There is something fantastically weird about giant ground sloths. Creatures from a not-too-distant past, close enough in time that their hair and hide is sometimes found in circumstances of exceptional preservation, these creatures have no living equivalent. Their arboreal cousins still live in the tropics of the western hemisphere, but they can hardly be considered proxies for the ground sloths of the Pleistocene.
The most famous of these ancient beasts was Megatherium, an exceptionally large ground sloth which has been fascinating paleontologists and the public for over 200 years, but what is less well known by members of the public is that there were many kinds of ground sloth. Megatherium was not a lone aberration but a part of a highly successful family, one of the few types of weird South American mammal that flourished in North America when the two continents came into contact a few million years ago. Not all of them were the same. While some made their living grazing in open habitats others preferred to browse among most forested environs, and a recent study published in the Journal of Morphology provides a way to tell which kind of lifestyle particular sloths might have had.
Everybody knows that teeth can often tell you quite a bit about what an animal eats, but it is not the only informative parts of the skull when it comes to diet. To ascertain the range of dietary habits in giant ground sloths, researchers M. Susana Bargo, Nestor Toledo, and Sergio Vizcaino looked at the muzzle shapes of the species Megatherium americanum, Glossotherium robustum, Lestodon armatus, Mylodon darwini, and Scelidotherium leptocephalum, a selection with a variety of skull shapes. Two of these, Glossotherium and Lestodon, had squared-off muzzles, and the rest had more narrow snouts, but to determine how this related the diet the authors looked at the muscle scars and other landmarks in comparison with the known tissue anatomy of living sloths in an attempt to recreate the soft tissues of these animals.
As illustrated by the restorations of the head of each sloth, figuring out their muzzle shape was a multi-step process. First was determining the extent of nasal cartilage that would have been present in life. This provided the complete framework on which to place the various muscles related to lip movement, and from there the head could be fully fleshed out. Once these restorations were completed it could be further hypothesized whether each species was a browser or grazer, with grazers being characterized by having wide, squared-off muzzles suited to taking in low-quality foods (i.e. grass) in bulk and browsers having narrower muzzles better suited to more selective feeding on high-quality foods.
The results were fairly clear cut. Lestodon armatus and Glossotherium robustum both had comparatively wide, spoon-shaped muzzles, with Scelidotherium leptocephalum, Mylodon darwini, and Megatherium americanum having narrow muzzles (the latter species having the narrowest of all). Overlain on top of each other, there is a wide gap between the muzzle shape of the grazers and browsers; the sloths selected are not grades between one extreme and another but occupy opposite, well-defined ends of the spectrum. Additionally, Megatherium americanum may have been such a specialized feeder that it had a prehensile upper lip akin to what is seen in the black rhinoceros which it could have used to grasp and selectively tear off particular plant parts. The grazing sloths Lestodon and Glossotherium, on the other hand, would have had lips more like that of the white rhinoceros – squared off and better suited to bulk feeding.
The hypothesized feeding habits of these sloths are consistent with what is seen in living herbivores – browsers and selective feeders have narrower muzzles than grazers within lineages of plant-eating mammals. This may have been a form of niche partitioning as sloths evolved through the Pleistocene, and may explain why there were so many genera and species present at the same time. Even though we think of modern sloths as peculiar, specialized animals, the ground sloths of old appear to have been more adaptable to a wide array of habitats, though this makes their disappearance in relatively recent time all the more mysterious.
Post script: This kind of niche partitioning did not only exist between closely related species, but could also happen within species as organisms grew up. A recently-described juvenile Diplodocus skull, for instance, suggests that young individuals were browsers while adult Diplodocus were better suited to grazing.
Bargo, M., Toledo, N., & Vizcaíno, S. (2006). Muzzle of South American Pleistocene ground sloths (Xenarthra, Tardigrada) Journal of Morphology, 267 (2), 248-263 DOI: 10.1002/jmor.10399