Time of the Tiny Otters

An earlier fossil
(The new one still has the jaw attached to the rest of the skull)

A few months ago, the discovery of the "largest dinosaur ever" was announced. Again. Indeed, the "largest dinosaur ever" always seems to be being discovered, and, while we must at some point, surely get to the real end of the sequence, I wouldn't put any money on this latest one being it. But speaking more generally, I'd expect that when most people think of dinosaurs, "big" is going to be a fairly common adjective.

Even when we look at prehistoric mammals, it's the big ones that get most of the attention. Mammoths. Prehistoric rhinos. Enormous tank-like armadillos. Giant wombats. Big animals are cool, and there were some pretty large ones in the distant past.

Yet, at any point since their first appearance, small animals will always have been more common than large ones. Lots of small and interesting things doubtless scurried about under the feet of the dinosaurs, and so it was with the Age of Mammals, too. So today I want to look at the recently described fossil of a small mammal, and how it too, can tell us something interesting.

The animal in question lived around 52 million years ago, and is thus quite a lot closer in time to the non-avian dinosaurs than it is to us. It lived in what is now Wyoming, in an area that was, at the time, the shore of a large lake. This is, incidentally, around the same time that British Columbia was covered in sort-of-jungles, and Wyoming, being a bit further south, could be fairly described as "subtropical".

The fossil is something like 16 cm (6 inches) long, and most of the surviving bones are articulated, still joined together in the positions they were in life, so that's not going to be far off the size of the living animal. Plus however long the tail was, because that's missing, as are the hind legs. (They may just be missing because 52 million years is kind of a long time, but for all we know, something ate them after the animal died. Or, given the shape of the slab the fossil came out of, the palaeontologists may just have missed them, and left them behind in the rock).

The fossil is easily complete enough to tell us that it belongs to a species of previously-known pantolestid named Palaeosinopa didelphoides. And what, you might reasonably ask, the heck is that when it's at home?

Pantolestids were first identified back in the 19th century, and quite what modern animals they were related to has been a matter of dispute for much of the time since. We can tell that they were placental mammals, not marsupials - despite the second half of the scientific name of this species meaning "opossum-like".  Our best bet today is that they didn't leave any descendants, and that, beyond being placentals, they aren't particularly closely related to anything we'd find in the modern world. They were, in other words, an early experiment in the great burst of placental evolution that followed the extinction of the dinosaurs and one that, in the long run, didn't work out.

Indeed, they first appeared something like 60 million years ago, not long after that great extinction, and went extinct themselves no later than 40 million years ago, and possibly quite a bit earlier. Physically, they looked rather like shrews, albeit the size of a small rat. On the other hand they were widespread, being known from Germany as well as from North America, so presumably, they were pretty successful at the time.

But, okay, so I've just said that this isn't a new species. For that matter, it isn't even the most complete skeleton that we've found of this particular species. And it lived when and where we already knew that it lived. So what's interesting about it?

Well, the researchers noticed that there a few things that were odd about the skeleton. For a start, there's its size. Pantolestids are supposed to be 20 cm (8 inches) or more in length, plus tail, so this one's pretty small as such things go. The skull is a slightly different shape, too, being shorter and more rounded. The first thought here might well be: is it a new species after all?

But if so, it's odd that the teeth look identical, not merely in shape, but also in size. And that's the clue. In most mammals, teeth don't get larger as the animal does - which is why you need to get more of them as you grow up. Further evidence comes from that the fact that the third lower molar teeth don't appear to be fully erupted; when it died, the fossil hadn't quite got its "wisdom teeth".

So that, the researchers believe, is why it's smaller, with a shorter, "cuter" face: it's not an adult. Once we know that, we can start looking for further clues, and they can tell us, not only what the animal was like, but how it grew as it aged. Now, the fossil does have, "wisdom teeth" aside, what appear to be a full set of adult teeth. It's not as if it still has milk teeth (though, to be fair, we don't know quite what those would look like in this species if it did). So it can't be all that young.

Or can it? When the researchers looked at the bones in the forelimb, they discovered that they hadn't quite fused yet. They were incomplete, with the ends not yet fully attached to the shaft in between. This is the way that long bones grow (see, for example, this X-ray of a healthy 12-year old child), so that's not terribly surprising. But, in this case, the limb bones, in particular the elbows, look to be rather less well-developed than we'd expect, judging by how close the animal was to achieving the full adult dentition.

Which implies that, either the animal grew very slowly (as humans do, for instance), or, more likely, that it developed adult teeth very quickly (as is the case in some other primates). Primates aside, the researchers point out the apparent parallels with bone development in otters. Which they don't think is a coincidence.

When pantolestids were first discovered, it was noted that they had robust, rounded, teeth, of the sort that look like they would be good at cracking open some sort of solid, crunchy food. Which, given that they live near lakes, probably meant shellfish, such as crabs or clams. There's no hard evidence that this is true, but we have found some fossils with fish bones where their stomachs would have been. So we do know that pantolestids ate fish.

Which means that they didn't just live around lakes, but they must have, at least some of the time, swum about in them. The shape of their limbs is consistent with either this or with doing a lot of digging (they might have done both, of course, if they lived in burrows), and muscle attachments on the tail bones suggest a strong, muscular tail that could propel the animal through the water. Just like otters, in other words.

It's less clear why otters should grow their teeth more quickly than their skeletons - although it may be relevant that this is even more true of seals - so we can't say that this isn't a coincidence, or seeing a pattern that's not there. But since we already knew that they resembled miniature otters, it's at least possible.

This is long, long, before the first otters appeared, around 11 million years ago. But it seems that something else, very similar, once shared their lifestyle back on the subtropical lake-shores of ancient Wyoming.

[Photo by "Archaeodontosaurus", from Wikimedia Commons]