Sunday 15 March 2020

The Smallest Mammal Ever?

When it comes to fossil mammals, or indeed, any other kind of fossil animal, our attention is inevitably drawn to the giants. We are often fascinated by the mammoths or glyptodonts or the largest of Irish elk or the most muscular of sabretooths. Outside of mammals, it seems there's a never-ending battle to find the "largest dinosaur ever".

Indeed, one might almost get the impression that everything prehistoric was larger than today. In a number of cases, that's because larger animals really did exist in the past, perhaps being wiped out by a combination of the harsh realities of the Ice Ages and the arrival of human hunters. It's also an artefact of larger bones being easier to find in rock layers and being less fragile and likely to fragment when they fossilise. And that's before we add in the fact that most popular books on the subject tend to have a focus on the biggest and most dramatic animals of their kind.

But, of course, if we were to travel back in time millions of years and, if we were the kind of person that goes around cataloguing the wildlife, we'd notice that there were a lot more small animals than large ones. That's always going to be the case, simply because of the way ecology works and of the demands of nutrition.

And these smaller animals do fossilise, so that we do have extensive fossil records of early mice, voles and so on. The smallest mammals alive today are the shrews, which turn out to be a very ancient group, although their small size means that their fossil record is patchy at best. They have likely always been among the smallest of mammals, and, given the adaptations needed for warm-blooded animals to survive at such a small size, probably evolved from something that was at least a little bit larger.

But, especially if we go back far enough, we can find many other small mammals in the fossil record, some of them related to shrews, but some rather more distant. Around 45 million years ago, during the middle of the Eocene epoch, the most common small insectivorous mammals in North America, apart from bats, seem to have been a group known as the sespidictines. I've talked about these before, and how they seem to be distantly related to shrews, but to have separated from their lineage some time before the first true shrews appeared.

Another group of exceptionally small animals living in North America at around this time were the geolabidids. A number of species of these tiny shrew-like animals have been described, mostly from North America, although one example is known from Mongolia. They seem to have first appeared around 55 million years ago, and while they declined after the end of the Eocene, a few managed to struggle on into the early Miocene, around 20 million years ago.

Crucially, the group includes Batodonoides vanhouteni, first described in 1998, and, by some estimates, the smallest mammal ever to have lived.

It's difficult to know quite how small that is, especially since pretty much all we have to go on are some teeth and a few bits of jaw, which requires us to make assumptions about how these scaled with the rest of the animal. But it may have been as light as 1.5 g (1/20 of an ounce), about two-thirds the size of the world's smallest shrew today.

But, given how little we have of the animals, and how fragile their tiny bones tend to be, they remain something of a mystery. For a start, what exactly is a geolabidid anyway?

The first geolabidid to be discovered was Centedodon in 1872, and since this is more common than its smaller relative Batodonoides, it's the one on which most of the subsequent studies have been performed. At first, it was assumed to be some kind of bat - which goes to show how little of its skeleton they actually had - but in 1928, it was decided that were some kind of ground-dwelling insectivore, related in some vague and unknown way to modern shrews and moles, while clearly being distinct from either.

In 1960, the geolabidids were recognised as a distinct group of animals for the first time, albeit at the time as a subfamily of hedgehogs. By this point, we had some reasonably complete skulls and even a few other bones, but our understanding of how even the living species of insectivore were related to each other was incomplete enough that many of the theories proposed for the evolutionary position of the long-extinct geolabidids are no longer thought to be possible.

We now know that shrews and hedgehogs, together with moles and a couple of odd animals known only from the Caribbean called solenodons, belong to a larger group collectively termed the Eulipotyphla. The solenodons are almost ridiculously ancient, by the standards of living mammal groups, with the three better-known families having probably diverged from one another in the immediate aftermath of the extinction of the non-avian dinosaurs.

Using this knowledge, along with more complete remains of Centedodon, a 2004 analysis confirmed that the geolabidids did belong to this larger group - rather than being related to other insectivorous mammals, such as the tenrecs. But, beyond that, the authors of the study had to hold their hands up and admit that there just wasn't enough evidence for them to be any more precise.

It would help if we had fossils showing more than teeth and a few bits of jaw from some of the other species of geolabidid... and now, we do. Last year, an analysis of tiny ankle-bones recovered from a Middle Eocene site near San Diego confirmed that they appear to belong to a species of Batanoides, also known from teeth in the same deposits. These are very small bones indeed, somewhere in the region of 1.5mm (0.06 inches) across at their widest, but even so, their detailed shapes can tell us a few things about these small and obscure mammals.

Perhaps the most significant finding is that the shape of the bones suggests a remarkably flexible ankle-joint, of the sort that can readily twist about so that the foot can grab onto things. This implies that they belonged to climbing animals, possibly even ones that spent most of their lives in trees. This isn't necessarily true of all geolabidids, of course; in particular, the previously known leg bones of Centedodon seem to be more suited to regular walking. But these very tiny animals at least may have hidden in bushes or trees, away from the sight of many predators.

In terms of how they relate to modern animals, the bones told a rather different story than the teeth examined so far. They imply a relationship to the nyctitheres, a group of primitive shrew-like animals that died out even before the geolabidids did. The position of nyctitheres in the great mammalian family tree is itself controversial and, in any event, the similarity could be down to the fact that nyctitheres too, are thought to have been small climbing animals, and so might be expected to have similar feet in a way that, say, most shrews do not. (Yes, there is such a thing as a "tree shrew", but these aren't actually members of the shrew family, being more closely related to primates).

It's perhaps more likely that the geolabidids represent a very early branch in the evolution of insectivorous mammals, arising before the shrews, hedgehogs and moles diverged, perhaps at a similar time to the solenodons. If so, they're something relatively unique, with no close relatives living today... utterly minuscule insect-eating animals hiding in the trees 55 million years ago while the larger and more famous animals walked by.

[Photo by "Randychiu" from Wikimedia Commons, showing an exhibit at the California Academy of Sciences.]

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