Miocene (Pt 28): Miniature Super-horses and the Dawn of the Guinea Pigs

Thoatherium

For most of the Age of Mammals, South America was an island continent, separated from its northern counterpart by a wide sea. Even at the dawn of the Miocene, 23 million years ago, this isolation had already lasted for a long time, and the seas to the north were already a formidable barrier, with just a few islands where Central America is now. In some respects, the variation in climate across the continent was less extreme, especially because the Andes were not so high as they are today and so cast less of a rain shadow. Even so, there was enough variety that many different kinds of animals lived across its great area.

Indeed, the long separation of the continent from the rest of the world meant that it had had plenty of time to develop its own native animals. Whereas, in most of the rest of the world, animals of broad types we would still recognise were already around (early cats, deer, elephants, and so on), most of the larger animals of Early Miocene South America would have been much harder to place, since most of their descendants died out long before the Ice Ages, let alone the present. 

That is, however, less true of many of the smaller animals. Rodents already had a long history in South America, where they had developed a separate branch of their great family tree distinct from the rats, mice, and squirrels of the northern continents. But the reality is that the reason that there are so many species of rodent in the world today is that their body form has been a remarkably successful one and there has been little need to make drastic changes to it. They also fared rather better than the large native herbivores when invaders from the north invaded across the Panama land bridge so that, if a hypothetical time traveller were to stop gawking at the more charismatic animals and examine the rodents instead, she would likely find them at least somewhat recognisable.

Among these are Prolagostomus, an animal very similar to the living plains viscacha, itself a close relative of chinchillas - another group that first appeared around this time. Rather better known, however, is Guiomys, originally discovered in southern Argentina, but now known to have reached at least as far north as Bolivia. All we have of this animal is part of the jaw, but this is enough to show the distinctive double-lobed teeth that are found only in the modern guinea pig family. The shape of the jaw itself, however, does not fit with guinea pigs, so it's likely that it represents a creature that branched off just before the modern family first appeared. 

While it's difficult to say much about its habits or overall appearance based on the jaw alone, it seems to have lived in open terrain, and so probably ate low-growing plants and looked something like a rabbit-sized hamster. It may represent one of the last steps towards the guinea pig family as we know it today, since a number of rather more distant relatives lived at the same time - these are collectively referred to as "eocardiids" on account of the heart-like shape of their teeth, but probably aren't a single natural group. The guinea pig family proper probably dates from the Middle Miocene with the oldest-known example, Prodolichotis, having lived in Colombia around 11 million years ago. It's too primitive to be placed into any of the three living subfamilies (that is, the true guinea pigs, capybaras, and maras) but these seem to have already been well-established just two million years later, so there's a good chance that they were already around at this time.

An animal that might have looked even more familiar was the early porcupine Steiromys. Surprisingly, however, it probably looked a lot more like the living North American porcupine than it did any of the porcupines living in South America today. True, the North American species is descended from ancestors living on the southern continent, but it wouldn't be a direct descendant, indicating that its distinctive lifestyle must have evolved twice. 

Specifically, Steiromys was larger than any living South American species, and the shape of its leg bones suggest that, while it could also climb trees, it likely spent most of its time on the ground. Its heavy and unusually solid teeth imply a diet with plenty of bark and tough vegetation, perhaps even more so than the modern North American porcupine, and not the soft leaves found high in the trees that living South American species prefer. Sadly, we don't have any tail bones, so we don't know whether it had yet evolved the prehensile tail of modern American porcupines... and, of course, we can't prove that it had spines, either, although it seems likely.

Primates had not been in South America as long as rodents, but they were already well established, and most of the monkey species of the time can be placed in living groups. The most complete South American fossil monkey of the period is Cebupithecia from the Middle Miocene of Colombia, around 12 million years ago. It is clearly identifiable as a saki monkey (or, at least, a member of the same subfamily) and already looked similar to modern species. We can thus say that it likely had a similar lifestyle, living in trees and feeding on hard seeds and large, fleshy fruit. It was probably highly agile, but, like living saki monkeys, did not have a prehensile tail. The tail was, however, more muscular than that of modern sakis, so, unlike them, it may at least have used it to brace itself while holding on to branches with its feet.

However similar these animals may have been to modern kinds, it remains the case that the more visible animals of the time would have been a different matter. South America had no cloven-footed animals, and none of the large herbivores related to horses, rhinos, elephants and so on that we see elsewhere in the world at the time. But other hoofed mammals had arisen to take their place, and these odd animals would have been very visible members of the Early and Middle Miocene fauna of the continent.

There were three main groups of large hoofed herbivore living in South America during the Miocene, two of which did not finally die out until some time after the continent collided with its northern partner. Among these latter two were the litopterns, which are thought to be more closely related to the group containing the horses and rhinos than to anything else alive today... but that's not really saying much, so distant was their origin.

One of the larger litopterns of the Early Miocene was Theosodon, which was about two metres (6 feet) in length and had long legs and an extended neck that would have made it resemble an oversized and muscular llama. Unlike llamas, however, it had three hooves on each of its feet - a common feature in litopterns (as, of course, it was in horses at the time). It is known from at least Chile and Bolivia, and survived well into the Middle Miocene.

The long neck and sharp teeth at the front of the jaw may indicate that it clipped twigs and foliage from shrubs and the lower branches of trees. A most unusual feature is the fact that its nostrils pointed more or less straight up from the middle of the snout, much further back than we'd expect them to be. This would have made it look strange in life, and it's unclear why it evolved; it has been suggested that it might have helped the animal avoid stabbing itself in the nose when eating prickly vegetation, but that's really only a guess.

A second group of litopterns known as the prototheres took the role of antelope and deer elsewhere. They were smaller and more slender than Theosodon and its relatives and many of them were probably fast-running. However, the group was varied and Megadolodus from Middle Miocene Colombia, for example, was instead about the size and shape of a wild boar, albeit with a smaller un-piglike head. In fact, it was so unlike other prototheres that for a long time it was assumed to be something else entirely, a late survivor of a much earlier group - until we found the leg bones and confirmed that it had three toes in the standard litoptern pattern. Its broad teeth suggest that it ate hard food of some kind, rather than the browse favoured by other prototheres.

In some respects, Thoatherium was more typical of Early Miocene prototheres, in that had a similar browsing diet and a slender build, probably looking somewhat like a small deer without any antlers. But there is one real oddity about it, that's particularly surprising given how early it lived - it had only one toe on each foot. In fact, the cannon bones in the legs were even smaller, proportionately speaking, than those of modern horses, arguably placing it even further along that particular evolutionary pathway.

It's doubly strange, because Thoatherium lived in what were then the Patagonian forests, and not in the grasslands we would associate with horses. Indeed, its teeth would have been ill-suited to grass, and it presumably lived like a deer. Running in open grassy plains is given as the usual explanation as to why horses lost their side toes, but whatever Thoatherium wanted horse-like feet for, it almost certainly wasn't that.

But the litopterns were certainly not alone in Miocene South America, and next time I'll be looking at the other hoofed mammals of the day, some of which were arguably even stranger...

[Drawing by Charles R. Knight, in the public domain.]