Saturday 30 September 2017

Miocene (Pt 3): The Elephants Reach Europe

Gomphotherium angustidens
At the dawn of the Miocene, Europe was still relatively isolated from much of the rest of the world. The only other continent to which is was attached by land was Asia, and even with that, perhaps due to the higher sea levels and narrower land bridges of the day, there was less exchange of animals than one might expect. As a result, for a few million years at the dawn of the epoch, Europe remained a haven for animals that had died out elsewhere, or at least retreated. Tapirs, for example, were still common; today they are restricted to South East Asia and the Americas. The European rhinos of the day were also relatively small and primitive.

While the millions of years that they survived might actually have been quite notable, had we been talking about the much shorter, later epochs, in the grand sweep of the Miocene, we can say that they were quite rapidly replaced by immigrants from the east. The early tapirs were gone (although more modern forms did return for a while, much later), but the rhinos were instead replaced by larger and more sturdy, distinctly Miocene species.

At the smaller end of the scale were animals such as Plesiaceratherium, a lightly built rhinoceros that still resembled its hornless predecessors, but with a longer snout. Their ancestors had had a reasonably full set of teeth, but, in a step towards today's rhinos, Plesiaceratherium had lost the front teeth of its upper jaw; it's possible that this meant it had developed the prehensile twig-snapping lips of the modern animals. Close relatives lived across Asia, being particularly common in China, and known to have reached as far as Japan (which, in fairness, was still part of the Eurasian coast; the Sea of Japan was only just beginning to form at the time).

Perhaps more significant were horned rhinos such as Diaceratherium, thought to have weighed around 1.5 tonnes, and its relative Prosantorhinus, which had a smaller body but a proportionately larger horn. Although we do now have some fossils of the first of these species that dates to the very end of the previous epoch, they seem to have become much more common as the climate improved in the early Miocene. Although they lived across much of Europe (and possibly even parts of Asia), they are best known from what was then the east coast of Switzerland, an area of warm swampland at the head of a long bay stretching far inland from the modern Adriatic. They presumably fed on swamp vegetation, and may well have been partially amphibious, resembling a sort of horned hippo.

These swamp-loving rhinos suffered as the climate became cooler and drier towards the end of the Middle Miocene, with the earlier species being replaced by Brachypotherium, which still had short, hippo-like legs, but seems to have been better adapted to tough vegetation. It must have done well enough, since species belonging to the genus have been reported, not just from Greece and Turkey, but from places as far afield as Pakistan and Kenya. Similarly widespread, but moving out of the swamps to relatively dry woodland, was the closely related Alicornops, which seems to have had a small horn, and which steadily became larger and more sturdy as the climate changed to suit it.

Further west, however, we see the arrival of the long-limbed hornless Hispanotherium, whose teeth suggest a much tougher diet of grass and scrubby vegetation. As their name suggests, this was native to what are now Spain and Portugal, we even have what appears to be a footprint of one, where it stepped in a pile of dung outside present-day Lisbon that later became fossilised. Whether it lived anywhere else is unclear, similar animals from China may or may not be very close relatives. Rhinos more closely resembling the modern sort also reached Europe around the middle Miocene, including the long-horned Lartetotherium, thought to be related to today's all-but-extinct Sumatran rhinos.

Rhinos were not the only animals to reach Europe from the east in the Early Miocene. A brief land-bridge between North America and Asia allowed horses to cross over into Siberia for the first time. While there were, in fact, quite a number of different kinds of horse in the Americas at the time, only one Anchitherium, seems to have made the crossing, and eventually reach Europe by way of Turkey. It was a small horse, by modern standards, standing no more than a metre (three feet) or so at the withers, if that, and had a shortish, almost deer-like head, with teeth that suggest a similarly mixed diet. Like the other horses of the day, it had three toes, and was probably more comfortable on soft ground than the dry plains that modern horses like to run across.

Anchitherium was common across much of Asia, and, while it probably didn't leave any direct descendants, horses remain well-known today. Other Asian arrivals of the early Miocene were not so lucky in the longer term, and among them we can count the chalicotheres. Distantly related to both horses and rhinos, these were odd-looking animals with a sloping back and a short, but otherwise vaguely horse-like, head. The first chalicotheres to reach Miocene Europe, Metaschizotherium and Phyllotillon (which are not always recognised as distinct), were relatively primitive, with teeth adapted to a moderately tough diet, such as twigs and bark, but not to full-on grazing. This is supported by the shape of their front feet, which had claws instead of hooves, and look to have been ideally suited to pulling down branches to munch on.

These early chalicotheres responded to the changes of climate in the Middle Miocene by becoming even more specialised. The early species died out, being replaced by the likes of Chalicotherium itself, from which the group takes its name. Unlike the more primitive forms, this was a large animal, about the size of a horse, and its forelimbs were so long, and their claws so large, that it could not possibly have walked in the way that hoofed herbivores (and its own ancestors) normally do. Instead, they seem to have knuckle-walked, as modern gorillas do, which, when you consider that they were otherwise more or less horse-like, must certainly have made them an odd sight to modern eyes. They also appear to have adapted to a softer diet, presumably eating a higher proportion of leaves pulled down from the tops of trees with their exceptionally long arms. Enough fossils exist, across Eurasia, for us to tell that males appear to have been larger than females.

