Saturday, 7 June 2014

Pleistocene (Pt 15): Ice Age Down Under

The island continent of Australia has today what is probably the strangest mammalian fauna of any continent. Yet it is also the continent that has, perhaps, suffered the greatest number of mammalian extinctions over the last 50,000 years or so. Many of those are relatively recent, or at least after the end of the last Ice Age around 10,000 BC. But even compared with the Australia of, say, the 18th century, the wildlife of Ice Age Australia looked pretty odd.

In climactic terms, Australia didn't suffer too badly from the Ice Ages. It's too close to the equator to have had ice sheets get anywhere near it, although doubtless there was rather more snow on the mountains. (Although perhaps not too much - even today, Australia is the only continent to lack glaciers). Then, as today, much of the continent consisted of desert, and the bits that weren't were mostly arid grassland, albeit with denser woodlands around the eastern and northern coasts.

A rather more dramatic difference was the lower sea level, caused by so much of the ocean's water being locked up in the ice caps around the poles. Not only Tasmania, but also New Guinea, were simply peninsulas of the Australian continent. The latter had nearly as much jungle as it has today, a densely vegetated paradise compared with most of the lands further south. This is partly why there are so many marsupials in New Guinea today, with a vast and perfectly hospitable Pleistocene land bridge having once connected it to the continental mainland - interrupted by, at best, a salty inland sea in the deepest parts of the Gulf of Carpentaria.

As we've seen on the other continents, a lot of animals were larger in the Pleistocene. Nowhere, however, does this seem to have been more true than on Australia. In its isolation, Australia had no woolly mammoths, no rhinos, and no true sabretooth cats. But it did have some really big marsupials.

Not that Australia doesn't have animals other than marsupials, of course. Even among mammals, small rodents snuck onto the continent long before the Ice Ages began, presumably by rafting across on floating bits of vegetation from nearby Indonesia. There are also bats, which likely found the seas even less of a barrier. And, naturally enough, invertebrates, amphibians, and freshwater fish. But none of these were exceptionally large, or at least any more so than today.

There were, on the other hand, some pretty big reptiles. The monitor lizard Megalania seems to have been at least 50% longer than the living Komodo dragon, and possibly quite a lot more (the skeletons aren't complete enough to know for sure). At at least 3.5 metres (11 feet) long it is, in all likelihood, the largest land-dwelling lizard ever to have lived. Although, yes, I really do mean to include the qualifier "land-dwelling" in that.

There were also some pretty big constrictor snakes and tortoises, but birds weren't being left out, either. Genyornis, which actually appears to have been an odd sort of flightless waterfowl, was at least 10% taller than modern emus - and quite a lot more heavily built. It survived long enough for early Aborigines to make what seem to be cave paintings of it.

But, of course, it's the marsupials we're interested in on this blog. Today, the world's largest marsupials are kangaroos, specifically the red kangaroo (Macropus rufus). That species stands around 2 metres (6 feet) in height, and some estimates show that its close Pleistocene relative Macropus titan may have been up to 25% taller than that. It's more generally accepted, though, that the largest of the Pleistocene kangaroos, and, indeed, the largest kangaroo ever, was Procoptodon goliah, the giant short-faced kangaroo, which lived in semi-arid environments across the continent.

Short-faced kangaroos were a now-extinct kangaroo subfamily that was distinct from all living forms. As the "giant" part of the name indicates, P. goliah was unusually large, and, in fact many other short-faced kangaroos were the size of wallabies. Their short, blunt muzzles would have easily distinguished them from any living species. This shape of muzzle is generally associated with grazing, rather than browsing, and suggests that they ate rather a lot of grass. The shape of the skull also suggests powerful jaw muscles, which would also make sense for an animal that eats a lot of tough grass rather than softer leaves.

