X is for Xenarthra
The Xenarthra is a group of mammals known for a “strange joint” (what ‘xenarthra’ roughly translates to) in their vertebral column. Xenarthrans include sloths and armadillos, and the extinct giant ground sloths and massive armored glyptodonts.
That some sloths were massive leads scientists to question what made up the diet of these giant mammals. There is evidence that most giant sloths were herbivores, but there are some characteristics that suggest that at least some giant sloths could have been carrion feeders or even active hunters. Bochrens et al. (2017) use stable isotopes to address this question.
Bocherens, H., Cotte, M., Bonini, R.A., Straccia, P., Scian, D., Soiblezon, L., and Prevosti, F.J., 2017. Isotopic insight on paleodiet of extinct Pleistocene megafaunal Xenarthrans from Argentina: Gondwana Research, v. 48, p. 7-14.
The authors start by comparing d13C from bone bioapatite with d13C from bone collagen of animals known to be herbivores or carnivores. Then, because many sloth bones still preserve collagen, they run the same test on fossils and compare this with the relationship they determined with modern mammals. This simple test (which isn’t all that simple) shows that it is unlikely that giant sloths (at least the ones they studied) are anything other than herbivores.