Pliocene (Pt 13): The First Carnivorans in South America

Macrauchenia

For most of the Pliocene, South America was an island continent. Over the millions of years of its isolation, it had evolved some strange forms of mammal unknown elsewhere. Some, such as the anteaters, armadillos, and sloths, survived the continent's eventual collision with its northern counterparts; most did not. A time traveller visiting the continent during this epoch would find much that was vaguely familiar, to be sure - there were monkeys, rodents, and bats, for example - but also much that was not. This is especially true when it comes to the larger animals of the day.

Even today, South America has no antelopes, zebras, or native goats; the role of "large hoofed herbivore" is taken jointly by deer and by llamas and their kin. Both are introductions from the north, arriving only after the rise of the Panamanian Isthmus in the very latest part of the Pliocene. Their appearance did not spell immediate doom for the native hoofed animals, some of which struggled on until the arrival of humans at the end of the last Ice Age, but the end result was the same, and they are all long extinct.

Looking back over the long history of South America's isolation from the north, we can identify as many as five major groups of herbivores that can broadly be described as "ungulates" (loosely speaking, "hoofed mammals", although the term includes such things as camels, as well as primitive ungulates that hadn't yet evolved hooves as such). The Pliocene, however, started around 5 million years ago, which is relatively recently, compared with the huge time frame we're talking about. As a result, by this time, only two of these native groups of ungulate survived. I've already described the notoungulates, such as the huge rhino-like Toxodon, but what of the other group, the litopterns?

The features that distinguish litopterns from other ungulates are primarily concerned with the shape of the teeth and the detailed structure of the ankle bones, but, in more general terms they were typically long-legged herbivores, with body forms that, by the Pliocene, were somewhere between a horse and a llama. Indeed, for much of the late 19th century, they were thought to be primitive llamas.

In retrospect, one of the reasons that they couldn't be was that they typically had three toes on each foot. As in living tapirs and prehistoric horses, the bulk of the weight was borne on the middle toe, which was much larger than those on either side. This, it should be noted, is entirely unlike llamas, which, in common with their cloven-hoofed relatives, bear their weight equally between two of their toes.

By the Pliocene, the original, somewhat wider, diversity of litopterns had been reduced to just two families. One, the proterotheres, had evolved more in a horse-like direction, greatly reducing (and, in some species, actually losing) the two side toes. They also had teeth that seem to have been adapted to a tougher diet, which may mean that, also like horses, they had a tendency to eat grass. They were mostly fairly small animals, although Diplasiotherium, which died out not long after the dawn of the Pliocene, may have reached the size of a large pony. At least one species, the gazelle-sized Neolicaphrium, outlived the Great American Interchange, only going extinct towards the end of the Ice Ages.

The second family was the more llama-like of the two. While there were a number of species, by far the best known is also the very last to survive, Macrauchenia patachonica. This made it through the Ice Ages to die out as recently as 10,000 years ago, long enough that at least some of them appear to have been butchered by humans. The first specimen was discovered by none other than Charles Darwin, during his voyage on the Beagle, although, not being an expert on fossils, he passed the specimen on to palaeontologist Richard Owen on his return, who made the first formal description and came up with the name.

Macrauchenia was one of the largest litopterns, standing around 2 metres (6 feet) at the shoulders and weighing around a ton. The name Macrauchenia literally means "long-necked", and it also had long legs  that further enhanced the physical resemblance to llamas. Apart from the three toes on each foot, one of its most distinctive features was that the bony nostrils were remarkably far back, towards the top of the head, rather than at the front, where you'd expect them to be. This is usually interpreted as evidence for a short trunk, since it's similar to the way that tapir nostrils are arranged. The trunk itself would, of course, not fossilise, so absolute proof is lacking, but such an arrangement is otherwise only common in animals such as seals that like to keep their heads submerged beneath the water as much as possible. To me, at least, that seems a bit unlikely for an animal shaped like a llama.

