Oligocene (Pt 14): The Southern Horses That Weren't

Scarritia

30 million years ago, South America obviously lacked horses, since those (still three-toed at the time) originated in the north, and the southern continents had long been isolated from their homeland. What it did have, however, were a group of mammals called the notohippids, a name that literally translates as "southern horses". While they might not be closely related, the name might lead one to suspect that they had at least a resemblance to the modern animals.

They didn't.

Well, not much. When the name was originally coined, for the Miocene genus Notohippus, back in 1891, it was assumed that they really were horses, or at least closely related. This is because of the shape of their teeth which, did indeed resemble those of equines. It only took until 1914 to realise that, teeth aside, they weren't very horse-like. That their teeth were similar suggests a similar diet with plenty of tough vegetation, and their head was elongated in an almost horse-like fashion to accommodate them... but that's pretty much where the resemblance ends. For one thing, they had claws, not hooves; their bodies were also stockier, albeit with long limbs that may have given them a certain agility.

Strictly speaking, the notohippids are no longer regarded as a true family of animals. That's because they don't have a single common ancestor that they don't also share with other groups. Even so, the term is useful enough that it's still often used, at least until somebody can get their act together and come up with a classification scheme for the wider group that everyone else can agree on.

Bearing that rather significant caveat in mind, we can say that the notohippids died out in the Early Miocene, but that their heyday was during the Oligocene epoch. Early examples include Eomorphippus, from Chile and Argentina. This was a relatively primitive form, without the long face that many later forms and the remains we have suggest an animal not much larger than a dog (and, for some species, quite a small dog at that). Because of these and other primitive features, it's a prime candidate for not belonging to whatever family Notohippus remains in once the current confusion is sorted out.

In the second half of the epoch, the notohippids became more larger and more varied. Pascualihippus, from Bolivia, may have been a close relative of Eomorphippus since it had the same short snout suitable for grazing, although it was quite a bit larger. The best-known genus, however, is Rhynchippus, for which we have a few relatively complete fossils. This was about the size of a sheep, but had the long face of the more advanced "notohippids" and had only three claws on its front feet, not the four seen on the earlier species.

Although the horse-like teeth suggest that Rhynchippus was perfectly capable of feeding on youth vegetation such as grass and doubtless did so at times, the narrow snout implies that it may have preferred to nip leaves and fresh buds from bushes. It has also been suggested that the claws on its front feet may have helped it pull down branches from low-hanging trees to feed on them. The shape of its brain, as inferred from the inside of the skull, indicates that the snout may have been sensitive, as one might expect for a browsing animal, and that it had a good sense of hearing - as also indicated by the comparatively large size of the bony chambers around the inner ear. In proportion to the animal, the brain size was comparable with the cloven-footed animals of the time, if well short of a modern horse.

Closely related to the notohippids, the leontoniids were closer in size to a modern horse, and in many cases, quite a bit larger. While the name might suggest some parallel with lions, that's a coincidence, since Florentino Ameghino simply named them after his wife. Hopefully, this wasn't meant to imply any physical resemblance, since they were heavily built, with short snouts and probably looked rather like hornless rhinos. 

They had three hooves on each foot and were distinguished from their relatives by having tusks formed from their incisor teeth. These resembled the canine teeth of other animals, while the real canines were either very small or (in the more advanced forms) missing altogether. In many species, such as Leontinia itself, the tusks are much larger in some specimens that others, leading to the suggestion that these might represent males and females

Scarrittia, for example, is estimated to have weighed around 500 kg (1,100 lbs), in the same ballpark as a modern riding horse, but, with its short limbs and stocky build, it would have been quite a bit shorter. Like other leontoniids, their teeth did not have the adaptations seen in the notohippids, so they probably ate much softer food, browsing on trees, shrubs, and low-lying vegetation. It lived on the coastal plains of  Argentina and Uruguay.

The closely related Anayatherium lived further north and inland, in Bolivia. While most leontoniids were reasonably common in the places they are known to have lived, this was rarer, possibly because the drier habitat meant that not so many could live close together. Taubatherium, from Brazil, had no such problems, living in a comparatively lush tropical habitat that must have been better for a leaf-eating animal. It was slightly smaller and not so heavily built as its relatives in Bolivia and Patagonia, although perhaps at least the weight of a typical pony. 

CT scanning of the skulls of Argentinian genus Gualta shows a relative brain size similar to that of notohippids, but with a larger section of the brain devoted to the sense of smell.

All of these animals are notoungulates, members of the commonest and most diverse group of large mammalian herbivores on the continent prior to the collision with North America. In later times, a second order, the litopterns, would rise to come a close second but they were less significant during the Oligocene. Which is not to say that they weren't around.

Perhaps the best-known litoptern was the one-ton vaguely camel-like Macrauchenia of the Ice Ages. The family to which that belonged had its origins either in the Oligocene, or late in the previous epoch, depending more on where you choose to draw the line than on any dispute about the age of the relevant fossils. The Oligocene forms included the anagrammatical Cramauchenia of Argentina. This, and its related forms such as the almost equally anagrammatical Coniopternium and Pternoconius, may or may not form an early branch within the family but they are much smaller than their later relatives, being smaller than many antelopes. The most distinguishing feature from later forms, however, was the snout, which had the nostrils where you would reasonably expect them to be - on the end, rather than further back towards the forehead as they were on the likes of Macrauchenia.

The closest relatives of the macraucheniids were a group called the adianthids. Adianthus itself was one of the last forms living in the Early Miocene, but several others lived in the Oligocene, one of which keeps the anagram tradition alive by being called Thadanius. These were generally small and slender herbivorous animals, with the largest being no bigger than an Irish terrier or cocker spaniel and some being the size of a cat.

The proterotheres were a third group of litopterns and one that would prove equally successful in the long run, with the last dying out towards the end of the Ice Ages. Oligocene examples include Protheosodon, an unspecialised herbivore the size of a small goat and the even less specialised Lambdaconus, which seems to be a late survivor of an earlier branch of the family.

These were not, however, the only large herbivores on the continent at the time, because other, arguably stranger groups, still survived from the previous epoch. It is to these, as well as to slightly more familiar groups, that I will turn next time.

[Photo by "Ghedo" from Wikimedia Commons.]