Pliocene (Pt 12): From Rabbits to Rhinos

Toxodon

Around three million years ago, as the Pliocene began to draw to a close, North and South America collided, the island chain that had previously stretched between them rising to become the Isthmus of Panama, and much of the rest of southern Central America besides. With a solid land bridge in place, land animals from both continents were finally free to mingle, merging two faunas that had, until then, been quite distinct.

The North "won" the resulting battle of competition, in that far more creatures successfully moved south than travelled in the opposite direction. These were, on the whole, creatures we're familiar with across the Northern Hemisphere, because the proximity of North America to Asia had already led to some blurring between the animal groups there. So, it's as a result of this, the Great American Interchange, that South America has foxes, deer, cats (most obviously jaguars and ocelots), mice, rats, weasels, otters, and rabbits.

Not only that, but some of the animals that headed south later died out in their northern homeland, so that today, we think of them as being uniquely or primarily "South American". These animals include the llamas and the tapirs, and, to a lesser extent, the peccaries or javelinas, which still survive in Mexico and the American south.

But this great northern invasion happened towards the tail end of the Pliocene. It naturally raises the question of what, exactly, was living in South America before it connected with its northern counterpart. The answer is a complex one, not least because the Great American Interchange, important though it was, wasn't quite the unique event that it's sometimes made out to be.

Prior to the Interchange, South America had been an island continent, just as Australia is today. The last time it had been joined to any other land mass, that had been Antarctica, and even that had been a very long time ago. The animals living there had had millions of years to evolve in splendid isolation, creating peculiar forms not seen elsewhere on Earth.

Mostly.

In fact, by the time the Panamanian Isthmus was fully raised, a few animals had already made the crossing using the pre-existing island chain, and had already been in South America for a few million years, in some cases even pre-dating the Pliocene. But even those early island-hopping arrivals didn't reach a world that was entirely bizarre - three major groups of land mammal had already beaten them to the punch. (As, although it's beyond the scope of this blog, had a number of reptiles).

It probably doesn't come as too much of a surprise to discover that one of these was the bats. The fossil record of bats is pretty lousy, limited not least by the fact that their fragile bones don't preserve very well. Nonetheless, while many South American bats do indeed descend from northern immigrants (many of them presumably having arrived before the final separation of the Atlantic and Pacific Oceans), we do know that many native groups have been there far, far longer than that, having arrived on multiple different occasions over the course of tens of millions of years.

Most of the bats seem to have come originally from Africa, and so far back in time that the Atlantic was considerably narrower than it is now. The two other groups also seem to have come from that continent, and at around the same time, meaning that they had been evolving in South America long, long, before it joined with the north.

The first of these two groups to arrive was probably the rodents. For much of the Pliocene South America would have lacked such familiar rodents as mice and squirrels, but it did already have its own native forms. Many still survive today, and include such animals as guinea pigs, chinchillas, and coypus, along with less well-known creatures such as degus, tuco-tucos, and pacaranas. Indeed, one group of rodents was among the few groups of South American natives to successfully head north during the Great American Interchange, bucking the general trend. Today, porcupines are found as far north as Canada and Alaska.

The remaining group were the monkeys, which had been present in the South American jungles for tens of millions of years prior to the Pliocene. While the evolutionary relationships of the different groups remains a matter of controversy, it's seems clear that all the main groups we're familiar with today were already in existence by the dawn of the Pliocene. If we were to wander through the Amazon four million years ago, we might not recognise the individual species, but we'd probably at least be able to identify them generally as being marmosets, spider monkeys, capuchins, and so on.

But what of the "true" natives, those creatures that, even back in the Pliocene, were found nowhere else on Earth? Some of them survive to this day, and three of them, the opossums, armadillos, and sloths, successfully headed north during the Interchange. Arguably, none did as well as the porcupines - the giant ground sloths, in particular, succumbed to the cold of the Ice Ages and the arrival of humanity, leaving only their tree-dwelling kin further south. But survive they did, along with the anteaters, and it has to be said that they're pretty odd animals.

Bur during the Pliocene, they were not alone.

