Sunday 19 September 2021

Before the Monkeys: Primates in North America

an omomyid from Wyoming

The primates are one of the larger groups of mammals, with literally hundreds of living species known. Naturally, there is a great deal of diversity across the primates but, at the highest level, we can divide them into two main suborders. These used to be referred to as the "lower" and "higher" primates, but that's misleading because the so-called lower primates have been evolving for just as long as the higher ones. It isn't as if they gave up one day and stopped evolving towards some ultimate goal of becoming human. 

So, instead, when we aren't using the technical terms strepsirrhine and haplorrhine, today we tend to call them "wet-nosed" and "dry-nosed" primates, based on whether their nose is moist like that of a dog or not. When most people think of primates, however, it's likely the haplorrhine, or "dry-nosed" primates that they think of first, since this is the group that includes all the monkeys and apes - as opposed to lemurs and the like.

Whether or not this split goes back to the very beginning of the primate fossil record rather depends on what we consider a "primate" to be when we're describing early fossils. That's because of the existence of a group of early mammals called the plesiadapiforms that are, undoubtedly, closely related to primates. While it's generally agreed that, as currently defined, they actually represent more than one lineage some studies have concluded that they are (or at least include) essentially very early primates, while others have concluded that they are merely close relatives.

The way around this problem, at least until it's all cleared up, is to instead define primates by reference to the crown group. This term refers to the last common ancestor of all living members of a particular group and all of that animal's descendants. So, in the case of the primates, we want the last common ancestor of lemurs and monkeys and everything descended from that creature. Since plesiadapiforms probably don't fit that description (they likely either include that creature's own ancestor or are cousins of some sort) this neatly removes them from the picture.

And, when we look solely at the crown group - technically referred to as the euprimates - we find that the split between the two suborders does indeed stretch as far back in the fossil record as we can go. 

And how far is that? Well, the oldest known euprimate fossils date from around 55 million years ago, a time of significant climatic change that was important in the evolution of a number of other groups, too. Even at that point, presumably only a very short time after that ultimate ancestor of the crown group lived, there were already two different groups of primate in existence. One of them, the adapiforms, appear to be early strepsirrhines, while the other appears to be haplorrhine.

This second group are the omomyids. Like the adapiforms, they lacked many of the features we would associate with modern members of the group - for one thing, it's possible that they still had the "wet nose" found in related animals, although obviously, this is hard to prove. Partly as a result of these differences, we can't consider omomyids to be monkeys (or, more precisely, simians, since we're including the apes as a sub-type of monkey for these purposes), the animals that we'd mostly associate haplorrhines with today. In fact, the first monkeys wouldn't appear until around 15 million years later than the first omomyids.

Instead, the first omomyids probably had some resemblance to living tarsiers - small nocturnal haplorrhine primates with large eyes and long tails found in Southeast Asia. Like tarsiers, many omomyids had large eyes and short snouts, although not to the dramatic extent seen in the living animals. This may mean that omomyids were particularly closely related to tarsiers, rather than to simians and simply became more exaggerated in appearance over time, but it has also been suggested that the common ancestor of the simians and tarsiers was nocturnal and that monkeys lost their large night-vision eyes as they shifted towards daytime living. Analysis of their leg bones suggests they mostly walked on all fours, but that they had a particular ability for leaping. There are even some hints from their finger bones that they already had primate-like nails, rather than claws.

While later species were larger (the omomyids didn't die out until 36 million years ago, some time after the first monkeys appeared), the early ones were even smaller than tarsiers. The oldest known example, Teilhardina, is estimated to have weighed just 48 grams (1.7 oz.) - about twice the weight of a house mouse. Perhaps more surprisingly, though, Teilhardina was remarkably widespread.

At least six species have been assigned to the genus, although at least some of them might be different enough to be given a genus of their own. Regardless of that, animals that are, at the very least, very similar appeared more or less simultaneously in Asia, Europe, and North America. This, of course, makes it difficult to say where haplorrhines first evolved, since the oldest known examples already lived on three continents, as did the plesiadapids from which they may have evolved. Although we don't yet have a definitive answer to this question, the best guess seems to be that true primates first appeared in Asia, and headed westward to what was then the European archipelago, before crossing the Atlantic to North America via Greenland (a much easier route then than it would be now).

There are no primates native to North America today, at least not north of southern Mexico and the Caribbean, unless one counts humans. But, back in the Eocene epoch, there seem to have been a number, with Teilhardinia merely the first. Most of the known examples come from the western parts of the continent, which were largely subtropical at the time. That's partly due to a lack of fossils of the right age from further east, and a rare fossil from Mississippi implies that they likely lived for some time along the Gulf of Mexico before heading towards the Rockies.

On the other hand, since one can't get directly to Mississippi from Greenland, it makes sense that they must once have lived elsewhere on the Atlantic coast. Earlier this year, a paper announced the first discovery of a fossil primate from this region, specifically from a site near Fredericksburg, Virginia just south of the Potomac River. Fossils at the site are mostly those of fish, but land and shore birds are also known, along with some snakes, so it must have been at least close to the land when it formed. Mammal fossils are rarer still, but in this case include part of a jawbone with a few attached teeth.

There aren't enough of the right kind of teeth in the fossil to be able to say exactly what it was, although the researchers argue that it's unlikely to be any known species. Nonetheless, it does appear to be a primitive primate and the early age (54.5 million years ago) and very small size imply that it was a close relative of Teilhardinia, even if we can't be certain of how close.

These early primates lived in North America for millions of years. In the Late Eocene, however, 38 million years ago, the climate cooled and dried out, the dense subtropical forests of much of the continent being replaced with more open woodland. Within another three million years, the last descendants of Teilhardina on the continent had died out. 

It was not the last pre-human primate in the present-day US, however, since an animal named Ekgmowecheshala (which one can only imagine is easier to pronounce if you're a native speaker of Sioux than it is for the rest of us) is known from South Dakota and Oregon. This appears to be an adapiform primate - and thus, a strepsirrhine not directly related to Teilhardina and its kin - and presumably crossed over from Asia by the more direct route, over the Bering land bridge. By the end of the Oligocene, 26 million years ago, it too had died out, leaving the region devoid of primates.

Until, of course, we arrived.

[Drawing by Ambrosius Hubrecht, in the public domain.]


  1. Question: Surely the monkeys of Mexico count as part of North America? Also, the Caribbean species? That raises the question as to why they did not head further north - as far as I know there were never any South American primates adapted to open country/semiterrestrial life as many Old World species are, so perhaps they were constrained by a lack of forest?

    1. That's a fair point! I'll amend my terminology.

  2. I don't understand the comment about the Fredericksburg site having to be partly above water. Land animals can fall into water - indeed it generally helps their chances at fossilization.