Oligocene (Pt 4): Time of the Giants

Paraceratherium

The Grande Coupure saw many new animals pouring across from Asia into Europe. Whether or not the tapirs were among them is not immediately obvious. The oldest tapir that we know of is a fossil discovered in Germany and dated to the early Oligocene, not long after the Coupure. Named Protapirus, we know that it, or its very close relatives were found not only in Asia, but also North America, by the second half of the epoch and managed to survive, with relatively little change, into the Early Miocene that followed, dying out about 20 million years ago. 

At first glance, since the oldest fossil is German, it appears that tapirs originated in Europe and then spread east, and it's purely a coincidence that they happened to do so after the Coupure - which, after all, was a time of climatic change. The problem is, there wasn't anything remotely tapir-like living in Europe before the Coupure, but there were plenty of potential ancestors elsewhere. So it's perhaps more likely that the first true tapirs were Asian, and we simply haven't found their fossils yet. Even so, we can at least say that Protapirus, and its later relative Paratapirus (which never seems to have left Europe) really were tapirs, rather than some close relative. A key feature here is that, unlike their earlier relatives, they already had the modifications to the nasal bones that suggest the presence of the short trunk that modern tapirs have, although it was probably less prominent than in current species.

There is less uncertainty about the rhinos; we know that they were present in Asia before the Coupure and crossed over to Europe as soon as the opportunity presented itself. Unlike the tapirs, they would become a significant element of European wildlife almost to the present day, dying out only when the last Ice Age glaciers began to retreat a mere 0.02 million years ago. Among the first was the hornless species Ronzotherium, which is known to predate the Coupure in Mongolia and China (although these may simply be very close relatives), and to have reached Switzerland almost immediately after the event. It was around the size of a modern rhino, although the shape of the skeleton suggests that it was rather slimmer, perhaps only weighing a ton or so. The teeth already resemble those of modern species, and it likely had the highly mobile grasping lip that they do, suggesting a browsing diet.

Not all of these early European rhinos were necessarily hornless. Epiaceratherium, an animal about the size of a tapir may have had small horns on its snout - although, since rhino horns are not formed from bone, and do not fossilise, it is difficult to be sure. It was unusual in having a full set of teeth in the upper jaw, and four, rather than three, toes on the hind feet, features that mark it out as primitive compared to more familiar forms. It died out at around the same time as Ronzotherium in the middle of the epoch. While that left no descendants, however, Epiceratherium was replaced by the related Molassitherium and Protaceratherium, with the latter surviving into the early Miocene. These were still small and lightly built by rhino standards and were probably fairly fleet-footed. They seem to have been among the dominant rhinos in Europe during the late Oligocene.

They were, however, not alone. Two lineages of rhino that would become more significant during the following, Miocene, epoch both had their origins in the late Oligocene. Mesaceratherium, although sometimes regarded as a close relative of modern rhinos, is more commonly thought to be the earliest known acerathere rhino, a group of relatively small hornless rhinos with powerful chisel-shaped incisor teeth and heavy jaws. Unusually complete fossils from the Agen region of France suggest that they were slim and fast-running.

The second group were the teloceratines, first represented by Brachydiceratherium, which is also known from Asia. They resembled the aceratheres in having tusk-like incisor teeth and strong, but narrow, jaws but they had heavy bodies and short limbs, still retaining the fourth toe on the front feet. They are thought to have had small horns, perhaps because, like the aceratheres, their bite was powerful not to need them for defence. Their larger relative Diaceratherium lived slightly later, and survived into the Miocene, when the group began to diversify further.

While, today, rhinos and tapirs are each other's closest living relatives, the group of which they are a part was more diverse in the Oligocene, including at least three different kinds of animal that we can broadly describe as rhinocerotoids, but that were too distinct to be placed within the living rhino family. Originally known only from Europe, Eggysodon has recently been discovered in China as well and may well have originated there. Previously regarded as a hyracodont, or "running rhino", they are now more commonly placed in their own family together with a small number of Asian relatives. Even so, the resemblance to the "running rhinos" is clear; Eggysodon was about the size of a sheep and with slender limbs adapted for fast running and large tusks in the males, probably looked nothing like any rhino we would recognise today.

