Oligocene (Pt 10): The First Elephants with Trunks

Barytherium

During the Oligocene, Africa remained an island continent, separated from Asia by the Tethys Sea that ran from the Atlantic, through what is now the Mediterranean, and then south of the Zagros Mountains to connect with today's Persian Gulf. The Tethys was narrower in the Oligocene than it had been in the preceding epoch, due both to Africa's slow northward movement and the fact that sea levels were lower, but it was still substantial. As a result, the large herbivores of Eurasia, such as pigs and rhinos, had yet to enter Africa, resulting in quite a different fauna there.

The same was not, however, necessarily true of smaller mammals, some of which had been there for some time, more able, perhaps, to be accidentally carried across on floating vegetation. (A rare event, to be sure, but tens of millions of years gives a lot of opportunities to get it right). There may not have been antelopes in Africa yet, but there were certainly rodents.

Even so, these were not the rodents of the north, but members of groups that were - and to a large extent, still are - unique to the continent. Many of the modern families, such as cane rats and dassie rats, likely only arose at the start of the following, Miocene, epoch, but their ancestors were around, often classified in a broad grouping called the "phiomorphs". These include various vaguely rat-like animals such as Monamys and Neophiomys, the latter of which may conceivably be related to the ancestors of the African mole-rats (the naked sort included) which molecular estimates suggest first appeared around this time. Gaudeamus, once thought to be an early phiomorph, and which probably looked similar to them when it was alive, is now thought more likely to be related to African porcupines - although the true porcupines would not appear until towards the end of the Miocene.

A second group of rodents found in Africa at the time were related to today's anomalures (gliding animals similar to flying squirrels) and included Nonanomalurus, which would survive relatively unchanged right through the epoch.

But what of the larger herbivores in the days before zebras or rhinos? While there were no antelopes, giraffes, or warthogs on the continent yet, it isn't quite true to say that there were no cloven-footed mammals at all. The anthracotheres are better known from Asia, where they first evolved, but the genus Bothriogenys lived in what is now Egypt surprisingly early on, almost at the beginning of the Oligocene - and may be the ancestor of later forms that crossed back into Eurasia when the continents rejoined. 

Bothriogenys was about the size of a pig, but, while the exact relationship of anthracotheres to the other cloven-footed animals is debated, it's more often thought to be related to hippos. Indeed, apart from the smaller size, it probably looked rather like a hippo, but with a much longer and narrower snout that would have made it quite unlike anything alive today. The reason that it managed to beat other cloven-footed animals to Africa by such a large margin is probably that it was semi-aquatic, again emphasising the similarity to hippos. For example, studies of the bones around the ear suggest that it could hear well underwater.

One group of animals familiar to us from modern Africa does, however, have a much more ancient history on the continent, having originated there long before even the dawn of the Oligocene. These are the proboscideans, represented today by the elephants. These were not, of course, actual elephants, because these only appeared much later, but, with few direct competitors, they were more diverse than the modern animals. 

Eritreum, which lived in Eritrea towards the end of the Oligocene around 27 million years ago, is, however, thought to be a particularly close relative of the living forms. Like most early Proboscideans it still had four tusks, with two short ones at the end of an elongated lower jaw. It probably had a trunk,and is estimated to have weighed around 480 kg (1,000 lbs) standing about 125 cm (4 feet) high at the shoulder. Significantly, it is also the oldest known fossil to display the conveyor-belt style replacement of teeth typical of modern elephants. As such, while it sits outside any named family, having lived before the split between the gomphotheres, elephants, and shovel-tuskers, at the time of its discovery it was considered a "missing link" between modern and primitive proboscideans.

Typical examples of those primitive animals on the other side of the missing link include Phiomia and Palaeomastodon, which are known from Northern Africa and Arabia during the Early Oligocene. (Arabia, it should be noted, since it was south of the Tethys Sea, and the Red Sea didn't exist yet, was a part of the African continent at the time, not Asia). Although they are no longer thought to be each other's closest relatives, as was once the case, they did look rather similar. 

They were somewhat larger than Eritreum, with estimates ranging all the way from 140 to 220 cm (4'6" to 7'2"), and had flattened tusks in the lower jaw that resembled a smaller version of those in the later (and not closely related) shovel-tuskers. These were probably used to either scrape the bark of trees or help to uproot plants; the upper tusks were short and pointed downwards. While it's impossible to know for certain whether or not they had trunks, the consensus is that they probably did, although it would have been much shorter than in modern elephants, perhaps looking more like that of a tapir.

An even earlier branch of the proboscidean family tree is represented by the deinotheres, which survived all the way through to the Ice Ages just one million years ago. They never seem to have been an especially diverse group, although they were no doubt impressive in their day and, once again, the oldest known example of the group lived towards the end of the Oligocene. Chilgatherium is known from Ethiopia, but only a few teeth have so far been discovered; it may plausibly have been around the size of a cow. We also can't yet tell whether or not it already possessed the most distinctive feature of later deinotheres: they had downward-curving tusks in their lower jaw, but none in the upper one where elephants have them today.

Barytherium, from Egypt and Libya, was stranger and even less related to modern elephants than were the deinotheres. It was a survivor from the end of the previous epoch, and is the oldest known proboscidean that we can reasonably describe as "large", standing about 190 cm (7'3") high at the shoulder, and sufficiently heavily built that it may have weighed as much as two tons. It had no fewer than eight tusks, four in each jaw, although these were flat cutting blades, more like those of a hippo than a modern elephant. Nonetheless, the close relative Omanitherium had begun to develop conical tusks, at least in the lower jaw - although it didn't necessarily have any descendants, as the barytheres died out in the mid-Oligocene. Analysis of the tooth enamel of Barytherium suggests that it, at least, often fed on freshwater plants, and may have spent considerable time wading in lakes or wide rivers.

However, other than the rodents, it's not the proboscideans, but the hyraxes that were likely the main ground-dwelling herbivores on the continent during the Oligocene. Today, there just four or five species of hyrax, small furry animals looking a little like a marmot and living solely in Africa and Arabia. But their evolutionary history is ancient, belonging to a very different branch of the mammalian family tree than the marmots - or any other rodent.

Many of the hyraxes of Oligocene Africa were not that different from those alive today, although the family to which all living hyraxes belong is thought to have arisen shortly after the end of the epoch. These included animals such as Saghatherium, Thyrohyrax, and Selenohyrax. The last of these had unusually delicate teeth, suggesting that it fed on softer food than its kin, a dietary specialisation that may help explain why so many such animals were able to live alongside one another. 

The group was, however, more diverse than this, with its most notable Oligocene member probably being Megalohyrax. Originally discovered in Egypt back in 1903, other fossils have since come to light across North Africa and into the Middle East. By the standards of hyraxes, this was an enormous animal, the best-preserved skull having been measured at 39 cm (15 inches), over half the total length of the largest living hyrax. In life, the largest species of Megalohyrax was probably about the size of a tapir; it likely fed primarily on leaves.

During the Oligocene, Africa was not so diverse in climate as it is today, not least because the Sahara had yet to develop. The great majority of it was probably covered in rainforest, interspersed with patches of dryer woodland. With so many trees, another group of herbivorous animals prospered here, and it's one that tends to draw a lot of attention from palaeontologists. Next time, therefore, I will be looking at Oligocene primates, along with some other African herbivores of the day...

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