disease, and, more recently, the impact or explosion of a meteorite,  each have their strengths and weaknesses, some more than others.

The overkill scenario is perhaps the most contentious, he says, but it has its difficulties. Chief among them is the lack of confirmed kill sites where fossil remains of Pleistocene mammals are found with human artifacts. Also, many species appeared to have gone extinct before humans first appeared in North America.

While the date for human habitation is now being pushed far earlier than the Clovis hunters, evidence to date indicates the first humans in North America  were not the kind of uber-hunters with the numbers or population dispersal to significantly affect mammal extinctions, he suggests.

The disease scenario, he says, has yet to be supported by concrete data of any such hypervirulent disease actually existing. It would have to have shown the ability to cross multiple mammalian orders – a feat  never previously documented for any disease. Even if this was the case, whey did it affect some species and not others?

Dire wolves (Canis dirus), for example, went extinct while grey or timber wolves (Canis lupus) and coyotes (Canis latrans) survived – and even thrived. Similarly, the mountain goat Oreamnos harringtoni died out, but the closely related  O. americanus survived.

Scott says the scenario involving a meteorite explosion or impact falters on several fronts. Evidence of embedded micrometeorite fragments in fossils do not line up with the extinctions, nor is there evidence of continent-wide wildfires at any time during the last Pleistocene warming.

No attempt has yet been 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.

Scott says the climate-change hypothesis differs from most scenarios because it does not suffer from a lack of evidence. However, some of the evidence of climate change appears to negate the proposition that it was solely responsible for the extinctions.

No major extinction pulses are seen in the fossil record from any earlier glacial–interglacial transitions of similar  intensity to the terminal Pleistocene transition, when so many large mammals died out.

Primarily for these reasons, climate change is routinely dismissed at the sole cause for the late Pleistocene extinctions in North America, Scott says.

Scott argues that if the variety of large mammals in North America at the end of the Pleistocene differed substantially  from earlier such communities at times of climate change, it could well stand up to scrutiny as a factor in the extinctions.

The one major difference between late Pleistocene large mammal communities and earlier ones – as revealed in the fossil record – is the abundance of bison in later Pleistocene large-mammal fauna throughout North America.

They first entered North America in the latter part of the Pleistocene from Eurasia around 220,000 to 240,000 years ago, meaning they were  present in North America during only three major glacial–interglacial transitions.

The fossil record suggests bison were increasing in abundance in North America before the reduction in numbers of other large mammals.

Modelling suggests they exhibited a sharp increase in numbers between 75,000 to 80,000 years ago, then a sharp decline around 25,000 years ago, with a reduced rebound in population size some time after 10,000 years ago.

Examinations of the fossil record in the southern Great Plains by other researchers noted that bison were numerically less abundant than other herbivores – horses, camels, and mammoths – more than 20,000 years ago, but became relatively more numerous near the Pleistocene–Holocene transition.

Evidence across the continent indicates that late-Pleistocene mammoths, horses and bison consumed essentially the same diet. It is clear, Scott says, that bison and other North American megafauna were competing for available resources.

Responses by species to such increased competition could reasonably be distilled down to some variant of one of three alternatives: adapt, move, or die.

Struggling large mammals seeking to move away would have faced the widespread abundance of bison in North America.

Even in areas where bison were rare or absent, the arrival of large mammals displaced by bison elsewhere would still have faced increased competition from each other for feed.

Scott says several bison species successfully colonized most of the continent, becoming the most abundant large herbivore in many areas.

He proposes that it was this competition with bison, in combination with climate-caused changes in vegetation abundance and distribution, that directly drove the extinction event.

The presence of these large, aggressive herd-forming animals, coupled with climate and vegetation changes, would have had a big impact on other large mammals, he argues.

He believe future studies exploring Pleistocene extinctions should continue to examine bison and other large mammals  as continuously evolving organisms in communities that were themselves constantly changing through time.

That, he says, may help generate a better supported, more widely accepted, and more verifiable explanation for the late- Pleistocene extinctions that changed the face of mammalian wildlife in North America.

 

Scott’s hypothesis was the subject of a paper published in Quaternary International, the journal of the International Union for Quaternary Research, in its April 2010 edition. It is entitled “Extinctions, scenarios, and assumptions: Changes in latest Pleistocene large herbivore abundance and distribution in western North America”. The abstract can be read here.