Sunday 14 April 2024

Oligocene (Pt 8): The First Tapir and the Last Hoofless Horse

A close look at the evolutionary history of horses reveals that it's more complicated than sometimes presented, with numerous side branches forming a bushy tree of different species, many of which ultimately died out without descendants. This applies both to the origins of the group in its early days and its period of great diversification through the Miocene and Pliocene epochs. In the Oligocene, however, the picture, at least so far as we can tell, was rather simpler.

We know of two genera of horse that made it into the Oligocene from the preceding epoch. Mesohippus had been around for a while, Miohippus was a relative newcomer, appearing towards the very end of the Eocene. The two are rather similar, to the point that it has been argued they should be treated as different species of the same genus (which would be Miohippus, as that was named first) and they are both found in fossil beds across the United States and southern Canada, with Miohippus being known from Washington state to Florida and Mesohippus primarily in the west. The similarity also led to proposals early on that the one directly evolved into the other, but it's now clear that they lived alongside one another for millions of years, which scuppers that idea.

Having said which, Miohippus was likely an offshoot of earlier Mesohippus stock. It was larger than its putative ancestor, although by exactly how much is difficult to say, given the lack of any complete fossils. But even so, it was much smaller than modern horses, probably no more than 54 kg (120 lbs) at the most, making it the size of a large dog such as a Doberman. It's thought to have lived in dry woodland habitats that would typically have been cooler than those preferred by Mesohippus, which may help to explain its larger size. Indeed, it may be the generally cooler climate of the Oligocene that eventually caused the latter to die out, while Miohippus made it into the beginning of the next epoch. 

Judging from the wear patterns on its teeth, it seems to have been a mixed browser, with a diet more closely resembling that of an elk than a modern horse. Isotopic analysis of the teeth also shows that it most likely gave birth in the spring. They were, of course, still three-toed, with the shape of the foot suggesting that they still had soft pads at the base of their feet, which perhaps looked somewhat like those of a modern tapir. This may make it the last genus of horses to lack hooves as we would normally think of them. Despite this, it already had some of the adaptations to the ligaments that would later become refined to allow a full "hoofed" posture, and limb bones that suggest it was a moderately fast runner - if nothing like its modern relatives.

Tapirs are not something we would associate with North America today, but it may be that they first appeared on that continent in the early Oligocene, in the form of Protapirus, which had recently crossed over from Asia. In many respects, it, and its later American relative Nexuotapirus, were remarkably like its modern descendants, with the group having changed far less than horses have over the last 30 million years or so. 

The primary difference is that its premolar teeth were still easily identifiable as such, while in later forms they have come to resemble the true molar teeth behind them, improving the animal's ability to chew on tough vegetation. Even so, it's unlikely that its diet was very different to that of its descendants, either, with tapirs remaining as browsers while horses eventually switched to grazing.

However, while Protapirus may be the earliest known tapir, that's not entirely certain, at least partly due to uncertainty as to where exactly to draw the line around the family. Specifically, the North American animal Colodon is slightly older, but it's less clear as to whether it really counts as a "tapir" in the modern sense. Given the lack of other significant changes, the dividing line between tapirs and not-quite-tapirs has generally relied on the shape of the skull, specifically of the bones around the snout.

Unlike Protapirus, Colodon was traditionally regarded as not having the skeletal adaptations that would imply even a small proboscis - one of the living tapirs' most distinctive features. Under this scheme, it was considered the last of the helaletids, the only one to survive into the Oligocene. These are generally regarded as including the ancestors of the true tapirs, although Colodon itself may perhaps be too late to fill that role itself - and probably on the wrong continent. 

However, it now seems possible that Colodon did indeed have a proboscis, which throws the old scheme into doubt. It's certainly possible to come up with an alternative definition, based on the more primitive form of its teeth, that keeps it among the helaletids, but some researchers now consider it to be a true tapir, perhaps even one that was more closely related to the modern sort than Protapirus was.

While a great many different kinds of rhinoceros lived in Eurasia at the time, for much of the epoch Diceratherium was the only North American example - albeit the genus had multiple species. It was smaller than the typical rhino of today, with the earliest species estimated to have weighed around 500 kg (1,100 lbs) and later ones around a ton which is on the small side in comparison to typical members of modern species, but not by much. They were clearly successful, not finally dying out until well into the following, Miocene, epoch. Features of the skull suggest that it had two horns, located side-by-side on its nose, instead of one behind the other as we might expect.

This was, however, by no means the first rhino in North America, with the earlier genus Subhyracodon surviving for a few million years into the earliest part of the epoch before being replaced. This was even smaller, being similar in both size and shape to a tapir at around 300 kg (660 lbs). Its resemblance to a tapir was probably enhanced by its lack of horns, and the comparatively primitive head shape, although it obviously didn't have the proboscis. Its diet may also have been similar to that of a tapir, with it seemingly specialised for relatively high fibre browse, while isotopic analysis suggests that it lived in open, relatively dry country.

Diceratherium, however, was not the largest animal in North America during the Oligocene. That honour probably goes to Metamynodon, a member of a now-extinct group of rhino relatives called the amynodonts, which had previously crossed over from Asia and were still diverse there. Like its counterparts elsewhere, it had a body shape more like that of a hippo than a rhino, and it was also about the same size, with an estimated weight of around two tons. 

Like hippos, it is thought to have been semi-aquatic and it is probably the best adapted to water of all the semi-aquatic rhinoceratoids, living in flooded swampland. Another parallel with hippos came in the form of its unusually large canine tusks, which it probably used for fighting, but the shape of the snout also suggests a prehensile lip that hints at the relation to rhinos. It died out late in the epoch, although less specialised relatives survived for a while elsewhere.

We cannot, however, conclude our look at the mammalian herbivores of Oligocene North America without discussing Ekgmowechashala, not least because it's difficult to pronounce. With fossils found across the Great Plains and in Oregon, this was not a hoofed mammal, but a primate. This is peculiar, because, while there had been primates in North America in the preceding epoch, they did not survive into the Oligocene, and Ekgmowechashala came and went, without leaving any descendants, millions of years after the last of its predecessors had vanished.

At the time of its discovery, it was also a unique-looking primate, leading to confusion as to how exactly it related to anything else, although the general opinion was that it was somehow related to tarsiers. It's now generally recognised to be an adapiform, a member of a group of early primates related to, but distinct from, the lemurs. Furthermore, we do now know of some apparent close relatives, such as Muangthanghinius from China, suggesting that its lack of apparent connection to its American predecessors is entirely genuine - it was an immigrant from eastern Asia not closely related to anything that had previously existed on its new continent.

That it survived in a land where all the other primates had long since been killed off by the cooling and drying at the dawn of the Oligocene may be down to its unusual specialisations. Specifically its teeth - and those of its Chinese relatives - are rounded in a way that no other primate teeth are, which is partly why it was so difficult to classify in the first place. This may well be an adaptation to eating hard seeds, likely including those of fruit, allowing it to prosper where more typical primates could not.

Of course, not everything in Oligocene North America was a peaceful fruit-eater, and next time I will turn to look at the predators of the continent, including a major development in the evolution of one particularly well-known group...

[Photo by "mark6mauno" from Wikimedia Commons.]

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