Sunday, 4 September 2016

Ancient Otters of Idaho

The common river otters of Eurasia and North America are remarkably similar animals. If you placed them side by side you might notice that the Americans ones are (on average) slightly larger, and have a slightly more bland colour to their coat... but, really, it's not easy. I, for one, wouldn't claim to be able to tell which species a particular photograph was of without additional evidence (most obviously, where it was taken).

It was therefore assumed for a long time, not unreasonably, that they had to be very close relatives. As early as 1843, they had been placed in separate genera, as Lutra lutra and Lontra canadensis, respectively. However, it's worth noting that this is 16 years before the publication of On the Origin of Species, so, when he did so, it's isn't obvious that John Edward Gray was necessarily thinking about how "related" they might be. Indeed, for much of the 20th century it was common to ignore Gray and place all of the fairly typical looking clawed otters together into the genus Lutra.

The first modern suggestion that "well, actually, maybe not" seems to have come in 1972, when a survey of otters across both American continents suggested that Lontra should be resurrected. As it became possible to analyse the genetics and molecular biology of these animals in more detail, starting around the late '80s, it turned out that this new idea was correct; animals like sea otters and the various kinds of clawless otter are more closely related to the clawed river otters of Eurasia than they are to those of the Americas. While they are still clearly "otters", they nonetheless represent two separate evolutionary lines within the wider grouping of otter species.

This, however, raises the interesting question of how the two groups of species separated in the first place. The general opinion, for much of the 20th century, was that the ancestors of the North American otter had snuck across the Bering Land Bridge some time during the Ice Ages. Over the next million years or so, they had spread south, giving rise to the various other species of otter found from northern Mexico to southern Argentina.

This made a lot of sense. These are, after all, mostly freshwater animals, so it's unlikely that they'd swim all the way across the Bering Straits. Being so closely related, whatever happened must have happened recently, so the last time that the Land Bridge was open, when sea levels were lower during the Ice Ages, was the most logical time for it to have happened. In fact, we even had a pretty good candidate for the culprit: Lutra licenti, a fossil otter living in eastern Asia at around the right time.

The oldest fossils definitively assigned to the genus Lontra date from no more than 1.8 million years ago, and are already indistinguishable from modern North American river otters. Further south, the oldest fossils of the Neotropical otter (Lontra longicaudis) date back to, at most, 1.2 million years ago. There are no significant fossil remains of the other two South American species of Lontra, nor, for that matter, of any "missing" species of American river otter that have since gone extinct.

Everything fit the picture, so, sure, why not?

Well, the reason why not was, of course, that this no longer made sense once we'd realised that the two types of otter were not that closely related after all. Indeed, our best guess, based on the degree of genetic divergence between the two groups, was that they had parted company no later than 2.8 million years ago, and probably quite a bit earlier than that. This is well before the Ice Ages, and, crucially, before Lutra licenti began to swim in the rivers of China.

In a way, this isn't a great problem. We certainly know of fossil otters in Asia quite a lot older than this, and there would have been earlier opportunities for them to cross over to America. Heck, we even know that they did, because the giant otter of Brazil (Pteronura brasiliensis) is not a direct relative of the common river otters, and it had long been agreed that that, at least, had descended from some much earlier migrant. Presumably, then, at some point, the ancestors of giant otters and the ancestors of regular American river otters had lived alongside one another in Pliocene North America. It' not 100% certain (they could have diverged in Asia before they crossed over), and they might not have inhabited the same bits of North American even if they did, but it seems reasonable.

But the fossil record is, by its nature, incomplete, and while we could tell that the missing piece of the jigsaw was around somewhere, we didn't happen to have it.

Except now, we do.

The new fossils were uncovered at two localities within the Hagerman Fossil Beds of southern Idaho, dated to the Pliocene around 3.8 million years ago. One of the two sites had previously revealed fossils of beavers, swans, and diving grebes, so it's a fair bet that it was once a freshwater habitat, and likely with moderately deep water at that. The other has supplied fossils of more obviously terrestrial animals, such as llamas and peccaries, but with at least some previous semi-aquatic fossil species too (such as muskrats), so perhaps it had smaller rivers running across it.

We only have the lower jaw and one of the arm bones, so we don't know the exact size of the animal in life, but it appears to have been fairly small - the authors estimate it to have been about the size of a marine otter (Lontra felina), the second-smallest species of otter alive today. The bones in question also appear to belong to an adult, so that's probably about as big as it got. From what we can tell, it was very similar to the common river otters of today, and, given how little living otter species vary in colouration and so forth, it would probably have been hard to distinguish in life, too. Sufficiently so, in fact, that if we didn't know it was North American we'd have been hard pressed to say which of Lontra and Lutra it should have been placed in.

But, North American it is, and it has been assigned the new name Lontra weiri; the second half of the name being a pun based on the word "weir" and the fact that the researcher in question was a big fan of the Grateful Dead. It represents, not only the oldest fossil assigned to the genus Lontra to date, but the only such species known to have gone extinct - as noted above, every other fossil belongs to species that still survive today. Being older than any of its close relatives, it could, in fact, be the ancestor of the four living species, although we obviously can't know that for sure.

What's also interesting, though, is that this was not the first fossil otter to be uncovered from these particular sites. Fossils of the extinct otter Satherium piscinarium have also been found there, indicating that two different kinds of otter inhabited, more or less, the same river valleys together. Satherium is a huge otter, almost the size of the modern giant otters of Brazil, so there's no way you would have confused the two had you been able to see them in real life. Indeed, Satherium is often thought to be the ancestor of the living giant otter, although it died out 2 million years before the first record we have of the modern species, so it's hard to be certain.

The shape of their teeth supports that, as we'd reasonable expect for otters anyway, both of these species would have primarily eaten fish. That they lived together, however, suggests that there was likely some difference in their diets, and one could well imagine that the giant Satherium ate large fish, leaving plenty of smaller ones for L. weiri to feed on. This, after all, is broadly how giant and Neotropical otters manage to live alongside one another in Brazil today.

We already knew that otters had a long history in the Americas. Enhydritherium, which resembled the living sea otter, the primitive, perhaps somewhat terrestrial Mionictis, and Limnonyx, which has remains too fragmentary to tell us anything much, are all considerably older than Satherium, and must represent very early arrivals. But this new species is the first that we can directly link to to the otters that inhabit North America today, and at one point it shared, not just the continent, but the very same rivers, with a giant species that has long since vanished.

[Photo by U.S. Fish and Wildlife Service Southeast Region, in the public domain.]

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