Sunday 16 February 2020

Miocene (Pt 18): Return of the Cats

For much of the Miocene, bears were represented in North America by the single genus Ursavus. This was relatively primitive, and small by the standards of modern bears, although still easily identifiable as such. It seems to have left no local descendants, but, around 7 or 8 million years ago, it was joined - and eventually replaced - by new arrivals from Asia.

The best known of these were Indarctos and Agriotherium, bears that were widespread across the Northern Hemisphere of the time, and common enough in both Asia and Europe. Indarctos was the smaller of the two, roughly the size of a black bear, and with what was probably a similarly omnivorous diet. It was likely more closely related to pandas than to other modern bears, and died out as the Miocene ended.

Although even larger species of Agriotherium lived elsewhere, those in North America were somewhere around the size of a modern grizzly, if not smaller. Their counterparts in Europe and Asia had a typically ursine omnivorous diet, but a recently described American species has teeth that suggest it may have had a more heavily meat-based diet than is typical for modern bears (aside from the polar bear, of course).

A third genus of bear, however, appears to be unique to North America. Plionarctos first appeared at about the same time as Agriotherium and Indarctos made their crossing from Asia. As a result, it's often supposed to be another migrant, but the lack of any fossils from the Old World suggests that, if so, it didn't last very long in its original homeland. It seems to have done reasonably well in North America, however, surviving the end of the Miocene, and only dying out towards the end of the Pliocene. By this time, it had already left descendants that would cross back over to Asia, and down into South America, creating an entire lineage of "short-faced bears", some of which grew to considerable size. Today, only one survivor of this lineage remains; the spectacled bear of South America.

While bears had a longer history on the continent, cats were a different matter. Indeed, for much of the Early Miocene, there were no animals in North America that even resembled cats, let alone the real thing. The first cats reached North America during the Middle Miocene, filling a gap that had remained empty since the extinction of the cat-like nimravids millions of years before.

The oldest cat fossil in North America was uncovered in Nebraska, and dates to around 17 million years ago. Its exact status is unclear, but it belonged to an animal very similar to Pseudaelurus, a primitive cat known in Eurasia from even older deposits. Whatever it is, it likely gave rise to Hyperailurictis, a group that contained a number of species, the largest of which reached about the size of a modern leopard.

Although they lived for a long time in North America, and were presumably successful predators, Hyperailuricitis represented an early side-branch in cat evolution, with the cats we are more familiar with evolving from its relatives left behind in Eurasia. By the Late Miocene, it was already extinct. Some of the animals that it replaced may have been its direct descendants. One such possibility is Pratifelis, a cougar-sized animal whose relationship to other cats is otherwise unclear. Other replacements, however, were new immigrants, some of which, such as Amphimachairodus and Metailurus, were also known from Europe and Asia.

These were sabretooth cats, distant relatives of the more famous (and much later) Smilodon. While they may were other representatives of some of these genera living in Eurasia at the time, some distinctly American forms are known. For example, while the Eurasian species of Amphimachairodus were roughly the size of lions, the American A. colaradensis was at least the size of a tiger, and possibly slightly larger, which would likely make it the largest cat ever to have lived.

Other American sabretooths seem to have evolved on the continent. Adelphailurus was a moderately large cat of this type, known to have lived across much of the American southwest. Unfortunately, we have so few remains of the animal that we know almost nothing about it, beyond the fact that its teeth, while larger, weren't really what you could call "sabres", being still at a relatively early stage in their evolution.

Nimravides was another native cat of the time, although its exact history and identity are somewhat confused. It doesn't help that it has an unfortunate name, since it rather sounds like it should be some form of nimravid - the 'false cats' that had lived in North America in the preceding, Oligocene epoch. But it's nothing of the kind, and is undeniably a true cat... of some sort.

At first glance, the picture doesn't seem too confusing. The earliest forms are relatively small, and lack sabre-teeth. Indeed, they look quite similar to Hyperailurictis, and so plausibly evolved from them. These lived around 12 million years ago, and a couple of million years later, we have a larger form with noticeably more prominent teeth. Later still, we have N. catacopis, an animal about the size of a tiger, but with a longer, more muscular neck, and very clearly sabretooth in form.

