Pliocene (Pt 15): Life on the Australian Grasslands

Kolopsis, a diprotodontid

At the dawn of the Pliocene, Australia was a relatively green continent, with plenty of rich, tropical and subtropical, vegetation. That changed as millions of years passed, with the continent becoming steadily drier and the grasslands and semi-desert of the Outback came into being. This was, of course, bad news for many of the animals that had lived there in the wetter past, many of which went extinct, but it also saw a noticeable increase in the number of grazing animals, for which wider grasslands were clearly a boon.

Elsewhere in the world, this sort of thing was benefiting animals such as horses, goats, and antelopes. But Australia was different. It wasn't, of course, the only island continent of the day, but it was the oldest by some margin, having separated from its neighbours long before South America split from Antarctica, or before animals stopped crossing between Eurasia and North America (even ignoring the Ice Age crossings of the Bering land bridge, which were, at this point, still in the future).

This isolation had given it a unique fauna, which, as we all know, it retains to this day. Nor were all of the Australian mammals of the day marsupials or monotremes, as one might expect. Even at the dawn of the Pliocene, the continent already had bats, and had actually done so for something like half of its history. At some, much later, point, however, as Australia edged closer to south-eastern Asia, they were joined by rodents.

There weren't very many, to be sure, and all were members of the mouse family, but there were some, and with no non-marsupial competition, they quickly adapted to the dry open country, creating a number of new species unique to the continent. Quite when this happened is still unclear. The oldest rodent fossils we have from the continent are around 4.5 million years old, placing them near the beginning of the Pliocene, but there were already a fair number of species by that time, so the original immigrants may well have arrived much earlier, towards the end of the previous epoch.

At any rate, their descendants are still there today. Around 50 species of native Australian rodent are currently recognised, in addition to those more familiar animals that we have brought across ourselves in more recent millennia.

But it was the marsupials that were undoubtedly the most prominent Australian mammals of the Pliocene, just as they are today. In the absence of hoofed mammals, the grazing animals that became dominant were rather different from those on other continents. Two groups, in particular, seem to have prospered during the Pliocene, although our knowledge of the individual species within them is relatively sparse compared with their counterparts elsewhere.

One of these groups was the kangaroo family. In the earlier part of the epoch, when Australia still retained some of its greenery, particularly in the tropical north, these included a number of forest-dwelling wallabies, such as Silvaroo, but these were pushed north as the continent dried out. From about the middle of the Pliocene onwards, they had vanished from Australia proper, and were found only in and around New Guinea, where similar animals still survive. Back on the continent, they were replaced by larger species belonging both to the modern kangaroo lineage, and to the now-extinct "short-faced kangaroos". The latter, which would later become far more numerous and grow to considerable size during the Ice Ages, weren't quite so impressive at this time. For much of the epoch, they were also probably outnumbered by the ancestors of modern kangaroos, such as Prionotemnus and Kurrabi, which were better adapted to eating the tough vegetation of the dry continental interior.

Rather more distinctive were a group of large wombat-like animals known as diprotontids. The most famous of these animals, the hippo-sized Diprotodon, lived much later, but the Pliocene forms were already quite large. Euowenia, for instance, which is known to have lived across the full width of the continent, had unusually sturdy wrists that suggest it also possessed a heavy, relatively low-slung, body, part way along the path to the later giant animals. Indeed, we have actually found fossilised footprints of what appears to be the same animal, made crossing a clay bed on the banks of the Warburton River in the continental interior. The way that weight seems to have been distributed in these footprints, also suggests a slow moving, somewhat cumbersome creature, while the number of them found together implies that it may have lived in small herds or family groups.

Euowenia was not the only large diprotontid of its day. Euryzygoma, with its flaring cheekbones, has been estimated to weigh around 500 kg (1,100 lbs), while others, such as Nototherium and Kolopsis, weren't exactly small, either. They appear to have been well adapted to grazing or eating tough vegetation, although this may well have included leaning up against gum trees - which were becoming more common at the time - in order to munch on the leaves.

Diprotodonts may have resembled wombats, and were, indeed, related to them, but they were not members of the wombat family that survives today. We know of relatively few true Pliocene wombats, but many of those we do know of were relatively large, and typically did not have the adaptations for burrowing seen in the more modern forms; it has been suggested that the ability to dig for shelter may explain why the living species survived through the increasing desertification of Pleistocene Australis when their relatives did not. Back in the Pliocene, however, even very primitive forms such as Warendja, which was not much bigger than a rabbit, lived alongside their larger kin.

At least, we assume so; we have fossils that date to shortly before the start of the Pliocene, and some that date from after it finished, but nothing from the gap in between. Unless one or other of these have been misidentified, which seems unlikely, they must have lived through the Pliocene, too. The same problem arises (probably), on a broader scale, with koalas, which haven't left any undisputed Pliocene fossils that I'm aware of, but which were certainly around in the previous epoch. Judging from those older specimens, though, we can say that Pliocene koalas probably looked very much like the ones alive today, although they may have had a broader diet.

We do know of at least three species of Pliocene bandicoot, which were so similar to the living forms that two of them aren't even placed in a different genus. That would make them omnivores, eating worms and insects alongside seeds and fruit, but true carnivores naturally also existed. These include Glaucodon, a relative of the Tasmanian devil, and Archerium, thought to be related to mulgaras, rat-sized marsupials that today live only in deserts and feed on insects and small reptiles.

Marsupial "lions" of the genus Thylacoleo also existed, although they weren't particularly lion-like. They were, however, deadly flesh-eaters, and would were the ancestors of an even more highly specialised species that lived in the succeeding, Pleistocene, epoch. And, if they were a forerunner of a better known creature that lived later, at the other end of the scale, the rather strange carnivorous wallaby Ekaldadeta still struggled on from the preceding Miocene.

Which brings me almost to the end of my survey of Pliocene mammals. However, we aren't quite done yet, as there is one quite important group of mammals from the time that I haven't looked at yet...

[Photo by Mark Marathon, from Wikimedia Commons.]