Sunday, 16 June 2013

Pleistocene (Pt 9): Before the Bison

The Pleistocene in North America can be divided into three 'land mammal stages', defined by the kinds of animal that inhabited the continent. The first, which was only recently added to the Pleistocene, is the one that comes before the Ice Ages proper, so that the Ice Ages themselves are, as I mentioned in part 8, divided into just two. As I said back then, the first of these starts with the arrival, not only of the ice, but also of the first American mammoths. The second, called the Rancholabrean after the famous tar pits, is much shorter, and essentially refers only to exceptionally bitter cold of the Last Ice Age. In terms of animal life, though, it is defined by the arrival of bison.

Today, when we think of America before the white man arrived, vast herds of bison are often part of our mental picture. And that, on the whole, is pretty much accurate, and is why their first appearance is deemed so significant in the ongoing evolution of North American wildlife. But what was the American wilderness like before there were any bison? What dominated the continent during the earlier, "Irvingtonian", stage?

As always, of course, the most common animals were the smallest. But, important though the teeth of (for example) voles are for the precise dating of geological deposits from this time, the eyes of any time travelling visitor would inevitably have been drawn to much larger animals. Obviously, mammoths and mastodons are a large part of the answer, and the presence of large elephantine animals crossing the plains would have been enough to tell our time traveller that, while he might still be in Kansas, it isn't really the same one he left. But there are many other beasts that would provide just as quick a clue.

Deer, peccaries, and horses are all examples of animals that aren't so unfamiliar today, although in many cases, the exact Pleistocene species were different from those alive now. Horses, for example, had, by this point, evolved to the modern single-toed form, but the last North American native horses died out around 12,000 BC, when the Pleistocene ended, leaving the ancestors of the domesticated forms behind in Asia. Exactly how many species of wild horse there were in America at this time, though, is unclear, with as many as fifty having been named - in my view, it's somewhat unlikely that they're really all distinct. There were, however, at least two broad types, one similar to the modern domestic horse, and the other, the so-called stilt-legged onager (Equus francisci, among others), which looked much more like a wild ass.

And, while bison themselves had yet to arrive, there were some large cow-like animals already wandering the plains. There were true musk oxen further north, but across much of western America, there were instead the closely related shrub oxen (Euceratheium collinum). These were large, heavily built animals with long curving horns that projected outwards and sideways, quite unlike those of bison. Analysis of their fossilised dung has revealed that, at least in Utah, they ate sagebrush, acacia, and oak. Further east lived Bootherium, another relative of the musk ox; between the two they were found essentially throughout America south of the ice sheets.

Today, there is just a single species of pronghorn alive in North America - or, for that matter, in the world. Back in the Pleistocene there were at least five. These included the dwarf pronghorn (Capromeryx minor), which stood around 60 cm (two feet) tall at the shoulder, and was the smallest and last of a lineage that had once roamed across America, but was now restricted to the south-west and Mexico. Larger forms included the "four-horned" pronghorns Tetrameryx and Stockoceros, neither of which genuinely had four horns. Instead, like the dwarf pronghorn, they had a single pair of horns, each of which branched into two near the base (rather than half-way up, as in the modern pronghorn), making it appear, at a first glance, that there were actually four.

In most cases, the two forward-facing tines were much smaller than those to the rear, but in Stockoceros they were about the same size, perhaps emphasising the illusion. From studies of the wear on their teeth, we can say that at least some of these species ate more grass and other abrasive plant matter than living pronghorns do, perhaps reflecting the harsher climate of the day. This may explain why modern pronghorns have teeth that look as if they would be good for chewing tough grasses even though they don't actually do that - during the Ice Ages they may have had little choice.

To confuse matters further, there really was a pronghorn species alive at the time with four horns. Hayoceros is known only from Nebraska and Texas, and was a fair-sized animal, at least four feet tall at the shoulder. The forward pair of horns were flattened and relatively short, branching into two near the tip, while the rear horns were much longer, narrower, and un-branched. They must have looked quite dramatic - and weird - when they were alive.

