Saturday, 23 February 2013

Pleistocene (Pt 7): Meanwhile, Across the Atlantic...

Columbian mammoth
(It's likely that the real animal was hairier than the one in
this reconstruction, but it shows the tusks effectively)
Over the last five parts of this series I have described the history of Pleistocene Europe, describing some of the ways that the animal life of the continent changed over those thousands of millennia, and looking at a few particular animals in more detail. But one doesn't need a degree in zoology to notice that today, the wildlife of North America, for example, is different to that of Europe. North America has coyotes, raccoons, cougars, armadillos, and pronghorn antelope, to name just a few animals that are simply absent in Europe. (Or, in the case of raccoons, were absent until somebody made the mistake of releasing some of the furry nuisances in 1930s Germany).

It's hardly surprising that that continent also had different wildlife during the Pleistocene. One notable difference, for instance, is that we humans weren't there. Obviously, neither Columbus nor Leif Ericsson were genuinely the first person to discover America. But even whichever long-lost group of Native Americans was actually the first to discover the great western continent, they did so long, long, after the first Europeans discovered Europe. While Europe had at least some species of human inhabiting it for about two-thirds of the Pleistocene, nobody reached America until the epoch was all but over.

The tale of America's Pleistocene history, therefore, is one of a truly pristine wilderness.

If we could have seen North America from space at the dawn of the Pleistocene, it wouldn't have looked much different to the way it does now. The coastlines were broadly where they are now, and, fields and cities aside, the vegetation was more or less what and where you'd expect it to be - at this time, the climate wasn't so different from the way it is today. The most notable difference would have been that the Great Lakes simply didn't exist. They, along with a number of smaller lakes, such as the Finger Lakes of upper New York state, were carved out by the advance of glaciers that, at this point, had yet to arrive.

However, this general similarity, at least of the coastline, masks a dramatic fact, and one that had a huge and lasting effect on American wildlife. To put it simply, this similarity was new. Specifically, at the dawn of the Pleistocene, it had been perhaps as little as 100,000 years since South America had collided with its northern neighbour. The collision is still ongoing; at as little as forty miles across, the Panamanian Isthmus that joins the two continents is almost ridiculously thin, even today.

That collision had important effects on issues such as hydrology and climate - that the Pacific and Atlantic Oceans could no longer mix in the tropics had a number of significant implications. But, for our purposes, it's the effects on animal life that are especially dramatic.

It's not quite true to say that there had been no communication between the continents prior to this time. Even before Central America became an isthmus, it was a chain of islands, and at least some animals could move between them. But once it became a full, permanent, land-bridge, the floodgates opened. Animals from both continents could, and did, invade the other in large numbers, re-shaping the distribution of American wildlife forever. This event is known as the Great American Interchange.

You see, up until this point, South America had been just as much an island continent as Australia is today. Just as happened on Australia, that meant that South America could develop some pretty weird animals, evolving in isolation from the rest of the world. In contrast, the North was much less so. Although it, too, was an island continent prior to the collision, it had been so for far less time, having had contact with Asia, and, if one goes back far enough, also with Europe. Sure, even then, North American wildlife was different to that in Europe and Asia, but at least the rough kinds of things that were there were broadly familiar: dogs, cats, horses, and so on, were already found on both sides of the Atlantic by this time.

But South America... that was just strange. Or, at least, it was prior to the Great American Interchange. That changed because, for the most part, it was North American animals that went south. Cats, deer, foxes, and rabbits all headed south to colonise new lands. It's because of them that South America has, for example, jaguars and ocelots. Indeed, some of the invading species have since vanished from the north - camels joined the migration, giving rise to today's llamas even as their ancestors in the north subsequently died out.

Movement in the other direction was less common. That may partly be for climatic reasons, with creatures from the south not being able to spread beyond the tropics, while the northern continent was already more varied. But it seems more likely that the oddities of the south just couldn't compete as effectively with animals that had more evolutionary experience at facing challenges from Asia and the wider world.

Still, some did head north, and, while many of these creatures didn't make it through the Ice Ages, some did. It's thanks to the Great American Interchange that North America now has armadillos, opossums, and tree-dwelling porcupines.

Scarcely had this upset settled down than the Ice Ages began, bringing their own changes. For North America, one of the more significant of these was that, with so much water locked up in the vast ice caps, sea levels dropped, allowing the Bering Straits to dry up. North America was now connected by land to two other continents, and while the connection to Asia was by no means as unprecedented as that with the south, it, too, allowed new animals to enter, and existing ones to leave.

What was the end result of all this mixing? In many respects, it's a fauna that's fairly familiar today, especially where smaller animals are concerned. American mice and voles seem to have diversified rapidly at this time, adapting to a climate that swung between frozen Ice Ages and warm interglacials on a timescale of dozens of millennia - nothing much on an evolutionary scale. Gophers, squirrels, marmots, rabbits, and beavers were all common, along with other small mammals like shrews, moles, and bats. Hares and jackrabbits, while apparently having originated in North America millions of years earlier, seem to have suddenly become much more common at around this time, before spreading out over the Bering land bridge to colonise Asia, and, eventually, Europe and Africa.

One of the first animals to cross over the land bridge in the opposite direction, however, was much larger than a jackrabbit. For this was the time that mammoths entered North America, in the form of the southern mammoth (Mammuthus meridionalis). Back in Asia, they would eventually evolve into the more famous woolly mammoth, but, in America, they evolved instead into the Columbian mammoth (Mammuthus columbi). This was the largest of the mammoths, although probably no larger than modern African elephants; the older alternative name of "Imperial mammoth" (Mammuthus imperator) may represent a particularly large subspecies. Like all mammoths, they had particularly large, curling tusks, much larger and more dramatic than those found on any modern elephant. Living in relatively cool forests, they were probably hairier than modern elephants, too, although we don't know by how much. Mastodons, the native, home-grown equivalent of mammoths, didn't die out when their Asian relatives turned up, and they, too, would have been an immediately noticeable part of the North American fauna.

Horses and camels had both originated in North America, long before the Ice Ages, and they initially survived the downturn in the climate, only to die off in the very coldest days of the late Pleistocene. There seem to have been a range of horse species living across the continent during the Pleistocene, although the exact number is debated. They range from ass-like animals such as the stilt-legged onager, to the largest wild horse we know of, Equus giganteus, which was probably as large, or even slightly larger than, the largest domestic draft horses. Camels were just as varied, with some species being similar to modern llamas, and, at the other extreme, the giant camel Titanotylopus being a whopping eleven feet tall.

As was the case in Europe, the colder climate meant that animals such as musk oxen prospered across far wider stretches of land than they do today, in this case accompanied by the similar, but slightly larger shrub oxen (Euceratherium collinum) - bison did not arrive until much later. And, of course, there were those weird South American creatures that had managed to survive the struggle northwards, including giant ground sloths and armadillos the size of a rhinoceros.

Feeding on all these creatures were many different kinds of carnivore. Where North America today has only one even moderately large species of cat - the cougar - Pleistocene North America had several more, including lions, cheetahs, and jaguars, as well as at least two different kinds of sabre-tooth. Then there were the dogs, including the ancestors of today's coyotes, and the truly impressive short-faced bear, an animal that frankly makes the modern Kodiak bear look a bit wimpy.

Over the next few months, I'll look at some of these animals in a bit more detail, to build up more of a picture of the untamed wilderness of North America before the coming of mankind.

[Image by Sergiodlarosa, from Wikimedia Commons]

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