Sunday, 12 April 2020

Miocene (Pt 19): When Rhinos and Giraffes Went South

Prolibytherium
At the dawn of the Miocene, 23 million years ago, Africa was a very different place than it is today. This was as much due to simple geography as anything else. For one thing, the Sahara Desert didn't exist yet - even the most extreme estimates suggest that it didn't begin to appear until the Late Miocene, and it may well have not have formed until at least the Pliocene around 4-5 million years ago. Before this time, much of the continent was probably covered in warm, damp, forests interspersed with patches of more open woodland and scrub.

More significantly, perhaps, Africa was still an island continent, separated from Eurasia by a body of water called the Tethys Seaway. It had been like this, isolated from the rest of the world, for millions of years, during which time it had developed its own unique animal life, quite different from that elsewhere. At the time, the same was true of South America (and it still is for Australia), but that continent retained its unusual animals for much longer, not contacting the north until as recently as 2.5 million years ago, shortly before the first of the Ice Ages. When that did eventually happen, it led to the Great American Interchange, which set the scene for much of the American fauna we see today.

Taking a longer view, however, that wasn't such a unique event; essentially the same thing had happened 17 million years earlier, when, moving steadily northward, Africa hit Asia. The initial contact was made somewhere in the Middle East, throwing up the Zagros Mountains of southeastern Turkey and western Iran. It still continues today, with Arabia (which is geologically part of the original Africa) pushing its way north by around 2 cm (1 inch) per year.

Exactly as would later happen in America, the creation of this new land bridge between the Mediterranean and the Persian Gulf allowed animals on both sides to cross over and reach new lands. In fact, a great many animals that we now associate with Africa first evolved in Asia, and didn't reach their current home until the interchange took place 19 million years ago.

Among the most significant of these were the rhinos, which already had a long history in the north. Many of these were not close relatives of the modern species, but medium-sized hornless rhinos such as the short-legged Brachypotherium, which is more widely known from Eurasia, but at some point reached both Kenya and Namibia. However, while research on these African fossils has been relatively limited, there do seem to have been quite a number of different rhinoceros species on the continent even during the Middle Miocene, and they were more diverse than they are today.

Among them is the two-horned Rusingaceros, first discovered in the 1960s on an island just off the shore of Lake Victoria. This was similar enough to modern forms to originally be considered a close relative of the living Sumatran rhino, but more recent analysis has shown that it really can't be (as one might expect, given the distance to Sumatra from Lake Victoria). Nonetheless, had we been able to see one in the flesh, it probably would have looked more like modern species than pretty much anything else at the time, even if it likely left no descendants of its own.

A third form is represented by the elasmotherine rhinoceros Victoriaceros, a relatively large-horned, grazing rhino, with fossils primarily known from a small island in a bay to the east of Rusinga.

Other large herbivores also made the same crossing, and not all were as familiar as rhinos. Among them were the chalicotheres, odd-looking animals, distantly related to rhinoceroses, but with horse-like faces, long forelimbs, and long claws on their feet. Chalicotheres were distinctive enough that their fossils can readily be identified as such, but most of the early African remains are too fragmentary to assign to particular species. An exception is Butleria, known from Kenya and Uganda. It was probably a very early migrant, and was a small and relatively primitive example of its kind, an early branch on the line that later gave rise to the more famous Chalicotherium and Anisodon.

The opening of the land bridge also brought the first hoofed mammals to Africa. Prior to this, the only ungulates on the continent had been a primitive group of animals known as anthracotheres, which looked vaguely pig-like and may have had a lifestyle similar to that of modern hippos. Possibly because there wasn't much else like them at the time, the anthracotheres weathered the arrival of the newcomers and, in fact, even spread out of the continent and into Asia, where they continued for some time.

Jaggermeryx is perhaps the best-known example of these early Miocene African anthracotheres. Known from Egyptian deposits around 19 million years old, these were the size of small deer and seem to have had the same semi-aquatic adaptations as others of their kind. Although the first bones of the animal were excavated around the time of WWI, it took over a hundred years for anyone to notice that they were truly distinct, because of the presence of a number of tiny holes along the side of their jaws. In life, these would have carried nerves and blood vessels, but there were so many more than we would normally expect to find in a skull that the authors speculated that Jaggermeryx must have had unusually large and mobile lips.

Which may give you a clue as to why they gave it that name...

Many of the hoofed mammals that entered Africa to join it were ruminants, including the first antelopes to reach the continent; animals virtually identical to living gazelles are known from as far back as 15 million years ago. Other ruminants included the tiny Dorcatherium, which was replaced by the native Afrotragulus species. Standing somewhere around 50 cm (18 inches) high at the shoulders, these resembled miniature antler-less deer and had teeth that had evolved for a different, and presumably tougher, diet than their ancestors. They were related to, and were probably the direct ancestors of, the water chevrotains that survive to this day in Africa's tropical rainforests.

Giraffes are much larger than chevrotains, and, while giraffes as we now know them didn't come along until later, some early relatives were also among the new arrivals. These included Palaeotragus, a short-necked "giraffe" that was nonetheless about 180 cm (6 feet) tall at the shoulders. Although it lacked the horn-like ossicones that the modern species has, it probably looked something like the living okapi (an animal only discovered 40 years after the fossil was named...) and it is known to have been widespread in Asia before it headed to Africa.

Yet another group of ruminants that joined them were the climacocerids, thought to be related to giraffes due to some unique features of their teeth, but that are now placed in a family of their own. The group is named for Climacoceras, an animal with the approximate size and shape of a llama, but that had long branching growths on its head that look, to modern eyes, remarkably like antlers.

Unlike true antlers, these structures were probably permanent, rather than regrowing each year, although it's hard to be sure. They may, like the ossicones of a giraffe, have been entirely covered in skin and hair, but, then again, it might have been more like the velvet that encases growing antlers in deer. Whatever they were like, they are a key feature of climacocerids, with their varying shapes helping to define different species.

Orangemeryx from South Africa and Namibia, for example, had short, flattened structures vaguely similar to those of a young moose. They also had unusually long limbs and necks, suggesting that they probably fed on tall vegetation - although at 120 cm (4 feet) at the shoulder, this would have been in a manner similar to that of modern long-legged antelopes than to giraffes.

Prolibytherium was even more bizarre, to the point that there has been considerable argument about what it actually was. As the name suggests, it was first found in Libya, but we also know of fossils from Egypt, so it was probably widespread across northern Africa. Our current best bet is that it was another climacocerid, but it's sufficiently peculiar that it's unclear how it relates to the other known species.

[Edit: I have been reliably informed that newer, but apparently as yet unpublished, data contradicts the findings of the source used here. Such is the nature of ongoing scientific enquiry].

Specifically, the growths on its head are flat plates, spreading out to either side like the wings of a butterfly. Or at least, for a long time, that's what we thought. It turns out, though, that this is only true of the adult males, with females instead bearing four horn-like structures in an X-pattern. Close examination of the plate-like structures of the males reveals that they are supported internally by four struts that branch out like the 'horns' of the females, suggesting that they originally grew like this before (in the males) merging and spreading to form the more dramatic final structure.

They don't look like the sort of thing you could fight with, so it's hard to avoid the conclusion that they were used primarily for sexual display.

Next time, I will take a look at some of the other animals of Early to Mid Miocene Africa, including yet more immigrants, but also some of the native African animals that travelled the other way...

[Picture © Nobu Tamura, from Wikimedia Commons. Low-res version used here available under CC-BY-SA.]

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