Not the Pig Family: Fossil Peccaries and More

Mylohyus

Today, there are only three recognised living species of peccary, the smallish pig-like animals that inhabit the Americas... and one of those is endangered. However, these are but the last remnants of a once much larger group with a fossil history that stretches back even further than that of the true pigs.

While pigs date back, at the best, to the end of the Oligocene epoch, the oldest known peccary fossil dates from the end of the epoch before that, the Eocene. This implies that the ancestors of the peccaries entered North America from Asia between 36 and 34 million years ago. The fossil in question belongs to a species known as Perchoerus minor, and it's also worth noting that it is also the smallest peccary known... in fact, it was about the size of a typical house cat.

Over the next few million years, this tiny peccary gave rise to a number of other species on the North American continent. Many of these are similar enough to be placed in the same genus Perchoerus, although they do grow larger over time, with the last species, P. probus, not far off the size of the living peccaries. By the time we reach the late Oligocene, the most common form is Thinohyus, which was similar in size, but had a narrower skull with teeth better suited for grinding up plant matter than its primitive relative.

During the following, Miocene, epoch, the peccaries underwent two rapid bursts of evolution, each producing a number of new forms. The first saw the appearance of a group of peccaries often placed within their own subfamily, the hesperhyine (literally 'western hog') peccaries. These first arose shortly before the dawn of the Miocene, but produced a number of new species through the early part of the epoch. One of the most notable is perhaps Floridachoerus, which lived around 20 million years ago in eastern Texas and northern Florida (southern Florida was underwater at the time, the sea level being higher in those days). The group is named for one of the last forms, Hesperhys, lived in Nebraska and Montana, and died out around 15 million years ago, during the Middle Miocene.

The second burst of evolution resulted in an array of really rather odd-looking animals, best represented by Skinnerhyus and the closely related Woodburnehyus. These were distinguished by truly remarkable cheekbones, which projected sideways out of their heads in an almost wing-like bony bar. The exact shape of these strange projections varied considerably between species and genera, and may well have served to identify different species to one another for mating purposes. They are much more noticeable in males than females, and were plausibly used while fighting for mates - if nothing else, they would probably have helped to protect the animal's eyes from the slashing tusks of their competitors.

These odd-looking animals died out at the end of the Miocene, and were replaced by three main groups. One of these descended from a group of peccaries that had crossed to South America not long after the Panama land bridge opened up, and these are likely the ancestors of the living forms on that continent. Another were the flat-headed peccaries such as Platygonus, which adapted to a more herbivorous diet than their ancestors, although they seem to have been capable of eating a wide range of foods.

They were most common on the grassy plains of the US and Canada, but some species assigned to the genus lived as far south as Uruguay and Argentina. While mainly known from the Pliocene and Pleistocene epochs, they seem to date back a little further than that, with the most primitive species dating back to the tail end of the Miocene. The males had larger tusks and cheekbones than the females, again suggesting some degree of violent competition for females, features that have been lost in the living species. Fossilised hoofprints that may belong to this animal have even been discovered in Mexico, dating back over 2 million years.

The third group, the long-nosed peccaries, was represented by Mylohyus. These lived in more woody environments, and had a more omnivorous diet, closer to that of the living species than their flat-headed kin. They date back to the early Pliocene, but are best known from Ice Age (Pleistocene) deposits across much of North America. They are named, as one might expect, from the elongated shape of their snouts, formed by the growth of a long toothless gap between their tusks and their cheek teeth. This, together with a relatively flexible neck and high shoulders, may have made it easier for them to snip leaves from the lower branches of trees. It was also larger than any living species of peccary, roughly the size of a female wild boar or razorback.

Both the long-nosed and flat-headed peccaries survived right through the Ice Ages, and even a little bit beyond. This means that they would have lived alongside humans, perhaps for a few thousand years, before dying out, perhaps due to hunting, perhaps due to climate change, around 9,000 BC or so.

While peccaries are not true pigs, they are the closest living relatives of that family, explaining their superficial physical similarity. Together, the two families form a larger group, or 'superfamily': the Suoidea, or "pig-like animals". But, especially if the peccary family is much older than the pig family, we have to ask the question: were there once other suoid families that no longer survive?

The answer, perhaps unsurprisingly, is 'yes'.

Over the years, our view of exactly which families these might be has changed significantly. Indeed, until relatively recently, it was considered that hippos were suoid animals, based on a number of physical similarities. We now know that they belong to a distinct, although admittedly rather close, lineage, that includes a number of other living animals that are manifestly not pig-like. A number of prehistoric animals with superficial similarities to pigs have also been included in the group in the past, but today there are just two extinct families commonly included in the group.

Of the two, the more obscure are the sanitheres. These were relatively small pig-like animals that first appeared in Africa during the Early Miocene, round about the same time that the first true pigs appeared in Asia. During the Middle Miocene, they briefly expanded into Europe and southern Asia, but they did not last long there, and the last species died out in Africa around 14 million years ago.

The rear part of the sanithere skulls do look rather pig-like, but their faces and jaws do not, aside from the presence of large tusks on the males. being closer to what we'd expect of other cloven-hoofed animals such as deer or antelope (although only slightly - for a start, they didn't have horns). Their teeth show a complex pattern, suitable for chewing up vegetation, rather than more general omnivory, and, from the sorts of deposits in which they are found, it appears that they lived in marshlands or along lake shores, perhaps feeding on water plants.

Their position within the family tree of pig-like animals has been controversial, including early suggestions that they might be related to hippos, which obviously have a similar diet and habitat preference. Today, however, the consensus seems to be that they suoids, and are closer to the pigs than to the peccaries, having diverged before the latter headed to North America.

Much better known, and with a longer history, the fourth family were the palaeochoeres. Because, unlike those of pigs, the tusks in their upper jaws pointed downwards, these were for a long time considered to be members of the peccary family. The big difference was they lived, not in America, but in Europe and Asia, and so they were commonly referred to as "Old World peccaries", thought to represent a larger group, of which only a few American immigrants survived into the present day.

It's now clear that they are neither pigs nor peccaries, although, depending on your definition of the group, the odds are good that both of those families (and, indeed, the sanitheres) are ultimately descended from them, making them the original suoids. The majority of them, however, are known from the Oligocene, making them older than the oldest known pigs, but not quite as old as the first peccaries. These include animals such as Palaeochoerus itself, from which the group takes its name. They lived across Asia and Europe, and lived alongside the true pigs for a long time, only dying out around 9 million years ago.

It is only in the last few decades that we have begun to uncover suoid fossils much older than this, including Egatochoerus and Siamochoerus from Late Eocene Thailand. These have a number of similarities, in the form of the teeth and feet, to the later palaeochoeres, and are often included with them, although some researchers consider them too primitive to be meaningfully placed in any family, preferring to consider them as part of a more general ancestral stock. On the other hand, they are also very similar to Perchoerus, the earliest known peccary, and might well include its ancestor.

These were small animals, without the adaptations for efficient rooting seen in virtually all later suoids. The oldest currently known is probably Huaxiachoerus from China, which dates back a little over 37 million years. This suggests the ultimate origin of the suoids as taking place in eastern Asia during the Mid to Late Eocene, with quite a rapid distribution to the New World not long after.

It's likely that these very early forms were already omnivorous, adapted to a generalist lifestyle that enabled them to survive and adapt as the climate slowly changed around them, never relying too much on any one foodstuff. It's likely this that was key to their unusually long-lasting success.

[Photo by "Reynosa Blogs", from Wikimedia Commons.]