Sunday 17 April 2016

Pliocene (Pt 10): Before There Were Zebras

At the dawn of the Pliocene, Africa, like Europe, was a much wetter place than it is today. As a result, it was also much greener, a place of lusher vegetation, and the animals that fed on it. While that likely made little difference to the heart of the Congo jungle and to the more tropical reaches of West Africa, which are about as green as they're going to get, elsewhere the changes would have be obvious to any putative time traveller.

The biggest difference was likely in the north, where the even the very heart of what is now the Sahara Desert was likely covered in arid scrubland - hardly hospitable, but a significant improvement over baking hot dune-fields. By one estimate, moist savannah and open woodland stretched as far north as 21°, covering what are now countries like Chad, Sudan, and Mauritania. Further east, Somalia would also have been covered by woodland, rather than its current dry grasslands, and, at the opposite end of the continent, there may have been small forests in what are now the Namib Desert and the Kalahari.

It didn't last, of course. Around 3 million years ago, as the world fell irrevocably into the long autumn of the late Pliocene, Africa became not only cooler, but drier. And, if the generally cooler climate did not make too much difference to a continent sitting on the equator, the loss of rain certainly did. It's at this time that the Sahara, and the other deserts we are familiar with today, began to form, and the wildlife had to either adapt to that fact, or die. What was good news for voles in Europe, promoting the tougher grasses on which they thrive, was bad news further south, where the grass gave way to open sand.

Unlike the other continents that lie (partly) in the Southern Hemisphere, however, Africa already had solid land bridges with the north at the start of the Pliocene. The Zanclean Flood closed off the western land route, separating Spain and Morocco as the Straits of Gibraltar opened, but the eastern route through Arabia remained available - and, of course, many animals had already crossed over before the epoch started. As a result, much of the wildlife of Pliocene Africa had close parallels with that in Europe at the time. And much of that which didn't had a distinctly "African" feel that we'd broadly recognise today.

This is, perhaps, particularly true when it comes to the large mammalian carnivores. The cooler and drier climate of the late Pliocene saw the first appearance of lions, leopards, cheetahs, and spotted hyenas, all in forms essentially indistinguishable from those alive today. In fact, the only major difference is that, back in the Pliocene, they weren't alone. Lions, leopards, and cheetahs, for instance, lived for over a million years alongside the sabretooth cats Megantereon, Dinofelis, and Homotherium. All of these had close relatives in Europe, different in the exact species, perhaps, but likely no more different from them than African lions are from Asian tigers today. (Which is to say, very little - lions and tigers are very hard to tell apart once you've shaved them).

Similarly, spotted hyenas lived alongside the running and giant hyenas, Chasmoporthetes and Pachycrocuta, both also known from Europe. Presumably, the interactions between all these large predators was a complex one, with Africa having at least twice as many species of large carnivore then as it does today. But with half of them being animals we'd recognise from zoos and wildlife documentaries, there were clearly many similarities, too.

The smaller mammalian predators of Pliocene Africa would also often have been familiar, at least in their general form. There were, for example, already animals such as civets, mongooses, and relatives of honey badgers on the continent at the time. These would have been preying on a range of small herbivores, with mice, gerbils, porcupines, and mole-rats (both naked and otherwise) all being present. On the whole, though, it was not these that the lions, hyenas, and sabretooths were eating, but larger animals, which were also in plentiful supply.

When we think of the sort of animal that African lions eat today, zebras are probably fairly high on most people's mental list. Indeed, the plains zebra, at least, is a very common animal in Africa, and could be said to be a key part of our image of the continent. Yet, while the Pliocene may have had lions, it did not have zebras until the following, Pleistocene, epoch. This is because single-toed horses, of which zebras are, of course, an example, first appeared in North America, and took some time to cross over to Asia, let alone Africa.

