Claws of the Bear-Dogs

Arctocyon primaevus

Over the four or so years that this blog has been running, I have covered a number of discoveries of or about fossil mammals. These have come from various epochs within, and beyond, the Age of Mammals, and so are a wide range of different ages. But I don't believe that I've ever previously discussed a fossil from the very first epoch of the Age of Mammals, the Paleocene.

This is partly because, being the oldest, it's not an epoch from which very many good quality fossils survive - at least, as compared with later epochs (as opposed to, say, comparably short periods during the Permian). On the other hand, though, it's obviously an important one, because this is the time during which mammals rose to inherit the Earth in the wake of extinction of the dinosaurs. The great diversity of mammals that we see today had much of its origin in the explosive radiation of new forms that first occurred during the Paleocene. Few modern groups are recognisable from this time, but we do see a number of immediate ancestors of those later groups.

One such group of animals are the "condylarths". This is a broad term for a range of animals related to, and, in some cases, probably ancestral to, today's hoofed animals, such as deer, antelope, and horses, and (perhaps more surprisingly) to whales and dolphins. Originally intended. when the term was coined in the late 19th century, to represent a closely related group, it's likely that they were rather more diverse, and they're often defined as much by features that they don't have, as by features that they do. The condylarths, broadly defined, only survived into the second epoch of the Age of Mammals, the Eocene, but they left many descendants - including a number of hoofed oddities that have themselves gone extinct in the million of years since.

Because they're probably not a single evolutionary entity, the classification of condylarths had changed considerably over the decades. However, one of the earliest families of condylarth that we still recognise today were the arctocyonids, a word that means something like "bear-dogs". As that name implies, they were originally assumed to be related to the ancestors of modern carnivorous mammals, specifically as an early kind of bear, until they were shunted sideways into a different group of carnivores in the 1930s, and ending up where they are now in the late 20th century.

Although we're still not entirely sure what should and shouldn't count as belonging to the group (in the sense of actual evolutionary relationships, rather than just happening to look a bit similar), modern schemes show around two dozen different genera, so that, as with many families of living animals, there was a fair bit of variety within it. Common features include the presence of large canine teeth (hence the initial confusion with carnivorans), simple premolars, crests on the skull that suggest strong jaw muscles, and relatively small brains, as well as rather a lot to do with the detailed structure of the wrists and ankles. They had five toes on each foot, each of which apparently had claws, rather than hooves.

The largest arctocyonid was also the first to be discovered, back in 1841: Arctocyon, for which the family is named. We now know that there were several species of Arctocyon, and the largest of those was A. mumak, which was about the size of a small bear. (And, yes, Tolkein fans, the name "mumak" is not a coincidence). As with so many Paleocene mammals, the remains of these animals are largely fragmentary, often little more than teeth and jaws, and, if you're lucky, a few bits of the skull. Certainly there are exceptions, including specimens belonging to to two of the smaller species of Arctocyon, but, when A. mumak was first described by Leigh Van Valen in 1978, it was on the basis of the left half of a lower jaw

However, it turns out that there was a skeleton available, even then - the only one known for the species that includes anything more than the jaw. This was unearthed in 1963, from Bighorn Basin in Wyoming, just a mile away from where Van Valen's jaw had been discovered. The fossil was between 56 and 60 million years old, and it hadn't been obvious at the time of its discovery what it actually was. Although that was rectified later, it languished in the Yale Peabody Museum for over fifty years before the first formal description was published last September. That description gives us new information about what this, the largest of the arctocyonids, was actually like.

Most of the focus of this analysis is on the proportions of the limbs. To take a simple example, an animal with long legs is likely to be good at running, compared with one that has shorter, stockier limbs. That Arctocyon mumak had proportions similar to those of a bear suggests that, while probably fast enough by human standards, it was unlikely to be a fast racer. There is, however. further evidence in support of this.

One of the hind feet and its associated ankle was well preserved, and the shape of the bones strongly suggests that the animal was "plantigrade" - that is, it walked on the soles of its feet, as humans and bears do. And, for example, fast-running creatures like dogs and deer do not. The backbone was also unusually sturdy and inflexible, with powerful muscles anchored on the pelvis, suggesting an animal for which strength was more important than flexibility.

All of this suggests an animal that spent much of its time on the ground, and that walked more than it ran. Having said that, bears are also not bad at climbing, and the feet of animals like pine martens are at least semi-plantigrade, so this isn't conclusive. Indeed, what we can tell of its toe bones suggests that it might have had large claws. (The claws, themselves, of course, aren't bone, and wouldn't have preserved). Such claws are the sort of thing you'd expect of an animal that climbs a lot, but there are other possibilities, and the authors suggest that, taken with other evidence, they might have been more suitable for digging in the soil than clinging to tree trunks.

It seems hard to imagine that an animal this size lived in burrows, but it could have dug in search of food, and, depending on the exact shape of the claws on the living animal, they might have been effective at tearing flesh instead. The animal's teeth might be some clue to the latter, especially with those large canines. Arctocyon does not appear to have been primarily a predator, and it lacks any of the flesh-slicing cheek teeth that predatory mammalian carnivores have today, but there's every reason to suppose that - relative of modern hoofed animals or not - it ate a fair amount of flesh as part of an omnivorous diet. You don't need canines that size to fight bushes, after all.

We know from a previous study that the close relative A. primaevus had a long tail that it could have used for balance. But this was a smaller, slimmer animal, and the authors of that study concluded that climbing trees was likely a key part of its behaviour. Assuming that both analyses are correct - and there's inevitably some guesswork and extrapolation involved - Arctocyon may have been a varied genus, with more than one favoured lifestyle among its various species. For example, while we classify them in separate genera, pine martens and wolverines are close relatives, even though they live quite differently. Perhaps, as with wolverines, A. mumak was an animal whose ancestors had recently given up a climbing life in favour of increasing size and life on the ground.

Or it could simply be something to do with the fact that we're looking at what is, after all, a fairly early stage in the diversification of placental mammals. A lot of specialisms we see in later animals just hadn't evolved yet. You don't need to be an expert at whatever life you lead, so long as you're better than anything else around at the time. One recent study concluded that the common ancestor of all placental mammals likely spent a lot of time in trees, so that it might not be too surprising if a number of early mammals had adaptations suitable for that, even if they were no longer strictly necessary.

At any rate, considering that this is one of the earliest members of the group that would eventually lead to antelope, rhinos, and dolphins... its relatives clearly still had a lot of evolving to do to reach their modern forms.

[Photo by "ghedoghedo", from Wikimedia Commons]