Sunday 11 August 2013

Pleistocene (Pt 10): Of Armadillos and Bison

When the continents of North and South America collided, shortly before the dawn of the Pleistocene, a range of creatures wandered across the newly formed isthmus of Panama. Most were heading south, giving us such "normal"-looking South American animals as jaguars, ocelots, and grey foxes. The ones heading north were a lot stranger, and many of them didn't do so well in the long term. These include such wonderfully weird creatures as the giant ground sloths, which managed to get as far north as modern Canada before dying out.

But they weren't alone, and not necessarily any stranger than some of their fellow immigrants. The same event that saw the arrival of the ground sloths also saw the coming of the armadillos. Today, there is only one species of armadillo in the United States, and only two outside of South America (the other gets no further north than southern Mexico). But, back during the Pleistocene, there were others, and some of them were not quite what we'd expect today.

At least three different kinds of armadillo-like animal lived in North America at this time. One of them, of course, was the actual armadillo family, including such animals as the beautiful armadillo (Dasypus bellus). Dating back to the start of the Ice Ages, this animal survived their end, and only died out around 9,000 BC. Closely related to the modern US species, they would have looked very similar, but were about twice the length, and presumably quite a bit heavier. They inhabited broadly the same area as the modern nine-banded armadillo, and may be its ancestor, although doubtless they retreated south whenever the climate became particularly cold.

Large though that is, it's no larger than the giant armadillos of present-day South America. The other armadillo families of the North American Pleistocene, however, were quite a bit bigger. Of the two, the pampatheres were the most like the living forms. The North American genus, Holmesina, had at least two species, and lived from South Carolina to Texas. It proved successful enough that, in the late Pleistocene, some migrated south again, so that we also know of related species from Colombia and Brazil.

Like modern armadillos, pampatheres had bony armour plating, hinged for flexibility with a series of bands between the solid shoulder and hip pieces. Where the modern US species has nine bands, Holmesina only had three, presumably sacrificing some mobility for greater protection. They were, however, a lot bigger; perhaps the size of rhinoceros. Unlike modern armadillos, these seem to have been grazing animals, well adapted to munching on long grass, and with powerful jaw muscles that we just don't see in their living kin. Doubtless this stood them in good stead on the North American prairie, which stretched further south during the height of the Ice Ages than it does today.

Impressive though a rhino-sized armadillo must surely have been, Holmesina was not the only giant armoured creature lumbering across the plains. The glypotodonts were also related to armadillos, but unlike them, had no flexible bands on their bodies. Instead, their carapace formed a solid shell, not unlike that of a turtle, although the long, armoured tail, had some bands, and often ended in a spiky club that they presumably swung at their enemies. The North American species belong to the genus Glyptotherium, and there seem to have been at least three of them in the US, and probably a few more in Mexico and Central America.

All three lived in the southern parts of the continent, broadly between Arizona and Florida, although there are small number of finds ranging from El Salvador in the south to Nebraska in the north. The smallest of the three, Osborn's glyptodont (G. texanum), is also the oldest, living before the Ice Ages started, but the other two (which might be its descendants) seem to have lived alongside one another, although not necessarily in exactly the same areas. Of the others, the more widespread was Gidley's glyptodont (G. arizonae), while Simpson's species (G. floridanum), lived further east, between Louisiana and South Carolina.

Glyptodonts were big animals, measuring up to six feet in length, and, with all the armour, likely weighed at least a ton. With exceptionally large jaw muscles, they were probably grazing animals, although we don't know too much about the details of their diets and lifestyles. (And how, exactly, did they breed? That weight and lack of flexibility can't have made it easy).  Another armadillo-like creature, Pachyarmatherium from Florida, was once thought to be a miniature glyptodont, but now appears to be in a group all of its own.

Other, less bizarre, animals that we might not expect to see in modern America, but that ran wild during the Pleistocene include camels and llamas. These, too, you might think, must have been recent arrivals from the southern continent, where llamas live today, but that's not all the case. In fact, they were already in North America, and headed south when the Pleistocene began. It just so happens that the ones in the North went extinct as the Ice Ages ended, but those in the South didn't - llamas are, geologically speaking, recent immigrants to South America.

