Sunday 29 October 2023

Oligocene (Pt 5): The First Cats

As new herbivores entered Europe at the Grande Coupure, carnivores were bound to follow. As with their prey, these Asian newcomers seem to have rapidly outcompeted the native European forms, leading to a sudden turnover in the types of animals we find on the continent. Although we can say that these newcomers were "carnivorans" - the sort of mammalian carnivores we're mostly familiar with today - where exactly they place relative to the living families is harder to say.

Plesictis is an example here. It looked, so far as we can tell, rather like a polecat and was about the same size, so for much of the 20th century it was thought to be an early example of a mustelid, albeit one no more closely related to actual polecats than, say, badgers or otters are. More modern analyses are more circumspect; it may look like a mustelid in some respects, but it probably lived before those animals diverged from the raccoons and so can't be quite either. Palaeogale, which looked rather similar and was also originally assumed to be a mustelid, in fact turns out to be more related to cats and mongooses, but probably so far down the family tree that it's not yet possible to say much more than that.

The same rich fossil deposits of Quercy in southern France that revealed the first fossils of Plesictis also gave us those of Amphicynodon. This was a similar size, but certain features of the skull made it apparent that it was related in some way to bears. For a long time, all we had was the skull but in the last few years we have been able to examine more complete skeletons. These reveal an animal weighing about 2 kg (4 lbs), similar to a large ferret, with grasping forepaws and mobile hips that suggest it would have been adept at climbing. It probably spent much of its time in trees, and the hind ankles were able to swivel so that it could, potentially, have hung upside down from its hind feet to grasp at prey and it would likely have bounded from branch to branch with alacrity.

Modern interpretations describe it as an "arctoid"; a general category of early carnivores that include the ancestors of bears, seals, and the weasel/raccoon/skunk clade. Although it isn't entirely certain, it's unlikely to be a direct ancestor of any of those groups itself, and is typically given its own family. Although earlier arctoids did exist in Europe, it likely came across in the Coupure, rather than evolving locally, and it may be related to the Mongolian Amphicticeps, a more purely carnivorous (rather than omnivorous, as Amphicynodon likely was) animal of similar size.

Giving them their own family does create some potential confusion, however, since, under the rules of such things, the family name ends up as "amphicynodontids" which sounds remarkably similar to "amphicyonids", a separate group more commonly known as bear-dogs. These are better known from the subsequent, Miocene, epoch, where they attained bear-sized proportions despite keeping their narrower heads and long tails. Oligocene examples from Europe include Pseudocyonopsis and Goupilictis, smaller, more dog-like animals with an omnivorous diet that already leaned more towards small mammals than plants.

By most modern definitions, the earliest known true bear was Cephalogale, with fossils from the Czech Republic, and which is known to have survived a short way into the Miocene epoch in Spain. This looked much more like a dog than it did a bear; the size of the different species was highly variable, but most seem to have been comparable to a mid-sized domestic dog of today. It was probably already omnivorous, and its teeth seem as well-adapted to crushing plants as to slicing meat.

Eusmilus was rather more deadly. This was the first nimravid to enter Europe, arriving shortly after the Coupure. The nimravids were related to cats, and looked remarkably similar, with their teeth in particular suggesting a purely carnivorous, cat-like diet. Eusmilus was about the size of a lynx, but with a sleeker build. More significantly, it had long sabre-like teeth, much like those of the true sabretooth cats that would appear much later, and a jaw adapted for the wide gape that employing these would have required. 

More nimravids followed, including Eofelis and Dinailurictis, some of which had canines that, while still large, lacked the great length and flattened sabre-shape of those in Eusmilus. This suggests that nimravids, like the true cats in later times, were evolving down two different paths, one sabretoothed and one (from our modern perspective) more "normal". The largest species of Eofelis reached the size of a leopard, but it was not quite the largest carnivore on the continent at the time. That honour goes to Quercylurus, which estimates place between the size of a lion and a small bear. It's unlikely to have been fast-running, presumably sneaking up on its prey to suddenly pounce out at them. Its feet had flat soles, closer to those of a bear than a modern cat, perhaps in order to support its relatively high weight.

The nimravids may have been the largest cat-like carnivores on the continent, but they were not the only ones. Stenoplesictis is another creature that appeared after the Coupure, although the lack of obvious ancestors elsewhere makes it harder to say whether it came across from Asia or evolved locally. It would have been more like a civet than anything else alive today and about all we can say is that it was more closely related to the living species of cat-like carnivores (cats, civets, mongooses, etc.) than it was to the nimravids.

In the latter half of the Oligocene, after the sudden cold snap of the Oi-2 glaciation, the large nimravids began to die out in Europe, although they survived elsewhere. Smaller carnivores like Stenoplesictis, however, did rather better, and began to diversify. Animals such as Haplogale and Stenogale began to develop a more explicitly cat-like form, with a shortened face and reduced cheek teeth suited for a highly carnivorous diet, but, while they are probably more closely related to cats than to, say, mongooses, they don't fit within any of the living families. 

Proailurus, however, is a different matter. First appearing in Europe around 25 million years ago, towards the end of the Oligocene, it is the oldest known member of the cat family. Its proportions were not quite those of the modern animals, being closer to those of living civets, but the head was already shortening, if not yet fully feline, and its teeth had evolved the highly specialised meat-slicing form seen in living cats. It was about the size of an ocelot, and thus larger than the domestic animal, and was adapted for climbing, and so probably spent a lot of time in the trees.

Not all of the carnivorous animals native to Asia crossed over to Europe at, or after, the Coupure although, for the most part, those that did not were reasonably close relatives of others that did. An exception worth mentioning are an obscure group called the didymoconids. Never leaving Central Asia, they had survived from the previous epoch, but continued on through the Oligocene, only dying out towards its end, with the best-known example, Didymoconus, living in Mongolia at this time. 

This was a slender animal, about the size of a cat, with sharp teeth and large canines that suggest a carnivorous diet. Some features of the forelimbs, including their strength and shape as well as unusually solid-looking claws, suggest that it may been a burrowing animal, perhaps with a diet similar to that of modern badgers. Quite what sort of animal it was is far from clear, although it is no longer thought to be a true carnivoran as was originally the case. More recent suggestions include that it might be a relative of the mesonychids, strange carnivores related to the cloven-hoofed mammals that died out at the end of the Eocene, or that it was somehow related to shrews and moles. The chances are, however, that it might not have been closely related to anything alive today.

One thing we don't find anywhere in the Old World at the time are dogs. This is because they didn't enter Asia until almost the end of the following, Miocene, epoch, many millions of years later. So what was going on on their home continent during their early evolution. Next time, I will cross the Atlantic to find out by taking a look at some of the mammals of Oligocene North America...

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

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