A Tale of Three Ground Sloths

Today, sloths are slow-moving tree-dwelling animals found only in South and Central America. For most of their evolutionary history, however, the majority of sloths have been ground-dwellers, in many cases too large to have climbed a tree even if they had wanted to. In comparison, tree sloths are a relatively recent evolutionary development. 

Ground sloths, however, are not a single type of animal in the strict biological sense. This is because the two-toed and three-toed sloths that we have today are not especially close relatives, last sharing a common ancestor at least 28 million years ago. In fact, tree sloths evolved twice, in separate branches of the sloth family tree, and some ground sloths were more closely related to one or the other. On this basis, we used to divide the ground sloths into two families, but the fact that some of lived recently enough that we can recover and analyse DNA from their fossils gave us some surprising detail and a key 2019 study suggested that we should consider there to be no fewer than six different families of ground sloth.

So whatever we can say about one sort of ground sloth isn't necessarily true of all the others, even on quite a broad scale.

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.