Sunday 13 May 2018

Miocene (Pt 7): Hornless Rhinos, Long-Tusked Elephants, and Three-toed Horses

Anancus arvernensis
As the climate cooled around 11 million years ago, the forests of Europe began to thin out once more, something that favoured fast-running animals such as horses. Until this time, the only kind of horse in Europe, however, was the small, three-toed, Anchitherium, which was likely adapted to dense woodland and not so suited to the new climate. Its own ancestors had reached the continent from the east, having crossed over during one of the periodic rises of the Bering Land Bridge, but now, not coincidentally, given the colder climate, the Land Bridge rose again, and a second kind of horse followed it out of the Americas.

These new horses had evolved on the lightly wooded plains  of North America, and found the Siberian climate of the day much to their liking. They spread rapidly through northern and eastern Asia, and reached Europe and the Indian subcontinent not much later. Anchitherium was doomed, and, while it struggled on in central Europe for a few million years, it seems to have died out almost immediately in Spain.

Its initial replacement was Hippotherium, although it had close relatives elsewhere in Asia, some of which joined it in Europe a few million years later. This was about the size of a modern zebra, and, unlike the older European horses, the toes on the sides of its feet were greatly reduced, so that only the central hoof was fully functional - not quite the one-toed horses of today, and probably more of an agile animal than a fast runner, but certainly well on the way.

Analysis of the wear on their teeth has suggested that, while there was likely some local variation, Hippotherium typically had a diet similar to that of modern impalas, including a significant amount of grass, but also considerably more bushes and shrubs than a modern horse would eat. Known from a number of places across Europe, they survived for around 5 million years before finally dying out when the forests truly gave way to more open grasslands at the end of the epoch. Towards the end, there is some evidence that they may have migrated regularly to make the best use of local water sources.

The cooler, drier climate was also driving changes among the rhinos, with a shift from the earlier, swamp-loving forms to descendants such as the hornless Aceratherium, which was particularly common at the start of the Late Miocene. This stood about 130 cm (4 feet) tall at the shoulder, and so was quite small by the standards of living rhinos, but had proportionately long legs.

Unlike modern rhinos, it also retained the fifth toe on its fore-feet, although these were already quite small, and may not have reached the ground. The males, though apparently not the females, had large incisor teeth, probably concealed behind prehensile lips, but the rest of the teeth appear suited to feeding on soft vegetation. We know of at least three different species, with the European one having been particularly common in Turkey and the Balkans, but also found as far west as France. Although there has been some suggestion that they may have reached Africa, the other definitively known species are both from Asia, with the most recent having been identified from as far east as Thailand.

As the climate continued to dry out, however, they were first joined, and later replaced, by other Asian immigrants, most notably Chilotherium, which had first appeared in China about 3 million years earlier. Although fairly closely related to Aceratherium, this had entirely lost the additional toe, leaving just the three on each foot found in living rhinos. Compared with its earlier relative, it had much shorter legs, possibly because it fed mostly on grass but had a moderately stiff and straight neck that would otherwise have made this difficult.

Chilotherium was a successful animal, with a great many different species having been identified - some of them varying enough from the standard form for some authors to consider them entirely different genera. This variation likely indicated a range of different habitats, with one species from central Asia, for example, thought to have lived mainly in the high elevation foothills of the (still relatively new) Himalayas. As with their earlier relatives, the males had much larger incisor teeth than the females, which, taken together with some differences in the shape of their skulls, suggest that they may have clashed with one another for dominance and access to mates.

Still later, a third group of rhinos reached Europe, this time from Africa. These were essentially modern forms, all but identical to the two-horned African rhinos of today. These include Ceratotherium neumayri, a 'white' rhinoceros from Greece and Turkey, and Diceros primaevus, a 'black' rhino whose existence around the Middle East suggests the survival of at least some more densely forested habitat in the area.

The elephant-like mastodons, however, seem to have been much less affected by the changes in climate that affected the other larger herbivores. The four-tusked Gomphotherium, previously very common, did become much less so around 11 million years ago, but later recovered in numbers, and survived, essentially unchanged, well past the end of the Miocene, only dying out when the Ice Ages hit.

Right at the end of the Miocene, it was joined by Anancus, which may have been descended from the earlier Tetralophodon. It's not totally clear that these animals are gomphotheres, not least since Anancus had lost the four tusks that one would expect such animals to have, and some modern proposals place them in a group of their own. Whatever they were, Anancus itself is notable for the immense size of those remaining tusks. The animal was somewhere between an African and an Asian elephant in body size, albeit with shorter legs, but had tusks a staggering 3 metres (10 feet) in length, if not more. That aside, it probably looked more like a modern elephant than its earlier relatives had, due in part to the shortening of its lower jaw now that it was bereft of the second pair of tusks.

While gomphotheres seem to be somewhere between elephants and the later, American, mastodons, in terms of their evolutionary origin, the deinotheres were rather more distant relatives of the living animals. They had a single pair of tusks, pointing downwards and slightly backwards from their lower jaws, and, by the dawning of the Late Miocene, they were represented in Europe by Deinotherium itself, from which the group takes its name.

Unlike the earlier forms, this was, at around 3.8 metres (12½ feet) in height, equivalent in size to some of the largest African bull elephants today. Europe was home to one, or perhaps two, different species at different times, including the largest one known, D. giganteum; related species lived in both Asia and Africa, but, unlike the true mastodons, never reached America. Found across the continent, a number have been found on the island of Crete, which, on account of the Messinian Salinity Crisis and consequent lowering of the Mediterranean sea level, was probably just a peninsula of Greece at the time.

Such large animals as these would have little to fear from predators, at least once they reached adulthood. But for many other herbivores, the story was not the same, and as they evolved, so did those creatures trying to hunt and kill them. Indeed, some significant changes were in store for the carnivoran fauna of Late Miocene Europe...

[Picture by Nobu Tamura, from Wikimedia Commons.]

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