Sunday 24 January 2016

How Fossil Rodents Met Their Doom

Tritemnodon, another early hyaenodontid
from Wyoming
A large part of palaeontology consists, as you might expect, of looking in detail at the fossils that are dug up. This can tell us a lot about the animal and how it lived. By analysing the shape of the teeth, we can get some clue as to what sort of things the animal ate, and from the shape and proportions of the limbs and other body parts we may be able to infer other details of the lifestyle. If it's a carnivore, for example, did it chase its prey, or pounce on it from hiding and overwhelm it with sheer physical force? If the former, it should have long, relatively slender limbs suitable for running, if the latter, we'd expect powerful limbs with attachments for strong muscles.

When it comes to fossil mammals, teeth are often the best guide. For one thing, being small, and made of material even harder than bone, they are more likely to be preserved, even if the entire rest of animal is missing. For another, for most mammals, the precise shape of the cheek teeth (molars and premolars) is highly distinctive, giving us clues as to how one particular species is related to others. Even if we did find, say, an isolated rib, it might be difficult to tell what it once belonged to, and it probably wouldn't tell you much about the animal in question even if you could - beyond, perhaps, some clue as to its size.

Finding a reasonably intact skeleton is a relatively rare event, but it has obvious advantages when it happens. An extreme example is given in the book All Yesterdays by Conway, Kosemen, and Naish, in which they point out that if all you had of a manatee was the skull, you might well assume that it was some sort of land-dwelling herbivore. After all, it's not as if large aquatic grazing herbivores are a common thing, much of the plant-life in the oceans being floating plankton.

A less speculative example comes from a recent analysis of the hyaenodontid predator Galecyon. Despite the name, hyaeonodontids were not particularly related to hyenas, although they did have teeth that were at least superficially similar, and the larger ones, such as Hyaenodon itself, may well have had a similar lifestyle. However, the group lived for a long time, and not all of the species fit this sort of description.

Galecyon is one of the earlier hyaenodontid species, living in Wyoming during the early Eocene, around 55 to 50 million years ago. It was by no means alone, since anything up to 20 species of hyaenodontid are known from the same general time and place, and it seems plausible that they much have lived slightly different lifestyles in order not to have got in one another's way. In the case of Galecyon, however, all we had until recently were the teeth and a few bits of jaw. That's enough to tell us that, like its relatives, it was a predator, one that, like modern cats, was sufficiently adapted to eating meat that it would probably have had difficulty eating plant matter even if it wanted to.

As for the rest... well, with no skeleton to go on, all we could do was guess. It's closest relative for which we do have a reasonably complete skeleton is an animal called Prolimnocyon, so the best we could say was that it was probably kind of like that. Assuming, of course, that the shape of the teeth is sufficient evidence for us to be right about what the closest relative is in the first place. An analysis of Prolimnocyon back in 1993, showed that it appeared to be well-adapted for climbing trees. In particular, its limbs had highly flexible joints, similar to the sort of thing we see today in animals like pine martens.

So, for lack of any better information, and in the absence of anything to the contrary, we could at least make a provisional guess that Galecyon climbed trees, too. In fact, another close relative, Arfia, has been described as having hind feet that can swivel backwards on their ankles, a feature that is also common in climbing animals - making it easier for them to climb down a tree head-first, so they can see where they are going.  On the other hand, there were some related animals that seemed to be more adapted for simply walking along the ground or even, in the case of Gazinocyon, for running (presumably after their prey). So, we really couldn't know for sure.

Now that we do have much more of the skeleton, however, we can say that, while running at any significant speed was likely beyond it, it probably wasn't that good at climbing trees, either. So far as the researchers could tell by manipulating models of its ankle bones, it couldn't have reversed its hind feet, and other features of the limb bones suggest that it was at least in the process of descending from the trees onto the ground - likely retaining the option to travel in either as the circumstances demanded.

This sort of study, examining the shape of the bones of long-gone animals, is a central part of palaeontology, but it's not the only source of information we have as to how extinct animals lived. For one, knowing how old the fossils are, and what the environment they lived in looked like can obviously provide us with a fair bit of useful information. Was it a desert, a swampland, a forest, or a coastline, for example? But we can also look at smaller scale details of the environment, examining the taphonomy of a fossil.

Taphonomy, as a field, concerns how animals get to be fossils in the first place, and, more generally, what happens to the remains after their death. This has a lot to tell us about the fossil record, and why some animals are less likely to fossilise than others, perhaps creating misleading impressions about what was actually alive at any given time. Sometimes, however, it can tell us not just about how an animal died, but how it lived.

