Sunday 12 June 2016

Pliocene (Pt 11): A Profusion of Pigs

Probably the most numerous large mammals in Africa today are the antelopes, of which there are a great many species. In this respect, the Pliocene was not much different, with antelopes providing a large part of the diet of the various carnivores that lived before the Ice Ages, from lions and hyenas to sabretooth cats. However, the mix of antelope species was different then that it is now, not least because of the generally lusher environment that existed in Africa during the earlier part of the epoch.

Even so, most of the antelopes of the day were related to species that we would recognise today. For example, even at the very beginning of the epoch, around 5 million years ago, South Africa boasted a gazelle similar in size to a modern springbok. Indeed, springboks proper appeared around 3 million years ago in the same general locality, where they were represented by at least two extinct species until one of them (probably Antidorcas recki) evolved into the iconic modern animal.

The gazelles of the early Pliocene were likely different in their habits from the modern sort, being more adapted to woodland than open plains, but, moreover, they were not as common, comparatively speaking, as they are now. Instead, it appears that, at least in East Africa, a group known as the spiral-horned antelopes (about which I will have a lot to say elsewhere on this blog over the coming months) were rather more numerous.

One in particular, Tragelaphus nakuae, seems to have particularly numerous, with large numbers of fossils having been found at sites in Ethiopia and Kenya. There is some dispute as to whether the older fossils assigned to this species really belong to it, but, even if they don't, they're probably an immediate ancestor. It was a heavily built antelope, standing about 120 cm (4 feet) high at the shoulder, with long horns that twisted in a shallow spiral, and were likely present in both males and females. Among living animals, it likely most closely resembled the bongo antelope (T. eurycerus), which could well be its descendant. Similarly, another Pliocene species, T. lockwoodi, has been identified as a possible ancestor for the living kudu.

The first relatives of the modern wildebeest appeared around the dawn of the Pliocene, although, like many other antelopes, they were initially less adapted to grazing than they would later become. A particularly impressive specimen was the giant hartebeest (Megalotragus isaaci), which lived towards the end of the epoch, and well into the Pleistocene. It physically looked quite a lot like today's hartebeests, but, at around 140 cm (4' 7") at the shoulder, was considerably larger and heavier, in the size range of an especially big wildebeest. It's known from, among other places, the Karoo, an especially desolate region of southern Africa that could never support a population of such large ruminants today.

Many other types of antelope date from around the same time. Impalas, bucks, and relatives of the modern sable antelope all date from the last million years or so of the previous epoch, the Miocene, and are well known even from the early Pliocene. The latter two of those groups were represented by a number of species, diverging into slightly different forms as the climate changed and they came to exploit different niches - the giant sable antelope (Hippotragus gigas), known from late Pliocene to Pleistocene South Africa and Malawi, would have been large enough to ride, had anyone been around to do so at the time.

Impalas, on the other hand, were a bit different. They actually don't seem to have changed much at all in the over six million years that they have been in existence, and there's still only one species today. The very earliest fossil impala we know of (Aepyceros premelampus) was a little smaller than the modern sort, with horns that curved a little less, but otherwise would have looked very much the same. This is likely because impalas are generalists, moderately good at living in a wide range of habitats, and therefore able to just carry on as the climate changed - never the best at anything, but always good enough to survive.

Around 2.8 million years ago, as the Pliocene neared its end and Africa (outside of the jungles) approached its more arid modern state, there is clear evidence that antelopes modified their diet, with spiral-horned antelopes, bucks, and impalas alike all shifting to eating more grass than they had previously. As time wore on, larger species began to appear - it's at this time that the giant sables and hartebeest make their debut - as well as those with more physical adaptations to grazing, such as the first true wildebeest. By the early Pleistocene, right on the eve of the Ice Ages, antelopes in general were more fully adapted to open country, able to run faster to escape predators, a feat especially associated with the gazelles.

Of course, antelopes are by no means the only cloven-hoofed animals associated with Africa. Pelorovis, an exceptionally large-horned relative of the buffalo, also came to prominence as the climate shifted towards something more suitable for grazing. Giraffes, on the other hand, had been around for longer. The immediate ancestor of the modern giraffe, Giraffa jumae, first appeared around the dawn of the Pliocene, and held on through most of the Ice Ages (insofar as they affected Africa, of course). They looked extremely similar to their living descendants, but had only a single pair of backward-pointing protuberances on their heads.

