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An Overview of the Presence of Osteoderms in Sloths: Implications for Osteoderms as a Plesiomorphic Character of the Xenarthra


The presence of osteoderms in the skin of some extinct sloths and in cingulates (armadillos, pampatheres, and glyptodonts) has often been considered a pleisomorphic character of the Xenarthra. While osteoderms are known from the earliest cingulates, they are absent in most sloths including the two extant taxa and only appear late in their fossil record. Osteoderms are currently only reported from five genera of mylodonts and two megatheres, out of the over 100 currently recognized genera of sloths. Consequently, rather than a plesiomorphic character of the Xenarthra, which has been secondarily lost in sloths, it is more likely that osteoderms in sloths are the result of parallel evolution to the cingulates that independently evolved in one, possibly two different sloth clades.

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North American Land Mammal Age


South American Land Mammal Age


Anza Borrego Desert State Park, Borrego Springs, California (specimen originally in the Natural History Museum of Los Angeles County)


Field Museum of Natural History, Chicago, Illinois


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Bill Simpson kindly provided the images of the mylodont osteoderms from Tarija at the Field Museum of Natural History, Chicago. John Simmons kindly provided the photos of the mummified skin of Mylodon darwinii housed at the Natural History Museum, London, which granted permission to use the images. I thank the three anonymous reviewers for their comments and suggestions.

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Correspondence to H. Gregory McDonald.

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McDonald, H.G. An Overview of the Presence of Osteoderms in Sloths: Implications for Osteoderms as a Plesiomorphic Character of the Xenarthra. J Mammal Evol 25, 485–493 (2018).

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