MEGAFAUNAL EXTINCTIONS | American Armageddon:

The Terrible Wipeout of Great Wonders from the Western Continents

Last month we saw how many of the great beasts of southern Asia and especially Africa managed to escape the clutches of extinction during the late Ice Age, while the month before we encountered a scenario that was the very opposite in Australia, where the vast majority of the continent's ancient titans were cast into oblivion. This month we will fix our eyes on those continents which lie to the west of Ireland at the other end of the Atlantic, the Americas, to investigate a die-off of large species which appears to have been every bit as bad as that in Australia.

The roll call of forms which suffered extinction in these regions during the late Ice Age, and possibly into the early Holocene (11,700 years ago–Present), reads like a ‘who’s who’ of the most spectacular beasts and giants of the Ice Age. Among the disappeared were mammoths and mastodons, sabretooth cats and dire wolves, American lions and short-faced bears, along with both more familiar and more bizarre mammals, including horses, giant ground sloths, giant armadillos, and their massive armoured relatives the glyptodonts. With a roll call of such wonders, this month we will simply concentrate on discovering when they went extinct, waiting until next month to uncover the possible causes of this American Armageddon.

We can start by looking at the nature of the extinctions that occurred in North America. During the Ice Age, North America was awash with giant mammals, being populated by great beasts which had not only evolved on the continent itself but had migrated from Eurasia at times of low sea levels and from South America after the formation of the Isthmus of Panama around three million years ago. Yet, towards the end of this great Age of Ice, the vast majority of these massive mammals simply disappeared, with around 70 per cent of the 54 species weighing over 45 kilograms going extinct.

But these large mammals did not suffer extinction in the very same way all across the continent. During the late Ice Age, North America was split into two different regions, with ice sheets which extended over the northern half of the continent separating the southern half from an ice-free area in the northwest comprised of Alaska and part of the Yukon. And each of these regions exhibits its own pattern of megafaunal extinctions.

In the Alaska/Yukon region, the pattern appears to be quite staggered, and one of the first giants to go was also one of the most impressive. This was the short-faced bear, Arctodus simus, which, at a weight of around 800–1,000 kilograms, was larger than any living bear, and far larger than its closest living relative the Andean bear, Tremarctos ornatus, males of which weigh only about 100–175 kilograms.

The short-faced bear disappeared around 40,000 years ago, and a species of horse, some type of ass, went extinct around 4,000 years later, while a sabretooth cat, Homotherium serum, suffered extinction around 25,000 years ago. In turn, it was another 10,000 years before some other species vanished from this region, modern-type horses and saiga antelope going extinct around 14,500 years ago, and woolly mammoths following them around a thousand years later (although they managed to cling on off the Alaskan shore on St. Paul Island up to as recently as about 6,000 years ago, around the time when the first farmers were arriving in Ireland). And not long after the disappearance of the woolly mammoth from the mainland, the cave lion (Panthera spelaea/Panthera leo spelaea) was ushered to the same grim fate.

As for the southern half of North America, the pattern also seems to be somewhat staggered, albeit in a much narrower time frame and heavily weighted towards the latest part of the Ice Age.

A number of species’ last appearances date to over 17,000 years ago, including the dire wolf, the sabretooth cat Homotherium serum, as well as the American cheetah (Miracinonyx trumani), which was not a true cheetah but had independently evolved a quite cheetah-like form. These large mammals were joined by a number of great beasts of stranger form which were descendants of mammals that had evolved in South America, and included a number of species of ground sloths, a ‘giant armadillo’, and a glyptodont, which was a little like an armadillo, being adorned in similar armour, but far larger, some being the size of a small car, while also possessing tremendous tail clubs.

However, the youngest dates for most of the megafaunal species fall somewhere between 15,000 and 11,500 years ago, and the long extinction list includes a number of truly gigantic species which were larger than any of their living relatives.

These included forms such as the American lion (Panthera atrox), which, at up to 420 kilograms in weight, was larger than any modern lion or tiger; the short-faced bear (Arctodus simus); giant beavers; and Jefferson’s ground sloth (Megalonyx jeffersonii), a three-metre-tall, one-tonne behemoth that was far, far larger than any living sloth, all of which weigh under 10 kilograms.

A number of the ancient relatives of modern elephants present were also larger than their living kin, the woolly mammoth being joined in extinction by its larger relative the Columbian mammoth (Mammuthus columbi) and the American mastodon (Mammut americanum), both of which could be far heavier than an African elephant. Horses also disappeared at this time, not to mention species of peccary, moose, musk ox, llama, camel, as well as many others, most notably the sabretooth cat Smilodon fatalis. Other species that went extinct in the late Pleistocene but are not reliably dated include a tapir and two species of capybara.

But if the Grim Reaper was hard at work in North America during the late Ice Age, if anything he was wielding his scythe with even greater vigour in South America at this time.

In the late Pleistocene (Pleistocene: c. 2.6 million years ago–11,700 years ago), South America had a rich and, in many ways, a very strange collection of mammals, composed of the unique forms that had evolved during its long life as an independent island continent as well as the relatively recent immigrants from the north which had migrated into the continent after the formation of the Isthmus of Panama. Yet, many elements of this exotic fauna have simply disappeared, as South America was perhaps the worst hit of all regions as regards the megafaunal extinctions, with around 80 per cent of all genera (genera being the plural of genus, which is the category above species) going extinct. Just how extensive this great extinction was is clearly illustrated by the fact that the largest native mammals left on the continent today are tapirs, none of which exceeds around 350 kilograms in weight.

The radiocarbon dates available seem to indicate that the South American megafauna went extinct at around the same time as that in the southern half of North America, albeit over a slightly longer time frame and in three phases.

In the first one, before 18,000 years ago, the mastodon Haplomastodon, the giant armadillo Holmesina, and the glyptodont Glyptodon disappeared. In the second phase, between 18,000 and 11,000 years ago, the spiral-tusked gomphothere Cuvieronius, an ancient relative of modern elephants, perished, as did four species of ground sloths, as well as horses belonging to the modern genus Equus and the ancient genus Hippidion. This phase also witnessed the downfall of the strange rhino-like, but hornless, mammal Toxodon, which, like the horses it lived alongside, was a hoofed form, although a member of a completely different group to them that had evolved independently in South America.

Similarly, the third phase, from around 11,000 to 8,000 years ago, saw the loss of another raft of interesting forms. These included the glyptodont Doedicurus, the ground sloth Catonyx, as well as its far larger relative, the aptly-named giant ground sloth Megatherium ('great beast'), which measured an incredible six metres long from head to tail and weighed a whopping four tonnes – more than a female elephant! However, suggestions that the great sabretooth cat Smilodon was among the megafaunal species to suffer extinction at this time appear to be based on incorrectly calibrated radiocarbon dates, and the most recent fossil from South America actually dates to just over 13,000 years ago.

In the early Holocene, then, as the ancient Irish were colonising Ireland and establishing a lasting presence in that land, thousands of kilometres away to the west, across the wide span of the Atlantic Ocean, there were still sabretooth cats stalking and giant ground sloths lumbering through the wide lands of South America. But while the early Irish were beginning a new chapter in the life of their new homeland, a great chapter was coming to an end in the life of the Americas. At the same time as Ireland's ancient people were putting down the first deep roots of a human occupation of the island which persists to the present day, the last branches were being lopped from the mighty trunk of the American megafauna, the life of the western continents being changed forever by this American Armageddon.