Sunday, 20 November 2016

Pliocene (Pt 14): Terror of the Marsupial Sabretooths

As I have described in previous posts, during the Pliocene, South America was nearing the end of a long period of isolation, having been separated from the other habitable continents for millions of years. In the very latest part of the Pliocene, it collided with North America, leading to the massive influx of northern animals known as the Great American Interchange. Until that time, many of the native animals were, to modern eyes, rather strange, evolutionary experiments, that, while evidently successful in their own right, could not compete with the invaders from the north, such as deer, llamas, bears, and big cats.

One of the oddities of pre-Interchange South America, though, was the absence of large placental mammalian carnivores. There were, as I have already noted, a few, since the raccoons somehow crossed the seas between the continents well before the rise of Central America. And, towards the end of the Pliocene, a few other animals narrowly beat the Interchange by island-hopping before the land bridge was completed. But, raccoons aside, none of the large mammalian carnivores we would associate with the continent today had yet arrived.

But, with so much mobile meat wandering about on its three-toed hooves, or whatever other arrangements they may have had, surely something must have evolved to eat it? Evolution had, after all, had millions of years to fill the rather obvious gap. But if there weren't any mammals we'd recognise acting as dominant predators, what did the Pliocene herbivores of South American actually have to fear?

Well, certainly something!

However, many of the main predators of the day were not, in fact, mammals at all. There had previously been some fearsome reptiles on the continent, including both constrictor snakes and crocodile-like animals (although neither belonging to any family that survives today), but, even by the most generous estimates, these were already gone by the dawn of the Pliocene. Instead, much of the top predator niche was taken up by birds.

I'm not referring here to things like eagles, although flying avian predators of some kind were present. Instead, I'm referring to the terror birds, more technically known as phorusrhacids. These were large, flightless birds, that resembled a sort of fast-running carnivorous ostrich with a muscular neck and a powerful, cutting beak. Probably the best known to the general public is the massively built Titanis, an early arrival in North America. Most of the South American species we know of were somewhat smaller, although Devincenzia, which lived in Argentina and Uruguay at the very dawn of the Pliocene, may have reached 250 cm (8 feet) in height.

The most complete skeleton we have for one of these creatures belongs to Llallawavis, a 120 cm (4 foot) bird native to the grasslands of Pliocene Argentina. The skeleton is so well preserved that we can even make some guesses as to the sorts of sounds it would have been capable of making in life. A number of similarly-sized species existed alongside it, across the continent, and prospered throughout the Pliocene, only dying out when more efficient, mammalian, predators, such as the ancestors of jaguars, arrived in the Interchange.

However, while these strange, flightless, birds were among the top predators of their day, the continent was not entirely devoid of mammalian carnivores. But these were not placental mammals, related to the majority of the land-dwelling carnivores we know today, but, rather, marsupials.

Most were small, relatives of the opossums that are still found on the continent today. There are quite a few species of opossum native to present-day South America, and more lived in the past. One species, of course, was lucky to make the crossing north during the Great American Interchange, becoming one of the few (although by no means only) South American oddities to succeed in the northern continent, where it still lives today.

But opossums are more accurately described as omnivores, and when they do eat fresh prey (rather than carrion), it tends not to be anything much larger than a mouse. Clearly, such animals were hardly competitors for the terror birds. The same is not true for the sparassodonts.

Sparassodonts had existed for a long time in South America, including a variety of different forms over the millions of years since their first appearance. Strictly speaking, they probably weren't marsupials, in the sense of being descended from the last common ancestor of any living species. They did, however, belong to the marsupial half of the mammalian family tree, the Metatheria, and were quite possibly the closest relatives of true marsupials that we know of.

One feature of marsupials is the presence of epipubic bones - sometimes actually called "marsupial bones" - in front of the pelvis. In living species, they often help to support the pouch in females, although they also restrict the birth canal, which is probably why placental mammals, with their need to give birth to relatively large, well-developed, young, don't have them. It's interesting to note that, in sparassodonts, these bones were greatly reduced, and often entirely replaced with cartilage, which might indicate that their females didn't have pouches.

