Oligocene (Pt 3): From Musk Deer to Hell-Pigs

Entelodon

The Grande Coupure that marked the beginning of the Oligocene in Europe did not have as dramatic an effect on the cloven-hoofed animals as it did on some other mammalian groups. But the change was nonetheless noticeable, with several primitive forms that had once inhabited the continent suddenly dying out as it merged with Asia. Others, however, survived through the break, and the absence of their earlier competitors may even have helped them prosper.

These include the gelocids, which first appeared close to the end of the previous epoch. Few of the known fossils of these animals are in good condition, and there is some debate as to whether they are a true group of animals at all, or just a vague term used to collect similar-looking creatures that we can't place elsewhere. That aside, we can at least say that they physically resembled (but were probably not related to) musk deer. That is, they were relatively small, hornless animals with long legs suited for running fast, but lacking the large fang/tusks that mark true musk deer. They did well enough that some, such as Pseudogelocus, are known not only from France and Germany, but also Mongolia, suggesting that they crossed over in the opposite direction to most other mammal groups.

Once considered to be a gelocid, Bachitherium is unusual in having a relatively complete fossil skeleton. This provided enough detailed information that, in 1985, the animal was separated from the gelocids proper to be placed in a family all of its own that suddenly appears at the Coupure, probably having descended from an Asian immigrant. While some studies suggest that they may be closely related to (at least some of) the gelocids, at least one recent example places them closer to the chevrotains - small hornless animals native to Asia and Africa. Like chevrotains, they seem to have been browsing animals that inhabited moist, perhaps even marshy, forested habitats.

The cainotheres are another example of primitive European cloven-hoofed mammals that survived the Coupure, and prospered in the aftermath - eventually surviving well into the Miocene. Oligocene examples include the rabbit-sized Plesiomeryx from France; they were once thought to be over on the camel half of the cloven-footed family tree, but are now thought to be primitive relatives of true ruminants. It's also worth noting that the oldest known true musk deer, Dremotherium, first appeared in France towards the end of the Oligocene; it had a longer neck than modern species and may have browsed on high bushes. Similarly, a recent study identified Nalameryx of northern India as possibly the oldest-known chevrotain, even though it lived in a much drier habitat than the modern species.

Some other primitive cloven-footed animals surviving into Oligocene Europe from earlier times are harder to place. Dichobune was a rabbit-sized, probably fast-running, animal with a long tail whose relatives had been quite common in the preceding epoch, and managed to last much of the way through this one. Diplobune was unusual in having three toes on the forefeet (albeit one of them greatly reduced and possibly non-functional) and lived in forested or marshy habitats. Early suggestions that the shape of the feet implied it was semi-aquatic do not seem to be borne out, although it probably walked at least partially on its soles, rather than on tip-toe as cloven-footed animals more commonly do.

There are relatively few pigs known from Oligocene Europe, but one of the more common seems to have been Palaeochoerus. These were small long-snouted animals and, while they had the four-toed feet that we find in modern pigs, they still had a full set of teeth, without the reduction in incisors and premolars that pigs have today. While there is reason to suppose they were already shifting towards an omnivorous, rather than herbivorous, diet, this makes it difficult to place them. Once referred to as early peccaries, that's no longer considered accurate, but they are sufficiently primitive that they're essentially on the dividing line between true pigs and not-quite-yet-pigs and can reasonably be placed in either category. Assuming they are the former, they're likely the earliest example, suggesting that pigs first evolved in southwestern Europe from an Asian ancestor shortly after the Coupure. Other Oligocene examples, such as Doliochoerus, have fewer pig-like features and are more likely close relatives, rather than true pigs.

In a time before antelopes, goats, and so on even existed, many of the other cloven-footed animals of Oligocene Europe were also at least partially piglike. The anthracotheres are thought to be more closely related to hippos than to pigs, and may even include the ancestors of that group (which first appeared in the following, Miocene, epoch). Like hippos, at least some of them are thought to have been semi-aquatic, which perhaps allowed them to cross between merging continents earlier than some other animals did. 

Bothriodon and Elomeryx are thought to be especially closely related to the hippos and would have looked rather similar, albeit they were no larger than a modern pig. Bothriodon was the larger of the two, by some estimates weighing around 200 kg (440 lbs); it lived from western Europe right across to Japan, which at the time had barely separated from the Asian mainland, if it had at all. Elomeryx was not that much smaller, and reached at least southeastern Europe sometime before the Coupure, while already being widely spread across Asia. With broad feet for walking on soft mud, and a relatively long snout, another notable feature is that the upper canine teeth of around half the fossil specimens are serrated - the suggestion is that these were the males.

There was enough diversity within the anthracotheres that is unlikely they were all as semi-aquatic as sometimes supposed, with some being adapted to a drier habitat. Microbunodon of late Oligocene France was one of the smaller ones, perhaps weighing around 20 kg (44 lbs) similar to, say, an English Setter. It appeared suddenly as the climate dried in the Late Oligocene, which may give some idea of its preferences. At the opposite end of the scale is Anthracotherium and its close relative Paenanthracotherium, which was close in size to a modern hippo, with some estimates suggesting that the largest species could have weighed as much as two tons, and was likely very reliant on water. Unlike the slender, long-snouted Elomeryx, these had powerful jaws and large, shovel-like teeth.

Although they may not have been quite so large, the entelodonts, or "hell-pigs", would have been decidedly more fearsome-looking in the flesh. The oldest entelodont fossils come from China, and date from slightly before the Coupure, but they soon crossed over to both Europe and North America, finally dying out on the latter continent during the Middle Miocene. Unlike some of the other pig-like mammals of the day, they may have been quite closely related to the true pigs, although clearly not examples of them. Some studies, however, have placed them as related to the common ancestor of hippos and whales - much hinges on the exact placement of some even earlier animals in the family tree.

The best-known Oligocene example, the European Entelodon, stood around 135 cm (4' 5") at the shoulder, while its late Oligocene Asian relative Paraentelodon was even larger. They had long heads with powerful jaws, wide flanges on the cheekbones, and bony tubercles on the base of the lower jaw. These dramatic features make them sound like specialised animals and, to some extent, they probably were, but in other respects, they seem comparatively primitive. The rest of their body, for instance, apart from the size, was unremarkable, although their cloven hooves had already lost the "dewclaw" digits still seen in modern pigs. 

The name 'entelodont' means "complete teeth" and refers to the fact that, like other primitive cloven-hoofed mammals, but unlike all modern examples, they still had a full set of teeth. The shape and wear patterns on these teeth suggest that, despite the fearsome appearance, hell-pigs had a diet not that much different from a wild boar, omnivorous, but hardly active hunters since they weren't adapted for fast running. Having said which, they almost certainly scavenged on dead animals, some of which may well have been quite large, since their jaws look strong enough to crush bone.

However, the cloven-footed animals were by no means the only herbivorous mammals in Oligocene Eurasia, nor even the largest. It is to some of those animals that I will turn next time.

[Picture by "concavenator" from Wikimedia Commons.]