While the original fossils were discovered in Patagonia, and most of those since have also been found in the south of the continent, at least some have also been found in mountainous Bolivia and tropical Brazil and Venezuela, suggesting that an animal that could readily adapt to different climates and terrain. Its teeth suggest a mixed diet, likely including both leaves and grass, which also might fit with an animal that was adaptable. The long legs and hooves suggest that it could run moderately fast, although an analysis of the ankle and limb bones suggest that it would also have been skilled at making sudden swerves and turns to escape predators, rather than relying on speed alone.

But what, exactly, would it have been running from? The authors of the study on Macrauchenia's agility were concerned primarily with its ability to escape from attacks by the sabretooth cat Smilodon. This makes sense, since an adult Macrauchenia was just about the largest animal that Smilodon could have taken down, and younger animals would have been an even more obvious target. But there is a problem here. While the two animals did live alongside one another for millions of years, with all that that implies, for most of Macrauchenia's existence, they didn't - sabretooth cats cannot have been what the animal originally evolved to flee.

Today, all large, land-dwelling, mammalian carnivores belong to the order Carnivora, a group of animals that includes everything from tigers to coyotes to bears. But carnivorans evolved in the Northern Hemisphere, and very few reached South America before the collision with the north. So, before that time, what was eating the strange herbivores of the day?

For a start, one thing to note is that I did say "very few"; it isn't true to say that there weren't any carnivorans in South America before the rise of the Panamanian Isthmus. Foxes and mustelids both managed to cross to South America shortly before the isthmus was completed, presumably by way of the chain of small islands that occupied what is now the southern end of Central America. (It's perhaps notable that one of the first mustelids to do so was likely an otter). This, of course, is during the Late Pliocene, but it still beats the arrival of the cats, including both Smilodon and the ancestors of today's jaguars, and of the bears.

But one group of carnivorans had managed to cross the Caribbean much earlier than that, arriving over 7 million years ago, two million years before the dawn of the Pliocene. These were the raccoons.

The oldest raccoon fossil in South America, and, by some margin, the oldest fossil of any mammal there whose origins can be traced to North America, belonged to a species called Cyonasua. Strictly speaking, it wasn't a raccoon as we'd now think of the term (they crossed to South America surprisingly late, during the last Ice Age), but it was a member of the same family as the living species, and would have looked at least vaguely similar in size and shape. By the Pliocene, this had already died out, but not before leaving a rather more impressive descendant.

Chapalmalania was not your typical raccoon. Standing something like 1 metre (3 feet) high at the shoulder, it was of similar size and bodily proportions to a black bear. It is known to have lived from Argentina to Colombia, and so may have been fairly tolerant of different climates. It seems to have died out not long after the Great American Interchange, presumably, as so many other animals did, unable to compete with new arrivals from the north - in this case, bears make an obvious culprit.

Living raccoons are omnivorous animals, and it seems likely that Chapalmalania was the same. For a long time, though, it was thought that it was closer to the herbivorous end of the spectrum than the opposite, and that it might even have eaten some specialised diet - a theory doubtless inspired by its physical resemblance to a panda. Some more modern research, however, suggests that this might not be the case. An analysis of Cyonasua in 2010 suggested that it might have been notably more carnivorous than modern members of the raccoon family, and that this might apply to its descendant, too.

More strikingly, a fossilised glyptodon described in 2013 showed teeth marks on its bones that match closely what we'd expect of Chapalmalania, but not of any other carnivore that would have been around at the time. It seems very unlikely that something as large and heavily armoured as a glyptodon would actually have been killed even by a really big raccoon, but the signs are consistent with scavenging, with the animal having ripped away some stringy bits of meat from an existing carcass. The teeth of Chapalmalania are by no means those of a pure carnivore, still less of a bone-cracking scavenger like a hyena, but it did have skull features that suggest particularly powerful jaw muscles, and may well have included soft-body scavenging as a significant supplement to its regular diet.

While this gigantic, flesh-eating raccoon was, for a long time, the only carnivoran that we know of in South America, it was not the only mammalian carnivore. So before I leave Pliocene South America, I have to take a look at what else lived there...

[Picture by Kobrina Olga, from Wikimedia Commons.]