Abstract
The rotation of the pes or pedolateral stance in the extinct ground sloths so the body weight of the animal is primarily supported by the fifth metatarsal and the calcaneum occurred independently at least three times and is present in the Megatheriidae, Nothrotheriidae, and Mylodontidae. In contrast, the pes in the Megalonychidae more closely resembles the primitive eutherian pattern. The pedolateral rotation of the pes thus represents an excellent example of parallel evolution in a closely related group of mammals. While the rotation of the foot occurs as a functional complex resulting in the modification of many bones in the pes, the astragalus is the one bone that shows the highest degree of departure from the primitive mammalian condition and the most distinctive changes in morphology. The morphological transition from a plantigrade foot as occurs in xenarthran anteaters and is essentially retained in the megalonychid sloths to the highly derived condition seen in the megathere, nothrothere, and mylodont sloths follows a similar pattern in all groups but there is still significant variation in the foot structure between the lineages. Despite these variations there are consistent patterns of change in the astragalus in all groups related to the progressive rotation of the pes and a change from dorso-plantar flexion and extension to a medio-lateral rotation of the pes relative to the tibia.
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Acknowledgements
I wish to extend by appreciation to the following people for their support of this paper. Vanessa R. Rhue and Sam McLeod (Natural History Museum of Los Angeles County, LACM) provided the images of the Nothrotheriops astragalus. Kevin Seymour (Royal Ontario Museum, ROM) provided the images of the Catonyx and Lestodon astragali. Mary Thompson (Idaho Museum of Natural History, IMNH) loaned the astragali of Megalonyx and Paramylodon. Emily Lindsey provided the images of the astragalus of the Eremotherium from the Museo Megaterio at the Universidad Estatal Península de Santa Elena, Ecuador (MM). I thank the reviewers and editors for their insightful comments, which have added clarity to portions of the paper.
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McDonald, H.G. Evolution of the Pedolateral Foot in Ground Sloths: Patterns of Change in the Astragalus. J Mammal Evol 19, 209–215 (2012). https://doi.org/10.1007/s10914-011-9182-x
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DOI: https://doi.org/10.1007/s10914-011-9182-x