The problem is that this last species looks remarkably similar to Machairodus, a sabretooth cat living at the same time in the Old World. So similar, in fact, that there's an ongoing debate as to whether they're really the same animal (or at least, two different species of the same genus, in much the same way that leopards and jaguars are today) or the result of convergent evolution.

If the former is true, it would imply that N. catacopis isn't really descended from the earlier species of Nimravides at all. Under this scheme, those early species really did evolve in North America, and, despite some increase in the size of their canine teeth, were never sabretooths at all. They died out at the end of the Middle Miocene, around 8 million years ago, and were replaced by the immigrant Machairodus from Asia, which rapidly evolved into the extremely similar N. catacopis.

Shortly before this, however, another sabretoothed animal had crossed over from Asia. Despite its huge and dramatically curved canine teeth, however, Albanosmilus wasn't a true sabretooth cat, but a member of a different lineage. Instead, it was a barbourofelid, or false sabretooth, belonging to a group of very cat-like animals that were undeniably related to the true cats, but distinct enough to be considered a group of their own.

Quite how the false sabretooths related to true cats is, however, another matter of ongoing controversy. They were originally considered to be genuine sabretooth cats, albeit slightly different from the others known at the time, but, in the 1970s, it became clear that that couldn't be true. Instead, they were considered to be a subfamily of the nimravids, those ancient cat-like animals that had lived in North America before the 'Cat Gap', and that survived for much longer in the Old World.

More recently, however, they have been thought of as being more closely related to the cat family than to the nimravids, without actually being similar enough to be considered as true cats themselves. This would make them a third family, alongside the cats and nimravids, and it's the usual position taken by palaeontologists today. But the controversy is not over yet, and a paper published just a few weeks ago takes the opposite view, thus demoting them to subfamily status again - it remains to be seen how well this will stand the test of time.

Whatever they were, in the larger scheme of things, Albanosmilus was definitely one of their number. It didn't last long in North America, instead likely evolving into Barbourofelis, a larger, and presumably more dangerous, local predator. This latter animal was successful enough that, at some later point, it crossed back over the Bering Straits, where it is known to have reached as far as Turkey.

Most species of this genus were about leopard sized, with greatly elongated, flattened sabre-teeth and a relatively short skull. As the Miocene drew to a close however, the last member of the group appeared in North America, both the largest and most specialised of its kind. This was B. fricki, and it was closer in size to a lion than a leopard, standing something like 90 cm (3 feet) at the shoulder and with skeletal attachments that suggest remarkably large muscles.

Its canine teeth were even larger than in the earlier forms, and partly protected on their inner surfaces by a bony flange stretching down from the lower jaw. The head was also shorter, albeit with a flat forehead that would have helped distinguish them from true cats. So muscular were they that some of the features we would expect for catlike animals had faded, to make way for adaptations for sheer power and strength. For instance, their back was short, and doesn't seem to have been very flexible, while their feet were almost flat to the ground, closer in shape to those of a bear than those of most cats.

Another oddity of these animals comes from a study of the skull of a juvenile individual. This showed that the deciduous canines - that is, the milk teeth, which would eventually be shed and replaced by the adult ones - didn't erupt until the animal was almost fully grown. It is usually suggested that, since the lack of sabre teeth would be a bit of a handicap for a sabretoothed animal, juvenile barbourofelids must have been cared for by their mothers for an unusually long time, being unable to hunt for themselves. On the other hand, lions aren't fully independent for a couple of years anyway, so this extended parental care, while clearly different from that of cats, might not be so dramatic a difference as would at first appear.

Perhaps, however, Barbourofelis was too specialised, since it died out as the Miocene came to an end, surviving at best a very short way into the following Pliocene epoch, and leaving no descendants.

Which brings me to the end of Miocene North America. Next time, we will be heading south and east, to see what was going on in Africa during this long and evolutionarily significant epoch...

[Photo by Dallas Krentzel, from Wikimedia Commons.]

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