But that is not to say that Africa had no horses in the Pliocene, just that they were still of the older, three-toed lineage. Perhaps the most widespread such animal was Eurygnathohippus, which is known from a number of sites ranging from Libya down to South Africa. It had first appeared at the end of the previous epoch, the Miocene, and, during the early Pliocene began to diversify into a number of different species. Apart from the presence of three toes on each foot, they would have looked remarkably like modern horses, with, for example, a similarly shaped head. (For all we know they had stripes, since they lived in a similar habitat to modern zebras, but it's perhaps more likely that they didn't). At one site, their hoof-prints have been preserved, showing that they used their middle toe almost exclusively, the side toes already being too small for anything other than steadying the animal when it slipped or had to brake suddenly.

The various species were not all the same size, but one of the larger ones, E. turkanense from Kenya, stood around 120 cm (4' or 12 hands) at the withers, which is broadly comparable with a living plains zebra. By examining the shape of their teeth and how they wore down, we can even make some good estimates of how they ate, and how this changes as the Pliocene wore on. The earliest forms, such as E. feibeli, had a mixed diet, mainly grazing on grass, but also including a lot of softer leaves, browsed from bushes and the like. But once the continent became drier, and the grasslands and semi-desert began to spread, so their diet shifted more towards grass, so that there seems to have been relatively little difference between the diet of the latest species and that of modern zebras. This, it seems, enabled three-toed horses to survive for much longer in Africa than they did in Europe, so that they didn't die out until the mid Pleistocene - long after they had been joined by true zebras.

However, when it comes to large African herbivores, few animals can be more iconic than the elephant. African elephants first appeared in the early to mid Pliocene in East Africa in the form of Loxodonta exoptata, which was replaced by L. atlantica towards the end of the epoch. This, in turn, likely gave rise to the two living species towards the end of the Pleistocene. Although the latter is said to have been slightly larger than the modern forms, in most other respects, they would have been very similar. Interestingly, the ancestors of Asian elephants also lived in Africa during the Pliocene, with some remaining there long after others had left the continent, heading in the general direction of India.

These were not, however, the only elephants in Africa at the time. One of the most notable other species was the African mammoth (Mammuthus subplanifrons), an impressively large species that lived from South Africa to Ethiopia during the early Pliocene. This is the oldest species of mammoth known, and may well be the ancestor of all the later species, with its descendants leaving Africa around 3.5 million years ago, in the late Pliocene, and eventually giving rise, likely via the southern mammoth, to the famous woolly sort.

There were mastodons in Africa, too, including Anancus, which was also known from Europe and Asia. Sometimes considered a mastodon, and sometimes an unusual true elephant, Stegodon was also found in Africa, where it apparently originated towards the end of the Miocene, although it is far better known from Asia, where it was the dominant elephant-like animal for some time. There are several species of Stegodon, including a miniature form found on the island of Flores that may have weighed as little as 300 kg (660 lbs) - large by most standards, but unusually small for an elephant. At the opposite extreme, the Chinese species S. zdanskyi has been estimated as standing 3.9 metres (12' 6") tall and weighing over 12 tonnes, putting it in the same general range as the largest individual African elephant ever recorded.

Roughly the same size was Deinotherium bozasi, the last survivor of an ancient lineage of elephant-relatives that vanished from other parts of the world around the dawn of the Pliocene. In Africa, however, this one remaining species survived, not only through the whole of the epoch, but half-way into the Pleistocene, dying out shortly before the worst of the Ice Ages began to bite. To modern eyes, although clearly an "elephant" in general form, it would have looked pretty strange. This is because the tusks were not in the upper jaw, as they are in true elephants, but the lower jaw, from which they pointed downwards, curving slightly towards the rear.

Alongside the elephants were rhinos, including some very closely related to the modern species. One of these, Ceratotherium praecox, has often been proposed as the direct ancestor of the living white rhinoceros, which it resembled in both size and general form. It has, however, also been suggested that it might be closer to the black rhino, and that its diet may also have resembled that of the latter, smaller, species.

In terms of number of species, the most successful large herbivores in Africa today are the antelopes. But they, and all their cloven-footed relatives, are a matter for next time...

[Photo by Paul Maritz, from Wikimedia Commons.]

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