Oligocene (Pt 9): Rise of the Dogs

Sunkahetanka

Many of the carnivorous mammals present in North America during the Oligocene were of types also found in Eurasia. However, this far back in time, they did not necessarily belong to any family of animals we would recognise. Cats, for example, first appeared in Europe at the end of the epoch and did not reach the Americas until much later. Other groups, such as raccoons, simply didn't exist yet. 

But we do, for example, have Palaeogale, an animal also known from Europe that looked somewhat like a polecat, but was actually more closely related to cats and mongooses without, so far as we can tell, being either. Corumictis looked similar, but analysis of the skull has shown that it much closer to true mustelids. About the size of a modern weasel, it lived in Oregon at least 29 million years ago, towards the middle of the epoch. It may make it the first mustelid to reach North America from Eurasia, home to the very similar, and slightly older, Plesictis. It's far enough back that it might, however, belong to an early musteloid group rather than to the modern family (that is to say, it may be equally related to mustelids and raccoons, and thus, strictly speaking, neither). Oaxacagale is almost as old, and lived in what is now Mexico; it's probably another close relative but, it too, has so many primitive features that it's hard to place precisely.

A similar story applies to the bear-dogs. These are well-known from the subsequent, Miocene epoch, but, for a long time, the earliest known examples dated from the late Oligocene of Eurasia. However, in 2016, researchers took another look at an unusual carnivoran skull unearthed in Texas that dated back over 30 million years, to the earliest part of the Oligocene. (Or perhaps slightly earlier; the date of the bonebed it was found isn't entirely clear). It had originally been placed as so close to the base of the carnivoran family tree that it didn't really belong in any identifiable group, but the new analysis showed that it was probably an early bear-dog and gave it the new name Gustafsonia. 

Although known from only a single skull, further research confirmed the placement, showing that the shape of its brain matched those of later, known, beardogs. Still, the early confusion was understandable, and not just because of the dearth of material. Bear-dogs are neither bears nor dogs, but they have features of both, and most of their Miocene examples are, in fact, dog-shaped, but bear-sized animals. This was much smaller than those, far more slender, and likely better at running. They may be related to a group of roughly coyote-sized long-legged bear-dogs from the subsequent, Miocene, epoch but could also simply be very primitive examples of their family. It, and its possible relative Paradaphoenus, may have had a similar lifestyle to modern foxes.

The same issues do not apply to the nimravids, or false sabretooths, which, even so far back, were distinctive enough to be readily identifiable. These had a similar relationship top cats and mongooses that bear-dogs did to bears and dogs, representing an early branch in the evolution of the wider group to which both of the living forms belong. In the Oligocene, these would have been the only catlike animals in North America, where they had been present since the previous epoch.

Nimravus, the animal for which the family is named, lived in both North America and Eurasia throughout the Oligocene. It was about the size of a large bobcat, but with a slimmer body perhaps resembling the caracal of Africa and the Middle East. It was closely related to the uniquely American form Dinaelurus; both had the large sabre-like teeth that we associated with extinct forms of true cat, if not quite to the extent of the likes of Smilodon. 

Belonging to a distinct branch within the family, Hoplophoneus was much larger, about the size of a jaguar, and had proportionately larger teeth. The teeth also had a greater density of serrations along the cutting edge, making them more effective for cutting flesh; it would certainly have been more fearsome than either of its relatives and one of the top predators of its day.

One group of mammals that was unique to North America is much more familiar to us today. The dogs had first evolved on the continent towards the end of the previous epoch, and would not leave for many millions of years after it finished. Prior to the Oligocene, the dogs had been a relatively minor group, with only a few species, but during this epoch they began to truly diversify for the first time. 

The dog family is divided into three subfamilies. For much of their evolutionary history, the dominant subfamily were the borophagines, best known for including the huge and heavily built "bone-cracking" dogs. The oldest known fossils of this group date from around the dawn of the Oligocene, and look quite different from their more famous descendants. Archaeocyon seems to be the most primitive; it was about the size of a small fox but was likely even more omnivorous, eating a relatively small amount of meat. The small animals they did on were more likely pounced on than chased, in the manner of a wolf; they may also have been able to dig burrows in which to seek shelter from larger predators.