There is some debate as to how accurate this is, especially in the case of the giant species, which has long arms that look like exactly the sort of thing you'd pull down branches with. Isotopic signatures in their bones certainly suggest that they ate plants that photosynthesised in the way that grass does, but, while there aren't many other plants that do this there are some, so it isn't a fool-proof indicator. On the other hand, the teeth are low-crowned, a shape that you wouldn't expect of a grass-eater, because they would wear down too quickly. Perhaps its smaller ancestors evolved for one thing, and it shifted to another.

Overall, the giant short-faced kangaroo wasn't really any taller than living red kangaroos, but it does seem to have been quite a lot heavier, perhaps reaching 200 kg (440 lbs), compared with 85 kg (190 lbs) for the red species. (Some internet sources, incidentally, claim that it was 3 metres tall - this is nonsense, although maybe if it stretched it could have reached branches that high). Its feet were also somewhat unusual, having only a single functional toe, instead of four. The toe in question was particularly wide, presumably to compensate for the lack of anything to either side of it, and ended in a claw that was almost hoof-like. These may have been adaptations for speed, and are about as close as marsupials have ever got to evolving hooves.

In a similar vein, there were also giant koalas (Phascolarctos stirtoni), although, to be honest, the name might be a bit of cheat. At twice the weight of the living species, they were certainly big for koalas, but likely not the most terrifying thing you've ever seen. Never a diverse group, the giant and modern species were probably the only two kinds of koala alive during the Ice Ages, and it's not entirely clear why the larger one is no longer with us.

On the other hand, not all giant extinct marsupials would have been so readily identifiable had you somehow wandered into Pleistocene Australia. Palorchestes azael is sometimes called the "marsupial tapir", which gives you only a hint of how strange it must have been. Indeed, our ideas about what on Earth it looked like have changed significantly down the years. It was originally identified only from its teeth, which look rather like the teeth of kangaroos. This led the first scientist to describe it, Richard Owen (better known for coining the word "dinosaur") to make the not unreasonable assumption that it was, in fact, a big kangaroo. Hence the name he gave it, Palorchestes, which literally means "ancient leaper".

Once we got more of the skeleton, it became fairly clear that, whatever else it was, it wasn't a kangaroo. We still don't have a complete one, and later reconstructions moved through something that kind of looked liked a llama, and then, via okapis and sloths, to... well, what we've got now. We now know of several related fossil species, but they're all quite a lot older, and quite a lot smaller. The group has no close living relatives, although, in the great marsupial family tree, they appear closer to wombats and koalas than to anything else alive today.

Our best estimate is that they were about the size of horses, and looked something like a cross between a tapir and a sloth. Which is possibly not the easiest combination to envisage. The sloth-like part comes from its bulky build, which certainly doesn't look like it would have been good for speed, and from the long, heavy, claws, which it probably used to scape at bark or dig up roots. The resemblance to tapirs comes from the suspicion that it had, like them, a flexible snout.

The shape of the snout is not certain, since it's not the sort of thing that would fossilise. There are, however, two bits of evidence that suggest that it may have included a short trunk-like proboscis. The shape and position of the bony nostrils is perhaps the key indicator, since they seem rather too far back to have been directly underneath the fleshy ones. One can't be sure, of course, since the nostrils may genuinely have been in an odd place, but the shape is suggestively similar to the skulls of tapirs. Supporting this, there are unusually large openings for nerves that would run to the snout, suggesting that it was sensitive, and had the complex innervation we'd expect of a short trunk.The shape of the lower jaw has also led to the suggestion that it had a long tongue, probably like that of a giraffe.

We can safely say that these animals were herbivores, and they were probably browsers, although possibly eating quite scrubby food, rather than soft leaves. At least one fossil was recovered from what used to be eucalypt-dominated swampland.

But, remarkable though the thought of a horse-sized sloth-tapir-wombat might be, even this was not the largest Pleistocene marsupial. And it was certainly not the most deadly...

[Picture by Nobu Tamura, from Wikimedia Commons]

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