Sunday 18 February 2024

Oligocene (Pt 7): Not Quite Camels, Not Quite Pigs

While the ruminants of Oligocene North America would have looked similar to the musk deer of today, some of the other cloven-hoofed mammals inhabiting the continent at the time were more distinctive. Protoceratids no longer survive, but they had already been around for millions of years at the dawn of the Oligocene, and would survive throughout the whole of the following epoch and a little way into the one after that - an impressive record. Despite this, they never seem to have been very common, and the only undoubted Oligocene example is Protoceras, known primarily from Nebraska, Wyoming, and South Dakota.

It remains unclear exactly what protoceratids were, beyond the fact that they were obviously related to other cloven-hoofed animals. Some features suggest that they were closely related to ruminants (as was assumed when they were first discovered in the 19th century) while others indicate a close relationship to camels; it may even be that they are some early branch that doesn't fit well with either. Despite being the animal for which the group is named, Protoceras is not so well known as its later relatives, many of which notable for possessing a third horn on their snouts in addition to those in the place we'd expect to find horns on a goat or antelope. 

Protoceras, however, was relatively primitive and had yet to develop this unusual third horn. Instead, while females seem just to have two small, regular horns, the males not only had larger ones but two extra pairs on the snout for a full set of six horns in total, plus a pair of fang-like teeth in the upper jaw. They otherwise had a similar shape and size to roe deer, being something like 1 metre (3 feet) in body length and plausibly had a similar diet; their low numbers may be due to them relying on limited patches of riverside forest for their food. Analysis of the bones in their ears that would have held their organs of balance suggest that they were as agile as many antelopes if not quite up to the standards of, say, a gazelle.

Peccaries had first entered North America towards the end of the previous epoch, but were still not particularly diverse, with only two genera clearly identified from the Oligocene (although some fossils are hard to place, so there were likely more). The earlier of the two was Perchoerus, a genus sufficiently primitive that it has been argued that it may be an early side-branch of the pig-peccary ancestral stock rather than a true peccary - although this does not appear to be the current consensus. Whatever it was, the larger of the two Oligocene species of Perchoerus was about the size of a modern peccary, and was joined towards the end of the epoch by Thinohyus, which was notably larger with larger teeth and a comparatively elongated head.

If neither peccaries nor protoceratids were especially common or varied, the same was not true of the oreodonts. Today, these are generally considered to be more closely related to camels than to true ruminants, although the relationship probably isn't very close, and most of them certainly didn't look at all camel-like. Instead, most would have looked somewhat like pigs, with squat bodies and stocky limbs and without any horns or other adornments. Fossil oreodonts are common across much of North America, being best known from the same extensive White River fossil beds as Protoceras, but also found across the western US and as far afield as FloridaMexico, and Central America.

This wide distribution suggests that at least some of the species were highly adaptable, able to eat a wide range of plants across many different habitats. The exact number of known species is debatable; it's generally recognised today that many of those named in the mid 20th century aren't "real" in the sense of being genuinely different from one another, but there are so many that it would be a major undertaking to sort it all out.

In the early Oligocene, the most common examples were Merycoidodon and Miniochoerus, both of which seem to have been present in large numbers. (Indeed, the technical name for oreodonts is "merycoidodontoids" due to some confusion over the original name). The former was about the size of a pig, and the latter somewhat smaller, but both were relatively unspecialised herbivorous animals with a full set of teeth in the upper jaw and biting canines. Notably, while they had the general cloven-footed pattern to their feet, they still walked on four toes on each foot. From around the middle of the epoch, they began to diversify, with a general trend to a diet of tougher food, perhaps switching from forest fruits to soft-leaved browse as a primary food source, and later still to a mixed browsing-grazing lifestyle. 

These later forms include Merycochoerus, which has a physique that may indicate that it spent a lot of time wading in swamps or swimming in rivers, and the physically large but short-snouted Eporeodon. One of the more abundant examples from this later time is Leptauchenia, notable for teeth adapted to chewing tough plants, large inner ear cavities, and eyes placed unusually high on the head. Deep pits in the skull in front of the eyes most likely held large scent glands, similar to those in some modern animals such as gazelles. The high position of the eyes was once used to argue that they may have been semi-aquatic, peeking up above the water surface like a hippo, but the geology of the fossil beds they are found in suggests that they lived in arid habitats dominated by wind-blown sand.

While many later oreodonts were larger than the earlier forms, reaching the size of a large sheep, a later relative of Leptauchenia went in the opposite direction. Otherwise looking similar to its relative and likely living in similar environments, Sespia, known from western Nebraska to the California coast, was about the size of a house cat.

Agriochoerus was closely related to the main oreodont family, and sometimes placed within it, but it's generally considered to have been weird enough to get a family of its own (and whether that still leaves as an 'oreodont' or just something similar is a matter of the precise definition). In many respects, it resembled the oreodonts proper, being similarly sized, having the full set of teeth, including the sharp canines, and walking on four toes on each foot, rather than two. But it had a shape closer to those of their assumed common ancestor, with a slender body and a long tail. 

More strangely, however, the hooves on its feet had sharp points and were shaped in such a way that it's hard to describe them as anything other than "claws". Since this is still a cloven-footed mammal we're talking about, and every other indication is that it was herbivorous, there has naturally been some debate as to what the purpose of these claws was and why the animal was, broadly speaking, shaped like a cat. Some early proposals suggested that it may have used the claws to dig burrows in the ground, but this is now thought unlikely, and the best bet remains the original one from the 19th century when it was discovered - that this was a cloven-footed herbivore that climbed trees.

It probably wasn't great at climbing, being the size of a large pig and completely lacking opposable thumbs, but perhaps it didn't need to be. It was, after all, still smaller than a leopard, let alone a bear, and those are capable of climbing. One unpublished analysis apparently compared the limb proportions of Agriochoerus to those of the herbivorous and equally implausible-sounding tree kangaroos, which are an actual thing that exists today, so perhaps those would work as an analogy.

All of the above groups were unique to North America, but the remaining group of Oligocene cloven-hoofed animals on the continent was also present in Europe and Asia at the time. The entelodonts, informally known as hell-pigs, were represented in Oligocene North America by Archaeotherium, a long-legged but heavily built animal standing around 1.2 metres (4 feet) high at the shoulder and likely weighing at least 280 kg (620 lbs). Despite the name, which comes from a superficial resemblance and older classification systems, they were probably closer to hippos than to pigs. 

Like other hell-pigs, Archaeotherium had a massive head with long powerful jaws and an array of powerful teeth that suggest it was an omnivore that often fed on tough food. While it lacked the sharp slicing teeth of true carnivores, this almost certainly included scavenging on carcasses. Fossils of smaller mammals such as Merycoidodon and the camel Poeboetherium have been found with tooth marks interpreted to have been caused by hell-pigs, and that might even suggest limited active predation. The skull also has large bony flanges and lumps, somewhat reminiscent of those of warthogs, and likely used in the same manner, in combat between males - again, there are scratch marks on some of their skulls that support this interpretation.

The hell-pigs died out in Eurasia towards the end of the Oligocene, but in North America, Archaeotherium was instead replaced by the even larger, rhino-sized, Daeodon although whether that was the result of a last-gasp migration from Asia or a direct descendant of its predecessor is unclear. That survived into the following, Miocene, epoch before it too died out bringing the lineage of the hell-pigs to an end.

However, not all large mammalian herbivores in Oligocene North America were cloven-hoofed and next time I will be looking at some of the others...

[Painting by Charles R. Knight, in the public domain.]

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