Sunday, 21 April 2013

Pleistocene (Pt 8): Mammoths v. Mastodons

American mastodons
The arrival of the first mammoths in North America was a significant turning point in the development of the local wildlife. It's so important that this date, 1.9 million years ago, marks the beginning of the first of just two 'land mammal stages' that define North American wildlife during the Ice Ages. It used to also mark the beginning of the Pleistocene itself, but for various reasons, that's now been shifted a little further back.

The mammoths in question arrived from Asia, crossing over the Bering land bridge, the recurring appearance and disappearance of which greatly influenced North American wildlife during this time. They were southern mammoths (Mammuthus meridionalis), the dominant species of mammoth in Asia at the time, but they quickly evolved into a home-grown American animal: the Columbian mammoth (Mammuthus columbi).

It used to be thought that, even ignoring any late-surviving southern mammoths, there were at least two different species of mammoth living in North America in the early to mid Pleistocene. We're now pretty confident that they're all just examples of Columbian mammoth. Nonetheless, you will often see references to the "Imperial mammoth" (Mammuthus imperator). Perhaps the biggest elephant that has ever lived - they were about thirteen feet tall at the shoulder - these were probably just really big Columbian mammoths. Not that that's anything to sneeze at, mind you.

Even for their impressive size, however, Columbian mammoths also had exceptionally large tusks. These may have reached as much as five metres in length (16 feet) in fully grown bulls, making them quite a bit longer than the animal was tall. They were also probably hairier than living elephants, although it's obviously difficult to say by how much. They were widely spread across North America, south of the ice sheets, although they probably preferred forests to some of the harsher, northern, environments. Fossils are known from most of the US states, aside from New England, as well as from south-western Canada. Further south, they ranged across Mexico, and, at their very furthest extent, Nicaragua - evidently, they could cope with a fair range of different habitats.

Back in Asia, southern mammoths had evolved, perhaps via the steppe mammoths, into the more famous woolly mammoths (Mammuthus primigenius) of Europe and Siberia. Around 0.15 million years ago, during the last, and most severe, of the Ice Ages, these woolly animals followed the route of their ancestors across the Bering land bridge and, they, too, entered America.

Although they occasionally seem to have wandered further south, woolly mammoths for the most part stuck to the frozen north, just beyond the edge of the glaciers: their fossils are most common in Canada and Alaska, and in a band stretching from the Dakotas to Wisconsin. With the Columbian mammoths inhabiting less hostile climes, the two species probably didn't compete very much, and both species seem to have done fairly well. However, there were some areas where the two were found side-by-side, perhaps focussing on different vegetation, since woolly mammoths were likely much more fond of grass than their Columbian kin.

Indeed, not only did the two species encounter each other from time to time, there's actually good evidence that they interbred. The so-called "Jefferson's mammoth" (Mammuthus jeffersoni) that lived in North America around this time, may even be a hybrid, although it's equally possible that it was just an odd-looking Columbian mammoth, and a minority of scientists still regard it as a distinct species.

Back when the very first southern mammoths crossed into North America, however, they had discovered that they were not alone. There was another, very similar, animal already there waiting for them. But this was no mammoth: it was the American mastodon (Mammut americanum).

Unlike mammoths, mastodons were not members of the elephant family. To modern eyes, they would certainly have looked as if they should have been, but that's because, today, there really isn't anything else that looks at all like an elephant. But it wasn't always so.

Mammoth tooth (top)
Two mastodon teeth (bottom)
Elephants (including mammoths) are members of the wider Proboscidean order of mammals. Today, they're the only ones left, but there used to be more - quite a lot more. The mastodons are, perhaps, the best known of these other Proboscideans. In evolutionary terms, they parted company from the elephants around 27 million years ago, over ten times further back than than even the very beginning of the Pleistocene. So, while, by the time that mammoths turned up, the American mastodon was the only one left in North America, they already had a very long pedigree of being not-quite-elephants.

One of the key differences between the elephant family and the mastodon family is the shape of their teeth. Apart from the tusks, elephant and mammoth teeth have a long series of narrow, parallel ridges. Mastodon teeth, on the other hand, have a set of rounded bumps that, unaccountably, reminded 19th century palaeontologists of human breasts (one assumes they didn't get out much). Indeed the word 'mastodon' literally means 'breast-tooth', and is related to medical terms like 'mastitis' and 'mastectomy'.

Fortunately, if you had seen a mastodon in the flesh, you wouldn't have had to open its mouth to prove it wasn't a mammoth. For one thing, they were quite a bit smaller. A woolly mammoth was marginally smaller than a modern African elephant, while all the other sorts of mammoth were even larger. The American mastodon, on the other hand, at around 2.3 metres (7'7") at the shoulder, was roughly the size of a modern Asian elephant. The tusks, while impressive, especially in the males, were shorter, and without the spectacular curving sweep of mammoth tusks.

While mammoths had a high, domed, skull and massive shoulders, mastodons had a flatter, longer head and a straighter, more gently sloping back. On the other hand, they probably had a similar pelt of brownish fur, and it's likely that the ears of both species were more like those of Indian than African elephants, since they didn't live in such a hot environment as the latter. All in all, though, it should have been fairly easy to tell them apart.

American mastodons seem to have lived across the whole of the North American continent, from the Yukon to Mexico, although they probably only ventured into the north during the warmer gaps between the Ice Ages. They lived, for the most part, in dense forests, and they seem to have had quite a liking for marshes and bogs, if the distribution of their fossils is anything to go by. All of this would suggest that they were far less cold-adapted than the woolly mammoth, at least. Which, given that they first evolved some time before the Pleistocene, is unsurprising, although it's worth noting that they survived all the way through it.

We can tell from the pattern of microscopic scratches on their teeth that, like Columbian mammoths, but unlike the woolly kind, they were browsers. That is, they ate particularly lush food, such as leaves, swamp plants and the occasional bit of fruit, rather than scooping up great bunches of tough grass. In some respects, though, their diet was more like that of modern rhinos than elephants, and they seem to have eaten a lot of bark and twigs. Most likely, they ripped the bark off trees with their tusks - that one tusk is often more worn down the other may suggest that individuals were either right or left 'handed'.

In fact, we can go even further, because there are a few examples of the animals' stomach contents being preserved. These support the evidence from tooth-wear analyses, although they do suggest that the more northerly mastodons ate a lot more coniferous twigs than those further south.. They seem to have been particularly fond of spruce, and spruce forests may well have been their preferred habitat.

In the late Pleistocene, therefore, there were three large, elephant-like species living in North America: the Columbian mammoth and the American mastodon across much of the continent, and the woolly mammoth further north, close to the ice sheets. All three died out at around the same time, just as the last Ice Age was ending. That's also around the time that humans first entered North America, and it's likely that hunting and climate change both helped to spell their doom.

Still, they do seem to have struggled on for quite some time, into the modern Holocene epoch. One mastodon fossil from Indiana, for instance, has been dated to around 9,500 BC, thousands of years after the first humans arrived. Which doesn't entirely let us off the hook, but at least proves we must have taken longer to wipe out the mastodons than we did the passenger pigeons.

[Painting by Charles R Knight, copyright expired. Photos by Remi Mathis and "Jstuby" from Wikimedia Commons]

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