The amynodontids were rather different, being heavily built, squat animals with short limbs. Sometimes called "aquatic rhinos" these are thought to have spent a lot of time in the water, with a lifestyle that may have resembled a modern hippo. Unlike the other groups, the amynodonts were already present in Europe before the Coupure, having crossed over from Asia at a much earlier date - their presumed swimming ability perhaps having made the trip easier for them than for others (although presumably no stroll in the park, either). 

Cadurcotherium was for a long time the only known European example from the Oligocene, but it has not been as well studied as its close relative Cardurcodon from Asia. The largest specimens, at an estimated 2 tons, were slightly larger than the average modern hippo and probably had a similar build, as well as large vaguely hippo-like tusks. One distinct difference from hippos, however, is that they are thought to have had a tapir-like proboscis on their snout. The recently discovered Sellamynodon from Romania, however, belongs to a different branch within the family, and likely didn't have this feature, instead relying on a flexible lip - which, admittedly, is equally un-hippo-like.

The third group of not-quite-rhinos that lived during the Oligocene have gone by various names down the years due to confusion about which genera were valid and which one the defining species belonged to. They originated in Asia during the previous epoch, when they had appeared as horse-sized long-legged animals somewhat similar to the "running rhinos". But, as millions of years passed, they kept growing in size, soon surpassing every other rhino. By the time we reach the Oligocene, they were gigantic. By some estimates, they were, in fact, the largest land mammals ever to have existed, although some recent studies have given that honour to a species of mammoth - much hinges on how we measure the weight of something we know only from a skeleton, and whether "big" refers to mass alone, or incorporates height as well.

The animal in question is now known as Paraceratherium, although older books still use previous names such as Indricotherium and Baluchitherium - they are all the same creature. Originally identified from deposits in Pakistan, they are known to have lived from at least northern China to Romania and their teeth suggest that they primarily fed on soft leaves, likely from treetops. Large tusks were previously thought to have been used to strip bark from trees, but these are now thought to have been present only in the males, suggesting that they may have had more to do with competition for mates.

But it's their sheer size that's their most famous, and notable, feature. Both Paraceratherium and its close relative from Central Asia, Dzungariotherium, stood about as tall as a giraffe, but, while some of that is due to their long legs, their neck was proportionately similar to that of a horse, so that, at around 5 metres (16 feet), their shoulder height would have been greater. With a heavy body, their legs were necessarily much broader than those of giraffes, too, and current estimates of their weight hover around 20 tons - twice that of an African elephant. By some estimates, this may be as large as it is possible for a herbivorous mammal to be, due to limitations of the digestive tract.

I'll note in passing that it has been suggested that, in order to cool down its immense body, Paraceratherium and its ilk may have had large, flapping ears, just as elephants do. Clearly, that's hard to tell from a skeleton, and, like much else that's been written about them, is largely speculative, so I'll merely note that other scientists feel differently, and move on.

All three of these groups - the acerotheres, amynodonts, and paraceratheres - died out at the end of the Oligocene, leaving only the living rhino family to make its way forward to the present day. Horses and elephants had, at the time, yet to reach Eurasia from their birth continents, while the palaeotheres, an ancient group related to horses that were previously known from Europe, died out during, or shortly after, the Grande Coupure. 

However, one other group of large, non-cloven-footed herbivores did cross over from Asia, although not in large numbers. These were the chalicotheres, animals distantly related to both tapirs and rhinos but that had clawed feet and were able to rear up on their hind legs to reach tree branches. Schizotherium and Borissikia are early European examples, although they were smaller and less specialised than those we are familiar with from later epochs, such as the American Moropus. Also known from China, Schizotherium had claws on the hind feet as well as the front ones, and, notably, they seem to have been retractable, which would have allowed them to walk in a fairly normal four-footed manner rather than knuckle-walking like a gorilla, as their later relatives were compelled to.

But, as all these various kinds of herbivore crossed over into Europe, predators were bound to follow. The Grande Coupure affected carnivores just as much as it did their prey, with many new animals reaching the newly available lands. It is to some of those creatures that I will turn next time.

[Picture by "ABelov2014" from Wikimedia Commons.]