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.

A History of the Strange-Jointed Beasts

All living species of mammal fall into one of three major groups, representing the lowest branches of the mammalian family tree: the placental mammals, the marsupials, and the bizarre, egg-laying, monotremes. Well over 90% of identified living species fall into the first of those groups, which prompts the interesting question of how we divide those up - what, in short, are the next branches on the tree?

In the strict, old-style, system of classification, the next major level down is that of the "order", groups of broadly similar creatures that are in turn, divided into more than one hundred "families". Primates, for instance, are an order.

What the orders are, and how many of them there might be, has varied a little down the years, but nineteen or twenty would be a good, modern estimate. But, of course, the orders themselves are entirely arbitrary, and, even if they weren't, it isn't as if they all sprang simultaneously from the same stock. In evolutionary terms, there must have been some early branches in the placental family tree from which the "orders" that we know today descended.

The question of how to group the orders together into some higher level pattern is one that mammalogists have been been pondering for well over a hundred years. The problem is that the orders are pretty distinctive - is a moose more like a wolf than it is a monkey? When all you have to go in is physical anatomy, these are not the simplest questions to answer, and it's not surprising that people have come up with different answers.