Mention of giraffes does, of course, bring us to the question of why their necks are so long - whatever it was, it was something that clearly already applied to G. jumae. The standard explanation, of course, is that it's to enable them to get at leaves high on trees that other animals can't reach, but a famous study in 1997 appeared to show that they actually don't do this to any significant extent. Instead, the authors pointed out that male giraffes fight one another by whacking their necks together, and suggested that getting better at this sort of thing led to longer and longer necks, a claim supported by the fact that the necks of male giraffes are, indeed, stronger than those of females. Debate continues as to whether this is a more likely explanation than the classic one, or, um... not.

(I'll point out, incidentally, that there's no reason both theories can't be at least partly true. There doesn't have to be only one reason for something).

But one group of animals that was more diverse in Pliocene Africa than it is now were the pigs. There are, of course, pigs in Africa today, most notably the warthog, but there are relatively few such species compared with those in Asia. In the Pliocene there were many more, perhaps in part because the wetter climate better favoured them. The changes in African pig species over evolutionary time is unusually complete, so much so that examination of their remains has been used for decades as a means for calibrating the ages of fossil sites on the continent.

At the dawn of the Pliocene, African pigs were relatively large animals, distinguished from their modern kin by thickened tooth enamel. They were represented by two genera, each with a number of species, perhaps indicating specialisation for different habitats. The older genus, Nyanzachoerus, is known to have been sexually dimorphic, with one sex (assumed to be the male) being larger and having a number of dramatic lumps and flanges on its skull that would probably have helped in fighting for access to females. Notochoerus, which likely evolved from the older form, nonetheless lived alongside it, and was much larger, with a long snout and relatively long, curving tusks. Adult males stood around 120 cm (4 feet) tall at the shoulder, and may have weighed as much as 450 kg (1,000 lbs), far more than the largest wild pig today.

In the later part of the Pliocene, these pigs were joined by another group of species, thought to be more closely related to those that survive today, and including the first identifiable warthogs, dating back to about 3.4 million years ago. These animals, belonging to the genus Metridiochoerus, and ancestors of the giant warthogs of the Ice Ages, were also joined by much smaller animals of the genus Kolpochoerus. These latter originally came from Europe, and clearly form a different evolutionary lineage to those previously living in Africa.

It's these later pigs that are particular significant for dating. While the older forms also seem to have shifted to eating more grasses as Africa dried, as much due to a lack of alternative as anything else, Kolpochoerus and its kind were already well adapted to this diet, and thrived, replacing their larger relatives entirely as the Pleistocene dawned. Once in Africa, they seem to have evolved relatively rapidly, and we have quite a clear progression through the Pliocene of one species being directly replaced by another, some of them apparently existing for as little as 300,000 years, which really narrows down the dates of any newly discovered deposits they happen to be found in.

Apparently not so suited to the cold, their relatives died out in Europe, presumably replaced by the modern wild boar, and, while they themselves died out at the height of the last Ice Age, they had already retreated to the very hottest parts of Africa, and are likely the ancestors of the modern jungle-dwelling hogs of the area.

There were other large herbivorous animals, too. Hippos already existed, with Hippopotamus gorgops being even larger than the modern species, and likely weighing well over 3 tonnes. The species is also notable for eyes that practically stuck out of its head on stalks, presumably enabling it to keep an watch of its surroundings while almost entirely submerged.

Even stranger was Ancylotherium, an animal with no close modern relatives, a holdover from the earlier, Miocene, epoch. Vaguely horse-like in their general proportions, these creatures had three clawed toes on each foot, a vestige of a distant relationship to rhinos and tapirs. They were among the last of a group of animals called the chalicotheres, although another genus, Nestoritherium, outlasted them in China. Unlike most chalicotheres, their claws appear to have been retractable, allowing them to walk in a fairly normal fashion, rather than in the gorilla-like knuckle-dragging style of their kin. Found through much of eastern and southern Africa, and even into southern Europe and west Asia, they probably browsed on the leaves of trees. Like so many other animals, they died out when the Ice Ages began.

Many of these animals are significant because they lived alongside our own pre-human ancestors. While the exact classification is debated, because many scientists prefer narrower divisions, our closest relatives during the Pliocene belonged to what we can broadly call the genus Australopithecus. By this point, we had already come down from the trees, and were walking on our hind legs, although we didn't yet have hand-crafted tools. True chimpanzees also first appeared during the mid Pliocene, around 3 million years ago, long after their ancestors had split from our own, likely around the end of the Miocene. As for ourselves, the climactic changes of the late Pliocene and early Pleistocene that brought changes to so many other species may well have spurred on our evolution too, with our own genus, Homo, arising from some unknown australopith around this time.

But now I will leave Africa behind, heading west across the Atlantic, to see what was happening in Pliocene South America...

[Photo by "Apotea", from Wikimedia Commons.]

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