Oligocene (Pt 13): The First Porcupines

Prosotherium

The dawn of the Oligocene is marked, in part, by the formation of a deep water channel between South America and Antarctica. To what extent there had been a true land bridge between the two continents in the preceding epoch is debatable, but the separation was certainly more complete than it had been previously, beginning a long period of isolation that allowed unique animals to evolve and flourish. Furthermore, most of the large mammal groups we are familiar with today originated on the northern continents, so much of the fauna of South America was already strange to modern eyes.

 

Much of it, but not all. The exact date of the oldest rodent fossil from South America has been disputed, but recent analysis suggests that it may have lived shortly after the beginning of the Oligocene. How it reached the continent is also unclear. Logically, it must have made a long sea journey, presumably clinging to some storm-tossed piece of debris, either from North America or Africa (the Atlantic being narrower then than it is now). The latter seems the more likely, since South American rodents are more closely related to the African sort than to their equivalent northern counterparts.

Once there, however, rodents began to rapidly diversify. The long isolation of South America that followed allowed them time to develop into an entirely new branch within the order, giving rise to all manner of unique rodents unknown elsewhere - even today, only one species within this group lives any further north than Central America. 

These are the 'caviomorph' rodents, including such animals as guinea pigs, degus, and pacaranas. Some of the families that existed in the Oligocene have since disappeared, such as the cephalomyids, which are probably (but not certainly) related to the modern chinchillas. The oldest known true chinchilla may be Incamys from Bolivia and Argentina, although quite where the dividing line between the family and earlier forms lies can be debated. Studies of the shape of the inside of its skull reveal that its brain already had some of the unique features associated with the group, and, significantly, that the areas related to hearing were enlarged - something that may suggest a need for auditory communication, and hence of some kind of communal living.

The only caviomorphs to have spread beyond the continent are the tree porcupines. Originating in South America during the Oligocene, these are thought to predate the ground porcupines of Africa, instead being an example of parallel evolution. Examples from the earlier half of the epoch include Eopululo and Shapajamys from Peru. These earliest known porcupines lived alongside animals that represent the earliest members of the agouti-guinea pig group and the earliest echimyid spiny rats, demonstrating that either the diversification of rodents in their new continent was very rapid or that even older fossils exist that we have yet to find.

In addition to diversifying, rodents also seem to have spread rapidly across the continent. The second-oldest fossil site in South America to produce rodent remains is from central Chile, some way south of the older Peruvian ones, and in what would have been much colder climes. Andemys is one of the better known rodents from this site, a rabbit-sized animal related either to agoutis or pacas - or, perhaps, to both equally.

But what of the unique South American mammals that these early rodents would have encountered? A high proportion of these would have been notoungulates. This group had already dwindled to a few specialised forms by the time South America collided with its northern counterpart, and the last died out altogether 11,000 years ago, but in the Oligocene, they were numerous and diverse - the largest order of mammals on the continent at the time, so far as we can tell.

Among the most common of the Oligocene notoungulates, at least in terms of the number of individual fossils, were the interatheres. These were likely common, at least in part, because they were small. One of the earliest examples is Santiagorothia from Chile, named for the palaeontologist who first coined the term "notoungulate". Like most members of its family, this has short limbs and a long body, giving it an appearance similar to a polecat. Similar in size to a particularly large domestic cat, it was actually a herbivore, with enlarged, clipping, incisors and cheek teeth suitable for grinding up tough plants. It probably lived in low-lying vegetation, and its body shape implies that it was likely agile and fast-moving.

Santiagorothia was, however, relatively unspecialised for an interathere, with later examples diversifying somewhat from the basic body plan. Protypotherium is an example of these later forms, first appearing in the Late Oligocene, but surviving well into the following epoch. It was about the size of a rabbit, and had the same slender body form that Santiagorothia had, but had four sharp claws on each foot, that may have enabled it to dig burrows; it was also better adapted for fast running than in the earlier form.

In some respects, the interatheres would have had a lifestyle similar to the newly arrived rodents and the same was also true of their close relatives, the hegetotheres. Prosotherium from the Late Oligocene of Argentina, was a member of this latter group. Like other hegetotheres, it had a pair of large and broad incisors at the front of the mouth, followed by a gap that had, at best, tiny vestigial teeth; this pattern would have looked strongly reminiscent of rodents, while the elongated hind limbs may have resembled those of rabbits, albeit without the similarly long feet. Significantly, it lived alongside other hegetotheres with a similar diet of tough plants, suggesting that there were plenty to go around, with each animal sticking to some particular niche whose details are now hard to distinguish. 

Another closely related group, the mesotheres, joined them towards the end of the epoch with their earliest representative, Trachytherus, being known from Bolivia. This was, unsurprisingly, a primitive member of its group, which would become more prominent in the later Miocene epoch, but it had already had the same superficially rodent-like teeth and digging claws that may have been used for burrowing or to get at tubers and edible roots.

While the hegetotheres would die out during the Pliocene, not long before the start of the Ice Ages, the mesotheres and interatheres would both survive contact with North America 2 million years ago, even if they failed to survive into modern times. The archaeohyracids were not so lucky, dying out at the end of the Oligocene around 24 million years ago. These were somewhat larger than their relatives of the day, with Pseudhyrax perhaps reaching the size of a large raccoon at around 8 kg (18 lbs) but they had the same high-crowned teeth suitable for eating tough or gritty food, as also seen in modern grazing animals.

As their name suggests, these animals resembled hyraxes, although they are not at all closely related (indeed, they were probably more closely related to us than to hyraxes, which is saying something...) Living in relatively open areas, they had a moderately long snout, but remains of the rest of their body are rare, making it difficult to say more about their lifestyle. However, their skulls and teeth are sufficiently common that it is thought they may have been social animals, living in small herds or the like.

The archaeohyracids are no longer thought to be a true biological family, lacking a unique common ancestor. Instead, they are an artificial group of similar-looking animals that probably include the ancestors of the hegetotheres. Nonetheless, they, and all the other smallish high-crowned grazing notoungulates do collectively form a single evolutionary branch within their order. Members of the other branch of the order were often larger, although, at this early date, not yet the giant rhino-like forms they would achieve in later epochs. It is to those animals that I will turn next time.

[Photo by Ryan Schwark, from Wikimedia Commons.]