Not all the Early Miocene immigrants to Europe came from Asia, however. Around 19 million years ago, Arabia and Turkey collided, allowing animals from Africa to enter western Asia, and thus Europe, for the first time. Having been an island continent prior to this time, Africa had developed its own, rather strange, animals, in isolation from the rest of the world. Now these odd creatures were free to spread more widely.

As it happens, most of them didn't take up the opportunity, perhaps out-competed by locals already better adapted to European conditions. Of those that did, the anthracotheres, strange pig-like relatives of hippos, died out in Europe in the Middle Miocene, although they survived further east and south. More successful, in the long term, were the elephants.

Which weren't actually elephants, of course, because they didn't exist yet. But the elephants we have today are merely the last survivors of a once much larger group, and no less than three different families of these animals arrived in Europe in the Early Miocene. We can vaguely lump them together under the broad term "mastodons", but they're more accurately "non-elephantid proboscideans"... it's just that, really, elephants are the closest thing to them that we still have.

Perhaps the most significant of these three families were the gomphotheres, represented by none other than Gomphotherium. So important was the sudden arrival of these animals that it has been referred to as "the Proboscidean Event", or even "the Gomphotherium Event", marking a radical change in European and west Asian ecologies that divides the local Early Miocene into two.

Gomphotherium was about the size of an Indian elephant, and it already had the trunk (and, probably, the large ears) that we associate with elephants today. The most obvious difference, however, was that it had four visible tusks, not just two. The tusks in the upper jaw, analogous to those of living elephants, were long, but pointed slightly downwards, not in the upward sweep that we are familiar with. The second pair were at the tip of a ridiculously long lower jaw, and were shorter and straighter, stabbing out like narrow spear-points. There is some evidence that, while they were of similar size in other respects, males had significantly longer tusks than females.

Quite what they wanted these extra tusks for, never mind a jaw that must have stuck out nearly as far as the trunk up above it, isn't entirely clear, although we do know that they had a reasonably mixed diet. But they were evidently very successful, being found across much of Europe, as well as India and China, and eventually even reaching America.

Arriving somewhat later than the gomphotheres were the deinotheres. It's possible that they arrived late because they actually came across from Asia, where some very early fossils are known, rather doing so direct from Africa as their relatives did. The first deinothere to reach Europe, Prodeinotherium, was only about two metres (6 feet) high at the shoulder - tall by most standards, perhaps, but on the small side for an elephant. In other respects, they probably looked much like the modern species... except, once again, for the tusks. Unlike the gomphotheres, they only had a single pair, but it was in the lower jaw, not the upper one (as it is in modern elephants) and, moreover, the tusks curved down and backwards. It's possible that they were used for stripping bark from trees, or something of that sort, but there really is no obvious analogy among living animals.

Like the gomphotheres, the deinotheres did spread across Asia, but they never reached America, and left a far smaller number of descendants, with the last ones dying out at the dawn of the Ice Ages. They seem to have been particularly distant relatives of both modern elephants and most other "mastodons", but the third group of proboscideans to come across belonged to the true mastodon family - that is, the same one as the well-known (but much later) American mastodon Mammut americanum.

Being true mastodons, this group, all belonging to the genus Zygolophodon (sometimes synonymised with Mammut), looked much more like the elephants we would recognise, with their tusks in the right place. But they were absolutely huge, quite a bit larger than today's African elephants, with full grown bulls reaching somewhere in the vicinity of 4 metres (13 feet) at the shoulders. It's one contender for the crown of "largest land mammal ever".

Such large animals were by no means the only ones to come up from Africa. However, the remaining major group to do so, the primates, did take a few million years longer. Primates had existed in Europe before, but had been gone for many millions of years, and, when they returned, about 16 million years ago in the Middle Miocene, they were a new kind of primate: apes.

Apes as a whole only date back to the dawn of the Miocene, but such is the length of the epoch that, by the time they'd arrived, they had already undergone some significant evolution. The first primate to reach Europe, Pliopithecus, was not a true ape but a sort of tail-less monkey. Despite being a monkey, it physically resembled a gibbon, although the first true gibbons probably did not evolve until much later. The major difference from gibbons was that the arms were still noticeably shorter, suggesting that, while they probably could swing through the trees, they likely weren't as good at it as living gibbons are. What they ate seems to be a matter of some controversy, suggestions ranging from "almost nothing but leaves" to "almost nothing but fruit", with some gradations in between.

Pliopithecus has been found from Spain to Slovakia, with related species inhabiting Asia. It was joined early on by Griphopithecus, known from Austria to Turkey. This really was an ape, although likely an early branch in ape evolutionary history that left no living descendants. It seems to have lived at least partly on the ground, and the thick enamel of its teeth suggests a diet with some hard components, such as seeds or scrubby vegetation.

Arriving rather later, although also known in Africa, was Dryopithecus, a roughly chimp-sized animal. It's exact relationships are uncertain, although it may be more closely related to the African apes (including humans) than it is to orang utans, placing it relatively late in the family tree. Its teeth had much thinner enamel than Griphopithecus, so is presumably ate softer food, such as fruit, but, while the shape of its hands suggests that it could hang from trees, as orang utans do, other features of the arms, back, and shoulders imply that it moved with its back held semi-upright, rather than knuckle-walking - although it would not have been fully bipedal. The face, on the other hand, is thought to have most closely resembled that of a gorilla.

But, with the primates out of the way, it's time we turned to the other major group of mammals in Europe at the time: the carnivores.

[Painting by Charles R. Knight, in the public domain.]

No comments:

Post a Comment