Sunday, 24 January 2021

Miocene (Pt 24): Of True Elephants and Three-toed Horses

Stegotetrabelodon

The first horses entered Africa towards the end of the Middle Miocene, about 10 million years ago. These have commonly been assigned to the same genera, Hipparion and Hippotherium, as were found in Eurasia at the time, although the fine details of the exact relationships are unclear. Although the latter in particular seems to have been reasonably successful on the continent, a more significant immigration from an African perspective took place later on, around 8 million years ago as the drier climate heralded the start of the Late Miocene.

This marked the first appearance of Eurygnathohippus, a genus of horse that seems to be native to the continent. It was not, of course, an ancestor of modern zebras, because those animals appeared much later and are very closely related to the living horses and asses. Indeed, like other horses of the day, this animal still had three toes, even if it put by far the most weight on the middle one. Certain details of the skeleton suggest that it actually evolved from Sivalhippus, a slightly older inhabitant of scattered woodlands in what is now Pakistan, or at least a close relative such as the Chinese Plesiohipparion. As this terrain type became more common further south, it likely entered Africa via the Middle East to give rise to the native African form. (Indeed, much later, Eurygnathohippus would retrace the route of its ancestor and reach southern Asia in the Late Pliocene).

A number of different species of Eurygnathohippus have been identified, becoming increasingly adapted to grazing in open country as time went on, and not dying out until shortly before the Ice Ages. They stood slightly shorter than a modern zebra, although the early species had proportionately longer legs and slimmer bodies so that they must have been significantly lighter. Later species remained about the same height but became more heavily built and physically powerful. 

Apart from the three toes, they would already have looked remarkably horse-like, and we also have a remarkable piece of evidence that their behaviour was similar, too. This comes from a set of fossilised hoofprints dating to around 3.5 million years ago (the Pliocene epoch) in Kenya. We can't, of course, know that the animal that left the prints was Eurygnathohippus, but it's at least one of the plausible candidates. At any rate, what's significant about the tracks is that they appear to show a foal crossing back and forth in front of its mother as they walked, exactly as baby zebras (and other horses) do today. At one point, the foal gets a little too close and the mother has to brake suddenly to avoid bumping into it, bracing herself by digging her side-toes into the mud, leaving a clear three-toed impression where, elsewhere in the track, only the middle toe reached the ground.

Horses were, of course, not the only newcomers to Africa as the climate changed. Another group of animals crossing over from Asia towards the end of the Middle Miocene were the anthracotheres. These already existed on the continent, but the newcomers are thought to be related to forms that first evolved in or around India. They gave rise to Libycosaurus, a relatively large pig-like animal that, like other anthracotheres, probably lived somewhat like a modern hippo. The exact number of species is unclear, perhaps because males were larger and differently shaped than females, but the fact that they are primarily known from Libya, Chad, and Tunisia suggests that that region was much wetter then than it is now. As the Sahara began to form, they would have been restricted to smaller and smaller regions (hippos still live in the Nile valley today) as well as migrating further south until eventually, they died out as the Miocene came to a close.

While Eurygnathohippus and Libycosaurus were unique to Africa, and are typically far less well-known than their Eurasian and American relatives, those elephant-like creatures that were native to the continent are typically more familiar. That's because, while their ancestors were indeed native to the continent, reaching more northerly locations by spreading out from their African home, they typically didn't change so much when they did so, and so are often assigned to the same genera as their kin elsewhere.

Late Miocene Africa saw a number of different kinds of elephant-like creature, far more diverse than the mere three species we have today. For example, Anancus, which looked somewhat similar to a modern elephant except for having staggeringly long tusks, was found across much of Eurasia, and probably evolved there. The European species, A. avernensis, is probably the most studied, although there are some known from eastern Asia as well. At least one species, A. kenyensis, was, however, native to Africa in the Late Miocene, before being replaced by newer forms such as A. capensis in the Early Pliocene. Despite only having two tusks, rather than four, it was traditionally considered a type of gomphothere, although more modern classifications prefer to give it a different - but related - family of its own.

Both mastodons and deinotheres, with their strange, downward and backward-pointing tusks, also existed in Africa at the time. Most such creatures shifted towards a more grassy diet, but the deinotheres, represented in Africa by the species Deinotherium bozasi, did not. Nonetheless, while never common, they survived through the following Pliocene epoch before being killed off by the arrival of the Ice Ages.

From a modern perspective, however, perhaps the most significant evolutionary development in the group during the Late Miocene was the appearance of a new family of such creatures: the true elephants. The elephant family is distinguished from mastodons, gomphotheres, and the like by the unusual shape of their molar teeth. (Today, they are also distinguished by the complete absence of premolars, but this occurred much later, and apparently on at least three separate occasions in different lineages).

The oldest known true elephant is the four-tusked Stegatetrabelodon, and it first appeared in Africa, before later spreading north into Europe and Asia. Even then, the oldest non-African fossils have been found in southern Italy - Calabria and Sicily - which it has been suggested might imply that these areas were more closely connected to Libya and Tunisia then than they are now. Other "primitive" species soon followed, including the two-tusked Stegodibelodon and Primelephas, confirming Africa as the likely point of origin of the group.

Despite their age and primitive appearance, these are not thought to be the ancestor of modern elephants. It has been proposed that Stegotetrabelodon might be the ancestor of mammoths, which belong to the same family... but there is also reasonable evidence that mammoths aren't distinct enough from modern elephants for this to be possible, making the primitive forms an evolutionary dead-end. The reason for this is that fossils of very nearly the same age as the primitive ones look much more modern, implying a "missing" ancestor rather further back. Indeed, these other fossils, which also date to the Late Miocene, are so modern that we can place them into the genera Loxodonta and Elephas - the ones that still survive today.

Indeed, genetic evidence suggests that these two genera diverged from one another as much as 7 million years ago, which is about as far back as their fossils go. For instance, the oldest known fossil of Elephas was found in Kenya, and dates back 6.5 million years, showing that what we'd now call Asian elephants (although obviously not the exact same species) first evolved in Africa before leaving for their current home. Loxodonta, on the other hand, does not seem to have left its home continent. While there is some dispute as to which (if either) of the two known Pliocene species is the ancestor of the living African animals, only one species, L. cookei, is definitively known from the Late Miocene. Apart from still possessing premolar teeth, it was probably very similar to modern African elephants and likely had a similar diet and habitat.

Iconic though elephants and zebras are in the modern African fauna, there are also carnivores of which we could say the same. But lions, of course, did not yet exist, so what did the likes of Eurygnathohippus have to fear? I'll turn to that question next time...

[Picture by "ДиБгд", from Wikimedia Commons.]

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