|The Altiplano - where ground sloths may once have grazed|
The ground sloths actually did reasonably well after the other mammals arrived, and a very wide array of fossil species are known, the last of them having died out as recently as 9000 BC. Indeed, they crossed Central America heading in the other direction, leaving fossils in the northern continent, and at least one species, Megalonyx, even crossed the Arctic Circle to live in the cold wastelands of northern Canada. Many inhabited forests, but others preferred grasslands, or even semi-desert, while Thalassocnus may have been semi-aquatic. There was also a huge range of different sizes, from some that were barely larger than modern tree sloths, up to the multi-ton Megatherium.
Most of what we know about the ground sloths comes from the last 23 million years or so. This takes us back to long before the joining of North and South America, although the first monkeys had reached the continent (presumably by island hopping) around 25 million years ago, and rodents earlier still. Yet the sloths were there essentially from the beginning, along with opossums and some rather strange herbivores. The oldest known fossil that might be from a sloth comes from, of all places, Seymour Island in Antarctica, although it is only one tooth, and we really don't get good fossils until much later.
A new fossil species recently described from Bolivia may add more to the story of early ground sloths. Authors Bruce Shockey and Federico Anaya estimate their fossil to be around 26 million years old, putting it firmly in the late Oligocene epoch. It consists of much of the skull, and possibly some parts of the tail and right foreleg. It belongs to the same family as Mylodon and its kin, and has been given the rather cumbersome name of Paroctodontotherium calleorum.
One of the first questions that's often asked about fossil animals is "how big was it?" As you might imagine, when you only have a skull to go on, that's not too easy, and even with a reasonably complete skeleton it isn't that straightforward. Nor it is unusual for a fossil species to be identified from no more than a skull - fossils are notoriously incomplete, and the skull is usually the most distinctive bit of the skeleton. In these cases, the best we can do is make some guesses based on the size of the bits we do have and the probable shape of the animal.
In this case, the authors used three different measurements to estimate the size that the whole sloth would have had when alive - and got three completely different answers. Based on the length of the row of teeth in the upper jaw, the animal would have weighed about 15 kg (33 lb), about the size of a Staffordshire bull terrier. Based on the total length of the skull, it would have been more like 47 kg (100 lb), similar to a Labrador retriever. Finally, using the width of the muzzle as the baseline, the usual calculations make it 354 kg (780 lb), at which point I run out of dogs, and have to refer you to a male grizzly bear.
Even allowing for a degree of uncertainty, and ignoring the difference in shape, its difficult to see how a Staffie could ever be mistaken for an adult grizzly, so what's going on? The animal obviously has an extremely broad muzzle, making any estimate based on that alone essentially useless, and leaving us with the two more reasonable figures. Its more likely that the 47 kg figure is closest to reality, but even that assumes that the sloth did not have a particularly long or short head relative to the rest of its body, so its really no more than an educated guess. The reason that the lower figure is probably wrong is that the equations used to work it out assume a relatively normal set of mammalian teeth, and sloths don't have normal teeth - in fact, they have far less than usual, meaning that the figure is probably an underestimate.
Aside from misleading us as to the size of the whole animal, that broad muzzle may, in fact, be very significant. Living tree sloths, especially the three toed ones, have fairly rounded heads, with short snouts, but the extinct ground sloths generally didn't. Indeed, only one other ground sloth - Lestodon, which lived much later, during the Ice Ages - had a broader snout than this new species. When we look at living herbivorous animals, we find that long, narrow snouts are associated with animals that carefully snip out high quality buds, fruits, and so on from the plants they are feeding on. Herbivores with wide snouts are generally grazing animals that shovel in large quantities of plant material without much in the way of precision. This makes sense if you're feeding on something like grass, because its not all that nutritious, and so you want to eat a lot in one go.
This suggests that the new sloth probably fed on grass or sedges, and this fits with the environment at the time, which was most likely a mix of pampas and altiplano, as well as with the known diet of the closely related species Mylodon. But there is a problem here. In addition to the shape of their muzzles, there is another difference between careful browsers and grass-eating grazers: the latter have very high teeth. They have to, because grass is relatively tough compared with most other green plant material, and because, if you're chomping stuff directly off the ground, you're probably going to end up with some soil and grit in your mouth from time to time. And these things wear your teeth down - if they don't start off high, they'll soon become useless, and you die.
This animal, like other sloths, and unlike most other fossil herbivores from the same neighbourhood, does not have particularly high teeth. Yet, here again, sloths are different from other big herbivores. Their teeth grow continuously throughout life, so, as long as that happens quickly enough, they will never wear down, simply replacing the worn enamel with new material from underneath.
Grasslands first spread across South America not long before this sloth and its relatives first walked. The climate had cooled, as Antarctica split away from the south, allowing the creation of a cold circumpolar current around the newly ice-bound continent, in addition to more worldwide effects. The fact that this new sloth does not have particularly primitive features, and is well adapted to a grassland habitat, suggests that its ancestors had been evolving for some time, probably longer than had previously been thought. Furthermore, we know of at least seven other species of ground sloth that lived at the time, and the fact that this one is (so far as we can tell) much smaller than any of the others suggests that they were already quite varied.
Indeed, it could even be that the change in the climate, and the spread of the grasslands was what first caused the sloths to diversify, and begin evolving into the even greater range of forms that they later took on.
[Picture from Wikimedia Commons]