Today, the hoofed herbivores and their kin are represented in South America by deer, llamas, tapirs, and peccaries. All of these, as I've mentioned above, were late arrivals from North America, and weren't known in the south during the Pliocene. But the continent did have its own large herbivores, often classified together under the umbrella term "meridiungulates", which means something like "southern hoofed animals". The great heyday of these creatures had already passed by the Pliocene, but two main groups still survived: the notoungulates and the litopterns.

These two groups have often been thought to be related, albeit distantly, to elephants. Some recent analyses of protein remnants in some of their younger fossils, however, point instead to a relationship with the group that includes the horses, tapirs, and rhinoceroses.  Either way, their fossil history stretches back almost to the extinction of the dinosaurs and the very dawn of the Age of Mammals, so whatever they really are, they've been distinct for a remarkably long time.

The notoungulates (notos, like meridianus, also means "southern", but it's Greek, not Latin) are officially distinguished from other animals by the structure of the bones and nerve canals around the ear, and by some peculiarities of the teeth. They were a very diverse group of animals, with some forms resembling rabbits, and others resembling rhinoceroses, with quite a few lying somewhere in between... which, yeah, is quite a wide range.

Down at the "rabbit-like" end of the scale we have the hegetotheres, with particularly high-crowned teeth suitable for eating tough vegetation. By the Pliocene, they were perhaps less rabbit-like than they had once been, with the shape of their hind-limbs suggesting that they were fast runners, rather than moving by leaping, as their ancestors had once done. They apparently lived in open, rather dry, terrain, and, judging by the size of their eye sockets, may have been nocturnal. They may even have had large ears, although not necessarily the same shape as those in rabbits. The last surviving forms, such as Paedotherium and Tremacyllus, died out not long after Great American Interchange, although it's plausible that the cooling climate, rather than competition from real rabbits, or the sudden arrival of cats and foxes, was what finally ended their line.

Rather similar, and thought to be closely related to them, were the mesotheres. These were much larger than the hegetotheres, with some forms approaching the size of a sheep. Nonetheless, their teeth closely resembled those of rabbits, with the same enlarged incisors, covered in enamel on both sides, unlike those in rodents. Along the course of their evolution, the mesotheres seem to have favoured increase in size over the sort of specialised herbivory of the hegetotheres, and they probably ate softer vegetation. There is also good evidence, from the shape of their limb bones and claws, that they were adapted for digging.

Digging is a fairly odd thing for an animal of this size to do, and it certainly seems hard to believe that they lived in burrows. Our best bet is that they ate roots, scratching into the ground to dig them up, although they presumably ate other plant matter too, when it was around. The last surviving form, Mesotherium itself, lived well into the Pleistocene, dying out in the Ice Ages, though neither it, nor any of its relatives, seems to have got any closer to North America than Peru.

Despite their wider diversity in previous epochs, however, only one other family of notoungulate survived through the Pliocene. These, however, were the toxodontids, and they were the largest herbivores of their day. Represented primarily by Toxodon itself, these probably looked somewhat like smallish rhinos, and may have weighed somewhere in the region of a ton-and-a-half, standing around 150 cm (4' 11") at the shoulder. Their teeth were similar to those of the rabbit-like forms, growing perpetually throughout their life, although this seems to have been a case of convergent evolution, since their last common ancestors with the other two groups don't seem to have shared the same feature.

This would suggest that Toxodon grazed on tough vegetation, like grasses, but analysis of the chemical composition of their fossils indicates that they had a more mixed diet. They may well have been adaptable, eating more grass in the southern pampas, and more leaves further north, in the tropical forests of the Amazon. Alternatively, their choice of grass in drier climes may have been a case of necessity, rather than preference, damaging their teeth in ways that may not have helped them in the long term.

At any rate, toxodonts survived the Great American Interchane, and even made it to the northern continent, where fossils of the genus Mixotoxodon have been found from Costa Rica to Texas. This was the largest notoungulate ever to have lived, truly rhino-sized at not much less than four tons in estimated weight. Also surviving in northern South America, it was also probably the last, living through most of the Ice Ages to die out as little as 25,000 years ago.

Next time, I will turn to look at their distant relatives, the litopterns, as well as considering what it was that ate these many strange herbivores...

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