On the other hand, the same bone is found in platypuses and echidnas, along with just about every other non-placental mammalian fossil we've found that's in a complete enough condition to judge, so supporting the pouch is presumably not what the bones originally evolved for. For that matter, thylacines, which certainly did have a pouch, had epipubic bones that were reduced in a similar manner to those of sparassodonts, so it may mean nothing at all.

By the Pliocene, however, the sparrasodonts were already in decline. Only two groups remained, of which the more numerous, in terms of number of species, was probably that of the relatively obscure hathliacynids. These were moderately sized animals, probably of more concern to animals the size of rodents than to any of the large herbivores of the day. Although they seem to have been specialised for meat-eating, in comparison to the omnivorous opossums, the increasingly cool climate apparently spelled their doom some time around the middle of the Pliocene.

The remaining group, the borhyaenids, were a different matter. By the Pliocene, just one known species of borhyaenid survived, from what had previously been a fairly diverse group. But that species was Thylacosmilus atrox, which was impressive indeed. It was about the size and shape of a leopard, and looked remarkably like a cat - despite, of course, belonging to an entirely different evolutionary line. Not only that, but the sort of cat it most resembled was a sabretooth.

In fact, it was a remarkably specialised sabretooth, perhaps, in some respects, even more adapted to the sabretooth lifestyle than Smilodon itself. The sabre-tooth canines were huge in comparison with the head, and had open roots that indicate that they must have grown continuously throughout life (if not necessarily very rapidly, after reaching adulthood). The roots are also remarkably long, stretching so far back in the upper jaw that they terminate above, and slightly behind, the eyes. The lower jaw is even more wierdly adapted, with long flanges of bone projecting downwards from the tip, allowing the sabres to slot either side of them, and presumably reduce the risk of them breaking. (It should be noted that similar, if less dramatic, flanges were also seen on some unrelated "sabretooth" species).

Analysis of the jaw muscles, as determined from their points of attachment on the skull, have shown that they were remarkably weak. That might sound counter-intuitive, but the neck muscles were very powerful, and the skull adapted to resist stress to an even greater extent than in real sabretooth cats. Those cats most likely delivered a precise killing blow, forcibly holding prey down with their bodies and fore-limbs as they punctured the throat, and it would seem that Thylacosmilus was even better adapted to this approach than they were. It killed, it seems, with a precision strike against an already immobilised victim, rather than simply ripping chunks out of it, and so risking breaking its long, slender teeth against some piece of bone.

This is supported by studies of the animal's forelimbs, the apparent strength of its shoulder and chest muscles, and its unusually rigid back, all of which would have helped hold down a struggling victim as the killing blow was delivered. All of this, of course, in a marsupial (sort of), not in a more typical mammalian carnivore. It's also perhaps worth noting that its brain was unusually large for an animal of its type - similar, proportionately speaking, to something like a kangaroo, but larger than we'd expect of an opossum - suggesting a reasonable degree of intelligence. On the other hand, it does not seem to have been a fast runner, and presumably attacked from ambush.

This ambush approach to hunting may have favoured wooded environments, which would helped reduce competition with the fast running terror birds, which likely preferred open grasslands. Even so, Thylacosmilus did not survive as long as the predatory birds did. It was long assumed that it died out following the Interchange, faced with competition from invaders from the north - which, in this case, likely means Smilodon itself, which did, indeed, enter South America by this route. However, it is now clear that the dates don't fit, and, unless some new fossils should present themselves, it appears that the marsupial sabretooth died out before the continents joined and its feline counterparts arrived. It is more likely, then, that th steadily worsening climate as the Pleistocene approached, destroyed the marsupial's native habitat long before it had to face a competitor.

Which brings me to the end of Pliocene South America, but not to the end of my discussion of Pliocene marsupials... because I have not yet looked at what was going on in Australia.

[Photo by Claire Houck, from Wikimedia Commons.]

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