Still, if you took the horns away, they would have looked much like any other pronghorn, animals that certainly aren't beyond our modern experience. But some of the creatures that had emigrated from South America were considerably stranger. Three groups in particular stood out as huge and bizarre-looking animals.

The first of these were the sloths. But not, of course, the modern sort of sloth, which live up trees (and are, themselves, pretty odd animals). These were ground sloths, large animals that lumbered across the plains. Although the true home of the ground sloths was South America, even in the North, there were many different kinds, and they weren't necessarily that closely related.

Some idea of the size of these animals comes from the fact that the smallest, the Shasta ground sloth (Nothrotheriops shastensis) was about the size of a black bear. Compared with some of their larger kin, they didn't travel so far north, and are known only from Mexico, the southwestern US, and Florida. There, they lived among juniper woodlands and pine-covered mountain slopes, feeding on scrubby plants, such as "Mormon tea", yucca, and agave. It has been suggested that, like modern sloths, they were solitary animals, only coming together to breed. The fossils of young Shasta ground sloths are often found in caves, and that may be because, again like modern tree sloths, they had difficulty coping with cold weather and so used the caves for shelter. The adults, being so much larger, would presumably have had less of a problem.

One site in California has yielded fossils of no less than three different kinds of ground sloth, apparently feeding off different plants in a complex environment so as not to compete with one another. One was the Shasta ground sloth, while the other two were much larger, both around the size of a Kodiak bear, and perhaps weighing about a ton. Jefferson's ground sloth (Megalonyx jeffersoni) is perhaps the better known of the two. It lived from Yukon to South Carolina, and is probably descended from the slightly smaller Wheatley's ground sloth (Megalonyx wheatleyi) which inhabited much the same area earlier during the Pleistocene.

It's named for Thomas Jefferson, an amateur palaeontologist more widely known for his day job. Like other ground sloths, it had huge sickle-like claws, which it presumably used to rip down branches and leaves as much as to defend itself against predators. The hind feet, however, were different, with flat soles, like those of bears or humans, rather than the strangely curved shapes that forced other sloths to walk on the sides of their feet. Maybe this made it more mobile than its kin, and it might explain why it was found in a wider range of forested environments. They also survived for a long time, apparently long enough for their last representatives to be hunted and eaten by humans.

Marginally the largest of those three Californian species was Harlan's ground sloth (Paramylodon harlani), which some older sources refer to as Glossotherium - which is a real animal, but one now known to be found only in South America. In fairness, the two are very similar, and it may even be that Paramylodon is actually descended from the South American form. With fossils found in places as diverse as central Mexico, New Jersey, and Alberta, it was nearly as widespread as Jefferson's species, and apparently fed on desert shrubs over at least some of that range. Like some of its kin, it could probably rear up on its hind legs, which must have been an impressive sight.

Large though these animals were, they were not the largest ground sloths of the North American Pleistocene. That honour goes, more or less as a tie, to the closely related giant ground sloths Megatherium and Eremotherium. More common in Central and South America, in the North, the former is known only from Georgia, and the latter from Florida to South Carolina. Although there have been previous suggestions that there are as many as three species of Eremotherium, it now seems there's only one (E. laullardi). Megatherium seems to have had rather more species, although only one of them (M. americanum) lived in the North.

These creatures were huge: at around seven metres (twenty one feet) long, it's been estimated that a fully grown adult could have weighed three or four tons - about the same as a female elephant. They lived among the plains and forest fringes of the region, and seem to have had a powerful bite, but, like modern tree sloths, not much in the way of powerful chewing action, suggesting that they mainly went for softer foods, most likely ripping the leaves off trees. Presumably, they had to eat quite a lot of the stuff - perhaps 130 kg (300 pounds) or more of food every day. Their narrow jaw also suggests a slender, elongated tongue, which would be quite typical for this kind of mammal, and could be used to manipulate food.

Like the other ground sloths, they died out around 10,000 BC, just as humans were becoming more numerous on the continent. As with other large North American animals that went extinct around this time, though, its worth noting that this was also a time of climate change, as the Ice Ages ended, and it seems likely that that can't have helped their chances of survival.

[Image by "ДиБгд", from Wikimedia Commons]

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