Sunday 13 March 2022

Vishnu's Otter in Bavaria

Otters are members of the weasel family, something that has been recognised since the dawn of taxonomy in 1758, when Linnaeus placed them in what was then the weasel genus (taxonomic families not yet being a concept). Their adaptations to the water, including their powerful muscular tails, were long thought significant enough that they were separated out into a distinct subfamily, a position they still retain, although it turns out that they are more closely related to the weasels proper than to some other members of the family such as, say, pine martens or wolverines.

Perhaps because rivers are a good environment for forming fossil-bearing sites, we know of a significant number of fossil otters and, most of them would doubtless have been instantly recognisable as otters were we able to see them in the flesh, there is perhaps more variety amongst them than we might at first expect. For example, a number of them are quite large - in some cases, larger even than the "giant otter" (Pteronura brasiliensis) of today's Amazon, the largest living member of the weasel family. (So, yes, it's bigger than a wolverine).

Another feature of the largest fossil otters, besides their sheer size, is that their teeth don't seem suited for eating fish. Where most living otters have a full set of sharp teeth ideal for biting into the slippery critters, the molars of these large forms are instead bunodont. This means that they are relatively flat and heavily built; ideal for crushing something hard rather than tearing into flesh.

Not all bunodont fossil otters are necessarily gigantic, although they are all pretty big by the standards of modern species. It's unlikely that they form a single evolutionary group, and this type of tooth probably appeared more than once in the history of the subfamily. It's most likely that the bunodont dentition was an adaptation to eating shellfish such as lobsters and clams and there's an obvious advantage to shifting to this different, but widely available, diet when other species of otter living nearby are eating all the fish. So it's easily the sort of thing that could have happened multiple times.

This makes it difficult to know exactly where they fit within the larger otter subfamily. The living otters that they most closely resemble are the clawless otters (Aonyx spp.) which are themselves relatively large and do, in fact, have a partially bunodont dentition, which they use for eating crabs and other shellfish. However, our current best estimate is that the clawless otters first appeared less than 4 million years ago, during the Early Pliocene... which is quite a lot later than the large bunodont fossils. Sea otters (Enhydra lutris) are also bunodont, again because they eat a lot of shellfish, and are physically large, so they're another possibility. The dates fit better in their case, but their specialised, fully aquatic lifestyle could be a confounding factor.

One of the smaller bunodont otters, Vishnuonyx was first described in 1932, on the basis of an approximately 13-million-year-old fossil from Pakistan. It's also known from Thailand, where it would have lived alongside the much larger Siamogale, and, more surprisingly, from Kenya. Two much later fossils from Africa have also been assigned to the same genus, although whether they're really Vishnuonyx or just a close relative has been questioned, and one is very fragmentary. Nonetheless, if the latter fossil is what it's been described as being, then otters of this genus survived in Ethiopia until the start of the Pliocene, 5 million years ago.

Either way, the Thai fossils is marginally the oldest and also seems the least specialised, suggesting that this particular otter first appeared in Asia before spreading to Africa. That may seem a wide range for a single species, but the modern clawless otters are found on both continents and, while those are two different species, it's not obvious that they would be classified as such if all we had of them were a few bits of fossil skull. (Which, incidentally, means that although the name Vishnuonyx literally means "Vishnu-claw", we have no idea whether or not it really had claws or if its feet resembled those of its modern counterpart).

Last year, however, a new fossil was described from a completely different location: Bavaria in southern Germany. Specifically, it had been discovered at the Hammerschmeide site, a clay pit with deposits dated to 11 million years ago that have already returned a number of fossil animals, including over a hundred remains of waterbirds, as well as catfish and giant salamanders. Among mammals, perhaps the most notable discoveries were fossils of the early panda Kretzoiarctos and the ape Danuvius, which had leg and hip bones that suggest it may have been partially bipedal.

Although the new fossil consists only of part of the lower jaw, there is enough of it to confirm not only that it is Vishnuonyx, but for the researchers to identify it as a previously unknown species, V. neptuni. It seems to have been larger than the two or three previously named species, and would have been quite impressive for an otter today, although hardly gigantic. As to how the animal could have got to Europe, the researchers speculate that it may have been at least partially marine, allowing it to use what was then the Araks Strait, running through present-day Syria to connect the bodies of water that roughly correspond to the present-day Persian Gulf and Black Sea. If so, the same route could have been used to allow the otters to reach Africa, which was probably still an island continent at the time.

This would make sense if the animal really was related to sea otters, but it's hard to say how likely that is. If anything, Vishnuonyx seems to have most closely resembled the modern giant otter although it's unlikely to have been a close relative of that, either. The teeth retained on the new fossil are more pointed than is typical of bunodont otters and the wear patterns on them suggest that they were mainly used for biting into soft flesh. This would most likely have been fish, like those that the giant otter eats, and not the mussels, lobsters, or whatever else we might expect a bunodont otter to eat - although there's nothing to say that it didn't eat some of those too, as its larger relatives likely did.

It's another missing piece in the history of these unusual otters, showing that they once lived in Europe as well as Asia and Africa. It doesn't really resolve that history, or show exactly how they evolved, or what parallels they may have had with living clawless and sea otters; for that, we'd likely need more complete remains showing us more of the body, or at least some other parts of the skull. But when the picture is murky, any new information can shed light on it.

[Photo by D. Nikolić, from Wikimedia Commons.]

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