Sunday, 9 June 2019

Miocene (Pt 14): Sabre-toothed Sea Otters

Ysengrinia
When bears first entered North America is really more a matter of definition than of our understanding of the fossil evidence. Going by what is probably the most common current definition, bears first appeared in Europe about 20 million years ago, in the form of Ursavus elmensis. Noticeably smaller than modern bears, this developed into a number of species across Europe and Asia, and to the closely related Ballusia in China.

Almost immediately, however, Ursavus also crossed into North America, probably arriving around 19 million years ago. It seems to have been far less common there than it was in Europe, with the local species, Ursavus pawniensis (there were probably others, but that's the only one we know of for sure) living in relatively small populations across the west, from Nebraska to Oregon and Saskatchewan. It doesn't appear to have left any descendants, being eventually replaced by more modern species of bear from Asia rather than evolving into anything uniquely American.

However, at around the same time as Ursavus, another group of related animals also entered the continent. These were the hemicyonines, a name that literally means "half-dogs", but which are commonly known as dog-bears. These days they are usually considered to be part of the bear family, but whether one could actually call them "bears" is perhaps debatable. They would have looked much more like dogs than bears, with legs adapted for fast running and teeth suited to a more heavily meat-based diet than most modern bears (apart from polar bears, obviously).

The early dog-bear Cephalogale seems to have arrived around the same time as Ursavus. A survivor from the previous epoch, it did not long survive in Europe and Asia, but may have held on in North America for rather longer. It was joined shortly after by Phoberocyon, a particularly wolf-like animal that is also known from older deposits in Spain, but seems to have prospered rather better once it reached the Americas. It was replaced by, and probably evolved into, the more advanced Plithocyon, which prospered during the Middle Miocene across at least the western half of North America, from New Mexico to California and Wyoming.

Unlike the bears and dog-bears, however, the bear-dogs already had a long history in North America when the Miocene dawned. Despite the similarity of names, these animals were probably more related to the actual dogs - a group known to be native to the Americas - than they were to their more bear-like cousins. While later forms became so large and muscular that to modern humans, the resemblance to bears is hard to miss, the species of the Early Miocene were still fairly dog-like in form, if considerably more widespread than actual dogs were at the time.

The bear-dogs of Early Miocene North America were particularly primitive, with some having changed little in the preceding 10 million years. These animals, such as Daphoenus, were about the size of a coyote, but soon gave rise to Daphoenodon, which was wolf-sized or larger. Compared with the dog-bears of the day, it was more heavily built, and is unlikely to have been an effective pursuit predator as living wolves are. Instead, its toes were splayed out, leaving the foot partially flattened so that the sole was partially in contact with the ground. This pattern is somewhere between that of living dogs and bears, and may indicate that the animal jumped from cover with a rapid burst of speed, rather than chasing prey down over a long distance.

Daphoeonodon is known only from Nebraska, Wyoming, and New Mexico, where it was probably the largest predator of its day - the teeth seem to suggest a strongly meat-based diet. At least three species have been identified, and one of the smaller ones is known to have used burrows in the ground for shelter, since one such fossilised burrow contained the remains of a mother and her pups.

The flood of new immigrants from Asia that brought the first bears across 20 million years ago also brought new bear-dogs from Europe. Some, such as Cynelos, were, at least superficially, not that different from the indigenous species, and survived alongside them for a while. But others were larger, evolving into a shape that was notably heavier and more bear-like - although without the omnivorous habits of true bears.

While other Asian species followed, the first of these new arrivals was Ysengrinia, which was perhaps similar to a modern brown bear in size. Although the genus is also known from across Europe and Asia, the American species happens to have the best-preserved fossils, which are sufficient to show that, as in living bears, the males were larger than the females - and presumably competed for access to them. Judging from the admittedly small number of fossils so far found, it preferred to live close to rivers or other plentiful sources of water.

Far stranger, however, was a dog-sized animal known from the coasts of Oregon, Washington, and possibly Alaska. It's not entirely clear what Kolponomos actually was, in terms of how it relates to anything alive today. When it was first discovered in 1960 it was thought to be related to raccoons, but that specimen consisted of only a very small portion of the snout, and further remains rapidly disabused scientists of that notion.

Some classifications consider it to be a very early member of the bear family, but many prefer to dodge the question altogether and consider it so weird that it belongs off on its own. It had an odd down-turned snout, with the nostrils unusually high up on the head, and eyes directed forward for effective binocular vision. The teeth are odd, too, with long upper canines and unusually flat molars - reminiscent of nothing so much as a sabre-toothed sea otter. Since fossils have only ever been found in coastal sediments, these features strongly suggest that the animal was semi-aquatic, even though the few limb bones we have suggest that it would have been only a middling swimmer.

This would explain the shape of the teeth, with the canines being used to prise clams and other shellfish free from the rocks, much as a walrus does, and the molar teeth then crushing up the shells. Indeed, the shape of the skull suggests that the jaws were very strong, ideally suited for just this sort of thing, and the neck muscles were also very powerful. Swimming along in shallow coastal waters, it could have held its nostrils up while scanning the seabed with its forward-facing eyes.

In terms of its wider relationships, it is at least clear that the animal belonged to the broad group of bear-like animals. If it was an actual bear, however, it has to be just about the strangest that's ever lived. Instead, it is more likely to be a close relative of the first seals and sea-lions, although it lived just too late to be their ancestor. While genetic evidence suggests that these animals do belong in this part of the mammalian family tree, it's still not entirely clear whether they are more related to bears than to, say, otters, so this doesn't truly help to fix down Kolponomos' position, either.

Back on the land, however, there were other changes coming. One of the last animals to cross over from Asia to North America during this particular rising of the Bering Land Bridge was Pseudaelurus, a primitive cat, and probably the ancestor of all cats alive today. Its arrival 18 million years ago brought an end to the "Cat Gap", although such animals as bear-dogs would survive for several million years to come, well into the Late Miocene.

But the Late Miocene, as North America, and the world in general, transitioned from hot conditions to ones that were at least comparatively cooler, was a different time. The Bering Land Bridge would rise again, once more allowing animals to cross between Asia and America, and to do so in both directions. Next time, I will look at how some of these changes affected the herbivores of the continent...

[Picture by Óscar San−Isidro, available under the Creative Commons Attribution 2.0 license.]

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