However, other borophagines are known that are at least as old, indicating that the group was already diversifying by this point and may trace further slightly further back in the past than we currently have evidence for. Rhizocyon was similar to Archaeocyon, with fossils known only from Oregon, while the latter was spread across much of the northern half of the Great Plains as well as the West Coast. This, and Oxetocyon from Nebraska and Dakota, are little known to the few fossil remains so far discovered. 

Otarocyon, which has been found from Dakota to Idaho, represents the oldest borophagine fossils, first appearing at the very dawn of the epoch, but is notably more specialised than Archaeocyon - again, implying an older fossil history we have yet to uncover. It's unusually small, similar in size to the smallest wild canid alive today, the fennec fox, probably weighing no more than 1 kg (just over 2 lbs). The unusual structure of the bones around the ear also resembles those of the fennec fox, so it may have had similarly acute hearing, enabling it to listen out for tasty insects - although there is, of course, no way to tell how large the non-bony parts of the ear were.

The borophagines declined at the end of the Miocene and died out altogether at the end of the following epoch, just before the Ice Ages began. The subfamily that replaced them, the modern canine dogs, do, however, have an equally long history. For most of their history, they were a minor group, overshadowed by the other two groups despite eventually developing a larger and more complex brain. In the Oligocene, only one genus is known to have existed. Leptocyon was another fox-sized animal, which would have looked superficially similar to its borophagine kin, but which had a narrower, longer snout that looks to have been better adapted for snapping at small, fast-moving prey. At least one species was even smaller than Otarocyon, and perhaps the smallest wild canid ever.

However, in the Oligocene, neither of these groups represented the dominant form of dog. That honour went instead to the third subfamily, which would dramatically decline when the epoch ended, dying out altogether in the Middle Miocene, apparently having become too specialised to cope with the changing climate. These were the hesperocyonines, thought to include the ancestors of the other two families although, by this time, already distinct and on their own (ultimately doomed) evolutionary trajectory.

Although they too, were initially fox-sized and at least partially omnivorous, the hesperocyonine dogs soon adopted a more exclusively meat-based diet. Perhaps the first of these animals to make that shift was Mesocyon. Appearing around 30 million years, this would have weighed something like 7 kg (15 lbs), similar to a red fox, and already had a diet that consisted primarily of meat. Over the course of the epoch, it was replaced by larger and more purely carnivorous forms, such as Sunkahetanka, before culminating in the 10 kg (22 lb) coyote-sized predator, Enhydrocyon. Despite the small size compared with modern wolves, this was powerfully built, with a short snout and attachment points for strong jaw muscles. 

Whether or not bears also lived in North America at the time is a matter of definition. By most definitions, true bears originated in Eurasia, and only began to reach the Americas around the end of the epoch, in the form of Ursavus. However, one group of carnivores unique to North America are sometimes also placed within the bear family.

They did not, however, look much like bears, at least as we could consider them today. For much of the 20th century, animals such as Nothocyon and Parictis were thought to be dogs or else were placed with even more primitive carnivorans near the base of the family tree. Today, we tend to assign them their own family, the Subparictidae, and describe them as "ursoids", animals closely related to bears but outside the bear family proper.

Certainly, if one could look at them in life, the relationship to modern bears would not have been obvious. For one thing, they were little larger than raccoons, and many were slender, lithe animals with long tails. The recently discovered Eoarctos is one such example, with limbs better suited for climbing trees than for running. About the size of a raccoon, and possibly looking rather similar when it was alive, its teeth show unusual adaptations for eating hard, crunchy food, suggesting that it might, perhaps, have had a diet heavy in shellfish.

Nonetheless, whether we place it in the same family or not, Eoarctos is thought to have been more closely related to bears than to anything else alive today. We see a similar pattern across the two northern continents, with creatures that are at least related to cats, dogs, weasels, and so on even if they belong to strange side-branches that didn't survive in the long run. But true carnivorans had not yet reached the other continents at all. Moreover, the predators that did live there at the time could not have fed on horses or cloven-footed beasts - both groups that were still unique to the north during the Oligocene.

Next time, I will turn to some of the herbivores that they could have fed on, at least on one of those continents. It's time to turn to Oligocene Africa...

[Picture by "ghedoghedo" from Wikimedia Commons.]