Saturday, 28 November 2020

Miocene (Pt 23): Giraffes Become Tall, Hippos Stay Dry, and Antelopes... Get Eaten

Palaeotragus, a short-necked giraffe
About half-way through the Late Miocene, around 8 million years ago, worldwide temperatures began to drop significantly, and even tropical Africa did not escape the effects. In its case, this didn't lead to cold and barren steppelands, and, indeed, the world may still have been slightly warmer than it is today, but there is evidence of the expansion of grasses across many parts of the continent. Perhaps enhanced by the closure of the Mediterranean Sea during the Messinian Salinity Crisis and disruption of the monsoons, North Africa also became much drier than before, although whether the Sahara as we know it today dates back quite that far remains controversial; there is some evidence of sand dunes that far back, but also of numerous rivers crossing the region.

These changes in climate also affected the animal life on the continent, to the benefit of some and the detriment of others. Pigs are omnivorous animals, and one might expect them to have survived such changes relatively unscathed. In a sense, this is true, since they remained common on the continent, but the nature of particular species living there did change.

The older African pigs of the Early and Middle Miocene, some of them related to the unicorn-pigs of China, were well-suited to lush vegetation and, while that didn't vanish, there may well have been less of it than before. Perhaps more significantly, new kinds of pig entered the continent from the north. In Asia and Europe, a group of pigs called the tetraconodontines had been particularly common, but, as that continent cooled, they headed south. They did survive back on their original continents (notably in Myanmar), but in smaller numbers than they once had, and not really for very long.

In Africa, however, they evolved into a new, native form: Nyanzachoerus. This was an exceptionally large pig, likely bigger than the largest species alive today, the giant forest hog of equatorial Africa. They had wide flaring lumps at the back of their snout, much like a warthog although their tusks - while impressive in the grand scheme of things - weren't quite as large as those of some modern species. Several different species of the animal lived across the continent, from at least Libya to Tanzania, with a tendency for them to gradually adapt to a grassier diet as time went on. Eventually, as the Miocene faded into the Pliocene, they were replaced by the even larger and more grass-specialised Notochoerus, which is clearly a close relative, and plausibly a direct descendant, of its predecessor.

While these pigs left no modern descendants, being replaced by the ancestors of warthogs and the like during the Pliocene, another Asian immigrant was more successful in its new African home. While members of the giraffe family did already live in Africa, they were quite radically different from the surviving forms. They were certainly bulky animals, but they were not exceptionally tall and some had huge antler-like bony structures on their heads. These latter animals, which likely grazed on grass rather than browsing on leaves, culminated in Sivatherium. This first appeared towards the end of the Miocene, but is better known from later deposits. With a more mixed diet, Palaeotragus was taller, reaching 3 metres (10 feet) to the tip of its head by the end of the Miocene, and is probably more closely related to the living okapi than to true giraffes.

But, as the climate changed, it had been joined by a second wave of immigrants from Asia, which could plausibly be described as actual 'giraffes', even if they were still part-way along the route to the extraordinary form of the modern species. Most significant among these was Bohlinia, which seems to have evolved in south-eastern Europe, with a number of fossils known from Greece. From there, it expanded eastward to leave fossils as far afield as China. Already noticeably elongated in form, and likely browsing from tree-tops, it was somewhere between an okapi and a modern giraffe in height and had large bent horn-like protrusions of solid bone on its head. As it grew taller, they grew shorter, leaving us with the comparatively stunted ossicones of living giraffes.

Quite where you draw the line between Bohlinia and Giraffa (the modern genus) is perhaps debatable, but at least some studies place the latter as evolving somewhere around China or India before they headed to Africa not long before the Miocene drew to a close. 

Hippos were another matter. So far as we can tell, hippos originally evolved in Africa, albeit from ancestors that must themselves have come from elsewhere (although quite where is hard to say). The dominant hippo of Late Miocene Africa was Archaeopotamus, which is thought to be closely related to the Asian genus Hexaprotodon, which lived at the same time. (Confusingly, the modern "pygmy hippo" is sometimes still referred to as Hexaprotodon even in modern sources; it's unlikely to be descended from the Asian fossils, however). 

Archaeopotamus was smaller than the common hippopotamus of today, but not drastically so, standing about 1.1 metres (3' 7") in height at the shoulder. Like Hexaprotodon (but not the pygmy hippo) it had six teeth in the front of its jaw, but in general terms it would have looked quite like a modern hippo. Closer examination of the bones, however, reveals that it was slightly slimmer, that its ankles were more rigid, and its toes did not splay so far to the sides. These latter points are significant, since they are adaptations to walking on soft mud. Together with the larger number of teeth, this suggests an animal that spent more time grazing on tough grass and less time feeding on soft plants in the water. 

The most numerous and varied large herbivores in Africa today are the antelopes, a term vaguely applied to any member of the cattle family that isn't identifiably a sheep, goat, or close relative of domestic cattle. Antelopes were among the many creatures that first entered Africa when it collided with Asia, and so had been there for a long time by the dawn of the Late Miocene. While they always seem to have been reasonably numerous (which was doubtless good news for the local carnivores) the earlier forms were less varied than today, and are often difficult to fit into modern groupings. Even by the Late Miocene, a number of types of antelopes could be found that have no modern descendants, apparently dying out in the transition to the Pliocene just as modern antelopes diversified.

Having said that, gazelles are something of an exception. The oldest fossil dates all the way back to 14 million years ago in the Middle Miocene, and is virtually indistinguishable from the modern species. By the Late Miocene, numerous species of gazelle existed, and they spread from South Africa to Spain and China. Relatives of the modern four-horned antelope, which lives in Asia, also seem to have reached Africa at a similarly early date, although the other "bovine" antelopes, ancestors of today's kudus and others, did not arrive until towards the end of the Late Miocene.

Impalas and relatives of today's sable and roan antelopes also date back to the Late Miocene, both appearing in Africa around 6.5 million years ago. Only one species of impala survives today, and few fossil ones are known, although they seem to have been fairly widespread; it's entirely possible that others existed but cannot easily be identified from their fragmentary remains. Damalacra, the earliest known member of the group that includes wildebeest, appeared just as the Miocene transitioned into the Pliocene; it doesn't appear to have much resembled its modern relative and likely had a more browsing diet.

These, of course, are just the "cloven-footed" animals. There were also many other large herbivores in Africa at the time, and I will turn to them next...

[Photo by Jonathan Chen, from Wikimedia Commons.]

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