Oligocene (Pt 16): The First Monkeys in South America

A modern South American monkey

Much like the rodents, the presence of monkeys in South America has long been a puzzle. We know that monkeys evolved in Africa and that the monkeys still living in the Old World share a common ancestor distinct from, but related to, the common ancestor of the American sort. Genetic evidence shows that the split between the two lineages, which must have happened in Africa, happened a very long time ago. At some point then, early monkeys from what we now call the 'New World' group must have crossed the Atlantic, likely rafting on a floating patch of vegetation.

The Atlantic was narrower then than it was now, and ocean currents were different, but it's still a remarkable feat. It may also have been a lucky escape, since the African relatives of this first American migrant died out not long after, perhaps outcompeted by the ancestors of today's langurs, baboons, macaques, and apes. In South America, however, its descendants got almost free rein, diversifying into the five families we have today.

The oldest South American monkey known from more than a few teeth is about the same age as the oldest rodent fossil on the continent, so the two groups may have come across at around the same time. This is Branisella, which lived in Bolivia around 26 million years ago, towards the end of the Oligocene. The presence of those older tooth-only fossils indicates that it can't have been the first monkey on the continent, with the initial arrival perhaps having happened at a time of intense flooding in the Late Eocene, but it's still very early. 

Perhaps unsurprisingly given its age, it's not clear which, if any, of the five families of New World monkey it belonged to. It was a relatively small monkey by modern standards, about the size of a lion tamarin and probably weighing around 700 g (1½ lbs). Analysis of early fossils, however, has shown that the very first New World monkeys were smaller still, perhaps weighing only 400g (14 oz.), which probably helped when they were crossing the Atlantic. The shape of the teeth, which bear some striking resemblances to those of Old World monkeys of the time, suggest that it primarily ate fruit, but that there was a fair amount of grit in its diet, wearing them down quickly. This may suggest that it lived on the ground, rather than in trees, with the patas monkeys of North Africa being one possible analogy.

For several decades after its initial description in 1969, Branisella remained the oldest known fossil monkey in the Americas. In the last decade or so, other contenders have been discovered, including Canaanimico, a much larger monkey, of similarly unclear family affiliations, from eastern Peru and Ashaninkacebus from Brazil, which appears even more primitive. 

If the primates were recent arrivals to South America, the marsupials had been there much longer. Strictly speaking, there are very few marsupials known from Oligocene South America, since the term is technically reserved only for those animals descended from the last common ancestor of all living forms. There are a few, such as Clenia, which is thought to be related to the modern monito del monte, but they are outnumbered by species that don't quite match the strict definition.

The polydolopimorphs may, or may not, fall into that category. Some analyses find them to be true marsupials, albeit an early branch within the group, but most modern descriptions tend to consider them close relatives, or "marsupialiforms". They were small animals, with the very largest being around the size of a guinea pig at 95 g (3 ½ oz.) and probably fed mostly on seeds. While they had been reasonably common in the preceding epoch, the worldwide drop in temperatures that heralded the dawn of the Oligocene killed most of them off, with the last example, Hondonadia, dying out no later than 29 million years ago.

Living slightly earlier, and likely surviving from the end of the Eocene, Klohnia is an unusual polydolopimorph in that its teeth resembled those of rodents, with enamel on the front surface only. This meant that the back of the teeth wore down, creating a bevelled edge ideal for gnawing on seeds, nuts, or similar hard vegetable matter. It was also unusually small for a South American marsupial, being shrew-sized and weighing only around 40g (1½ oz.)

The sparassodonts were once thought to be true marsupials, but they are now known to be another example of "marsupialiforms", albeit one of the earlier branching members of this group. The sparassodonts were carnivorous animals, and among the top predators of South America during their day, along with such things as crocodiles and terror birds. They are, perhaps, best known for the large, sabre-toothed forms of later epochs, but those had not yet evolved during the Oligocene.

The borhyaenids were another sparassodont family better known from the subsequent, Miocene, epoch, but in their case, we know of at least one example from the Oligocene. Australohyaena is thought to have weighed around 70 kg (150 lbs), placing it in the size range of a jaguar. Known from southern Argentina, it would have had a stocky, powerful build, with a short snout, powerful jaws, and large canine teeth. All of these features indicate a pure carnivore, much like big cats, and there is some hint that it might have been able to crack bone, hence the "hyena" part of the name. ("Bor-", in this context, means "flesh"). Being related to marsupials, it seems plausible to suggest that the females carried their young in pouches, although this is obviously difficult to prove when all you have are incomplete skeletons.

Impressive though those would doubtless have been, they were not the largest mammalian predators of their day. That honour goes to the members of a different family, the proborhyaenids. This was, as its name implies, older than the borhyaenids proper, but modern analysis shows that they are not their ancestors, but a separate group that lived alongside them. Significantly, however, they may include the ancestors of the sabre-toothed thylacosmilids of later times and, if not, they were certainly close relatives.

The proborhyaenids were possibly the largest marsupialiform carnivores ever to have lived. Although some were smaller, Paraborhyaena and Proborhyaena, known from Bolivia and Argentina, respectively, were especially large. While earlier estimates were larger, the current best bet puts them at up to 200 kg (440 lbs), similar to a modern black bear. The skull of Proborhyaena was itself 60 cm (2 feet) in length, which suggests that you really wouldn't want to have been bitten by it.

And, unlike black bears, they are thought to have been purely carnivorous. They were powerfully built with a hyena-like head and large canine teeth already beginning to shift towards the true sabre-shape of their later relatives. Unusually, these teeth seem to have had only thin enamel and do not look especially sharp, but this may have been compensated for by the fact that, unlike the canine teeth of most other mammals, they grew continuously throughout life, so that they could never wear away.

The same cannot be said for the teeth at the back of their mouth, which were sharp and clearly evolved for slicing flesh. These did wear out, but while placental carnivorans only have one pair of these 'carnassial' teeth, the sparassodonts had three, and, in the proborhyaenids at least, these appear to have been used sequentially throughout life, the ones to the rear replacing those further forward as the animal aged and they wore down. The one saving grace may have been that, given their size and build, they probably weren't very fast.. but then neither were the animals such as pyrotheres that they likely fed on.

So far as we know, the proborhyaenids died out at the end of the Oligocene, ceding the continent to smaller, if possibly more efficient predators. Marsupials are, of course, still present in South America today, if nowhere near as deadly, even if it isn't the continent we most associate with those animals. But, speaking of which, it's finally time to cross the oceans to Australia, to see what was living there 34 to 23 million years ago...

[Photo by Горбунова М.С., from Wikimedia Commons.]