Indeed, the last llamas only died out in the north around the very end of the Pleistocene. The last native camels died out around the same time, and looked not that different to their relatives that headed west into Asia - becoming the ancestors of the camels we now know from Asia and North Africa. A somewhat different looking example, dying out much earlier, at 0.3 million years ago, was Titanotylopus, which reached eleven feet in height, and had a rather smaller head than living forms. Nonetheless, judging from the shape of its backbone, it did have a hump - presumably just the one, although it's hard to be sure.

With all of these - to modern eyes - unusually large animals, it's perhaps no surprise that there were giant rodents, too. The largest of the day in North America was the giant beaver (Castoroides spp). This lived across at least the eastern half of the continent, as far west as Nebraska and Oklahoma, and in the warmer spells between the Ice Ages, also moved quite a way north, with one specimen being found as far from its regular grounds as the Yukon. Roughly the size of a black bear, in other respects, it probably looked quite a lot like the modern species. Its teeth don't seem quite so well adapted to gnawing on wood, and we don't have any evidence that it constructed dams, but many of its close relatives certainly did, so its not implausible that it did so, too.

All of these creatures, along with those I've described in the last three posts, lived during the main part of the Pleistocene, a time called the 'Irvingtonian'. Around 0.24 million years ago, there was a dramatic change in the local wildlife, marking the beginning of the final part of the North American Pleistocene, the Rancholabrean. Named, of course, for the La Brea tar pits, this began when the advancing ice sheets lowered the sea level sufficiently for a new influx of creatures to enter the continent, this time by crossing the Bering Straits from Siberia.

Many of the Irvingtonian animals survived the new influx, and, in that respect, the differences aren't so great. What really changed were the new animals that hadn't been there before, and the most significant of those were the bison. The bison found in such numbers when the white man arrived in the sixteenth century, and which still survive today, were merely the last in an array of bison species that were descended from these first immigrants. Bison first appeared in Asia, and, even today, a tiny number cling on in Europe, and were surprisingly late arrivals on the continent with which we now most associate them.

The first bison to arrive in North America were steppe bison (Bison priscus), animals also known from Europe and Asia. They survived for a long time in the north, and we even the frozen carcass of one from the Northwest Territories. Long before that particular bison died, however, others further south had already given rise to a new species, one that may be the largest bovine that ever lived.

These huge creatures, sometimes called palaeo-bison (Bison latifrons) stood around eight feet tall at the shoulder, and had enormous sweeping horns, quite unlike the small ones of the modern species. Measurements of these horns reach as high as seven feet from tip to tip - and it's worth remembering that we can only measure the bony cores inside, so the tips of the outer horny sheath were almost certainly further apart than that.

The palaeo-bison died out around the height of the Last Ice Age, a mere 0.03 million years ago. They were replaced by the somewhat more conservative "ancient bison" (Bison antiquus), which has left fossils across the modern United States, as well as southern Canada. Somewhat smaller than their predecessors, they were still over seven feet tall, although, with a spread of no more than three feet or so, their horns - while large - were much less impressive. One study suggests that at least one population had a broader diet than living bison, including a good deal of softer food, along with the grass. They seem to have been very numerous, with huge numbers recovered from the La Brea tar pits, and may have evolved into the even smaller, modern, bison, as recently as 7,000 BC.

Of course, bison were not the only creatures to enter North America during the Rancholabrean. The land corridor between Alaska and Siberia was, as we might expect, rather cold at the time, so the others were also animals well adapted to harsh environments. It was at this time, for example, that the ancestors of mountain goats reached America, along with caribou, musk oxen, moose, and, for that matter, woolly mammoths.

Quite when humans first made the same crossing is the matter of some debate. Certainly, they had done so by 11,000 BC, and there is growing evidence that it may have been many thousands of years before that. What we can say is that, around 10,000 BC or so, many of the larger, more dramatic, native animals of North America began to die off. Something like 73% of the large mammal species died out as the Last Ice Age ground to a halt. Climate change doubtless had some effect on that, because many of the animals would have been adapted to much colder weather.

On the other hand, most of them had managed to survive the warm periods between previous Ice Ages without much difficulty, so it's a fair bet that something else acted as a final blow that finished off populations struggling to adapt to a warming world. Something, no doubt, that carried spears...

[Picture by "Sergiodlarosa", from Wikimedia Commons]

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