Sunday, 7 June 2020

Miocene (Pt 20): Unicorn-Pigs and the First Hippos

Pigs seem to have been another example of animals crossing over from Asia when that continent collided with Africa around 17 million years ago. The oldest fossil pigs in Africa date from about that time and are represented by Kenyasus from Kenya and Nguruwe from Namibia and South Africa. These were relatively small, short-snouted pigs with a primitive appearance similar to that seen in their presumed Asian ancestors.

Because of these primitive features, determining exactly where they fit in the pig family tree isn't a simple matter, but they are often placed with a group called the kubanochoerines. Assuming this is correct (and it's far from settled) they must have evolved fairly rapidly into much larger and more distinctive animals. The best-known member of the group is the giant "unicorn-pig" Kubanochoerus, which lived in China during the Late Miocene. This was an exceptionally large, long-legged animal, perhaps standing 120 cm (4 feet) high at the shoulder and - even more dramatically - sporting a long pointed horn that projected from the centre of its forehead.

Whether this really did evolve from the likes of Kenyasus, it undeniably did have relatives in Africa, which must have crossed over the landbridge in one direction or the other. How distinct these relatives are from the Chinese animal is another disputed issue. They were long considered to belong to the same genus (and still sometimes are) but are more commonly considered to be different enough to be given their own names, as Libycochoerus and Megalochoerus.

These were also very large pigs, with long snouts and narrower faces than many modern species, as well as various distinctive features of the teeth. Perhaps the most significant argument for considering Libycochoerus as distinct is that the only reasonably complete skull that we have shows that it didn't have the horn, although there are large bumps above each eye. You might think that's a slam dunk for it being a different species than the horned animal, but it's entirely possible that it just happened to be a female...

A number of other pigs seem to have existed in Africa during the Middle Miocene, but most seem fairly similar to better-known animals living in Europe and Asia at the time. Examples include Lopholistriodon and Namachoerus, both of which seem to have specialised in eating more leafy material than pigs normally do. The latter has an unusually short snout and tusks that flare sideways, which may reflect a more distant relationship to the leaf-eating Eurasian pigs than might be expected, perhaps representing a uniquely African sub-branch within the group.

Hippos, on the other hand, appear to have first evolved in Africa, the only continent where they still survive today. Quite what they evolved from is another matter, partly because we don't have very good fossils of the earliest examples of the family. Indeed, only one definitive species of hippo, Kenyapotamus ternani, is known from prior to the Late Miocene, and it seems to have been a rare animal even then. It dates from around 16 million years ago, and while we have enough remains to identify it as a member of the hippo family, they remain too fragmentary to say anything meaningful about it, beyond the fact that it's unlikely to have stood much more than 3 feet or so high at the shoulder.

I say that Kenyapotamus is the only definitive species of hippo known to live at this time, since there is some debate about the status of two earlier animals, which have sometimes been included within the hippo family, but more often not. These are Morotochoerus, which dates all the way back to 21 million years ago, and Kulutherium which lived around 17 million years ago. Both are African animals that seem to have had at least some resemblance to modern hippos, with the latter even being a similar size, but whether that's what they really were, or just some other creature with a similar lifestyle is another matter.

Proboscideans, the group of large animals with trunks and tusks to which the modern elephants belong, have an even older history, and were still unique to Africa at the dawn of the Miocene. For them, the arrival of the landbridge with Asia was an opportunity to spread out into the wider world, where they survive in southern Asia today, and were even more widespread prior to the end of the Ice Ages.

Even before that happened, however, there was a reasonable diversity of proboscideans in Africa. The modern elephant family cannot be traced back any further than the Late Miocene, but at least three main groups of related animals predate them on the continent and experienced something of a surge in diversity in the Early to Mid Miocene.

The mastodon family, or mammutids, were represented at the dawn of the Miocene by Eozygodon, a short-tusked animal somewhat smaller than living elephants, although still quite impressive. Remains are known from sites in both eastern and southern Africa, suggesting a fairly wide distribution. By the Middle Miocene it was joined, and eventually replaced by, Zygolophodon, a larger animal with more rounded tusks. This rapidly crossed the newly formed landbridge, and spread out across Europe and Asia, from which a number of species are known. For a while, the only African fossils were known from the north, in Tunisia and Egypt, but some teeth from sub-Saharan Africa have also been assigned to the genus.

The second group were the large, four-tusked gomphotheres, many of which were very similar to the species known from Middle Miocene Europe and Asia and are typically placed in the same genus, Gomphotherium. Related to these, however, was Archaeobeladon, an animal about the size of a female Asian elephant today, but with a remarkably long jaw ending in wide flattened tusks. Known from Egypt 17 million years ago, as well as from France and Spain, this is now thought to be an early example of a separate family, the "shovel-tuskers" which went on to develop even more dramatic jaws and spread through much of the Northern Hemisphere.

Completing the set were the deinotheres, another group that originated in Africa and later spread much more widely across the world. During the Early Miocene, they were represented by Prodeinotherium, an animal that may have stood up to 270 cm (8' 10") at the shoulder, although some estimates make it rather smaller. Dating back at least 20 million years, in many respects, it probably looked quite like a modern elephant, despite not being a terribly close relative as proboscideans go.

The most dramatic difference, however, was that it only had tusks in the lower jaw, which pointed down and slightly backwards - it's really not clear what the purpose of these was, although the shape of the bones in the neck suggest that the animal regularly made strong downward motions with its head implying that it might have been scraping or hacking at something. Clearly, it worked, since not only did Prodeinotherium itself spread to Europe in the Middle Miocene, but it went on to give rise to even larger animals with a similar form that went on to survive into the Ice Ages.

Such large animals may have had few natural predators, at least once they were fully grown. But many other creatures were less fortunate, and next time I will look at some of the carnivorous animals that may have taken advantage of that...

[Photo by "Ra'ike", from Wikimedia Commons.]

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