To look at a recent example of this kind of study, we don't even need to leave Wyoming. Elsewhere in the same state that was the origin for the fossil of Galecyon described above, lies part of the White River Formation (most of which is over the border in South Dakota), a fossil site dating to a much later part of the Eocene epoch, between 38 and 34 million years ago. Within that are two pockets with particularly rich collections of fossils.

The fossils are, as one might expect, of a wide variety of different animals. Most of them are of mammals, although there are some reptiles and amphibians in the mix, and while most of the mammals in question are quite small, there are a few larger ones, such as three-toed horses and early dogs. But one particular animal dominates: Adjidaumo minimus.

Adjidaumo is a kind of rodent, somewhat resembling a cross between a squirrel and a gopher. Indeed, gophers are thought to be amongst their closest living relatives, along with kangaroo rats, although, evolutionarily speaking, the two parted company a long time ago. When it comes to this particular species, prior to the find in Wyoming, the largest number ever discovered in one place was four bits of jaw discovered in Montana in the 1960s. An examination of the fossils at these two sites, however, reveals a total of 150 specimens.

To be sure, only ten are reasonably complete, although that's still a huge advance on our previous knowledge, and enough to provide a detailed description of this previously obscure animal. But where the taphonomy comes in is questioning how quite so many of them got to be in one place at the same time.

There's basically two ways that this could happen. Firstly, we have to remember that, when we say that they all died at about the same time, we mean this in a geological sense. On a human timescale, it could have taken a thousand years, and we wouldn't necessarily be any the wiser, so long after the fact. So one explanation is that there was something about the area that caused rodents to visit it and die there. An obvious example here would be the famous La Brea Tarpits, which preserve a huge number of herbivores that fell into the tar and the predators and scavengers that followed to try and eat them.

A second possibility, though, is that they really did die at the same time, even from a human perspective. In other words, something horribly catastrophic happened that killed lots of animals together, burying their remains for us to find millions of years later. These particular animals weren't buried in volcanic ash, but, if they had been, it wouldn't take a genius to figure out what might have happened.

When you have such a large number of fossils of the same species, it becomes possible to take a good stab at answering this sort of question. If it's the first possibility, we should expect that most of the fossils will belong to weaker animals, the sort that are most likely to be trapped by whatever it is. On the whole, they'll tend to be either young and inexperienced, or old and feeble - there shouldn't be many from the middle age range. If it's the second, we should get something that much more closely resembles the population mix that the animals had in life. Which means relatively few older animals, because not many make it to that sort of age in the world of nature red in tooth and claw.

To estimate the age of the animals, the researchers looked at their teeth. Rodents' incisors grow throughout their life, so they aren't much help, but the molar teeth don't, which means that the older the animal is, the more worn down they will be. Using this as a guideline, the researchers estimate that one of the two sites held fossils of animals that were mostly in the middle of their lives, while in the other, there were very few such animals.

So, one site where something catastrophic happened, and another that seems to have built up over time. If anything, the 'catastrophic' site held less young animals than might be expected - conceivably, the rodents bred in (say) the spring, and whatever happened, happened in the autumn, by which time the infants had either grown up or been eaten. But that, of course, is just speculation.

The authors of the paper offer no explanation as to what that disaster might have been. A flash flood, perhaps? It could be difficult to say. But further examination of the skeletons might explain how the animals reached the other site. The exact way in which the bones had been broken suggests that they belonged to animals that had been attacked and eaten, and most closely resembles the condition of bones of the prey of birds, rather than of mammals. In particular, the researchers estimate that the attacking animal was probably either a large owl, or something like a hawk.

The structure of the rock around the fossils suggests that it was once a burrow of some kind, but, while there are burrowing owls, they tend not to be the large ones, and, anyway, none of the fossils were in the form you'd expect of an owl pellet. Since hawks don't dig burrows either, it seems unlikely that the burrow belonged to whatever killed them. Perhaps it was an old burrow, in which the bones happened to wash up over a period of years every time there was a heavy rainstorm.

Either way, this tells us more about how these particular rodents lived - and met their grisly ends - than we could tell from the shape of their skeletons alone.

[Drawing by Robert Bruce Horsfall, in the public domain.]


  1. Re:
    "Rodents' incisors grow throughout their life, so they aren't much help, but the molar teeth don't, which means that the older the animal is, the more worn down they will be."
    I believe that there are some modern rodents in which the molars ARE open-rooted and ever-growing. But this may only have evolved in the past few million years, so probably for an Eocene rodent the statement will hold good.

    1. Yes, I believe that's right. Interestingly, one of the rodent groups with ever-growing molars is, in fact, the Geomyidae (pocket gophers) one of the closest living relatives of the eomyids. However, the other probable close relative - the kangaroo rats - does not have this somewhat unusual feature, so it's likely a modern one, as you say.