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The Concept of a Pedolateral Pes Revisited: The Giant Sloths Megatherium and Eremotherium (Xenarthra, Folivora, Megatheriinae) as a Case Study

Abstract

The concept of a pedolateral pes in many extinct sloths began effectively with Owen’s mid-nineteenth century descriptions of Glossotherium and Megatherium. Pedolaterality denotes a pes that is habitually inverted, with the digital plane oriented nearly vertically so that weight is borne largely by the lateral digits (mainly metatarsal V) and the plantar surface faces almost entirely medially. Subsequent researchers were strongly influenced by Owen’s interpretations. Astragalar morphology, with the medial and lateral portions of its trochlea forming, respectively, a peg-shaped odontoid process and a discoid facet, came to be viewed as a proxy for pedolaterality and, eventually, horizontal rotation around a nearly vertical axis as the main movement of the pes. Such motion necessitates a nearly vertical orientation for the odontoid process. However, analysis of the pes of the Pleistocene megatheriines Megatherium and Eremotherium, the astragalus of which conforms to the type usually interpreted in the literature as indicative of pedolaterality, suggests that the pes was not strongly inverted. Rather, the digital plane was about 35o to the horizontal plane, so that weight was borne largely by metatarsal V, but also by metatarsal IV and possibly the ungual phalanx of digit III. The astragalus was positioned so that the odontoid process was oriented obliquely to the vertical axis. With this element so positioned, mediolateral rotation in the horizontal plane was minor, and the main movement of the pes produced dorsiflexion and plantar flexion in nearly the parasagittal plane, the usual movement of the pes in terrestrial mammals.

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Acknowledgements

The authors thank collection managers and chairs of institutions housing specimens analyzed in this work (AMNH, FMNH, USNM, MACN, MLP, ROM, YPM) for kindly granting access to materials in their care; to J. Nyakatura, J.C. Fernicola, R. Kay, G.H. Cassini, and A. Racco for providing valuable suggestions; and to Eli Amson and one anonymous reviewer for greatly improving the quality of this work. This contribution was presented during the ICVM 11 meeting in Bethesda (Washington, D.C.) in July 2016, as part of the symposium Morphology and Evolution of the Xenarthra organized by M. Susana Bargo and John A. Nyakatura. Attendance at the congress was partially granted by UNLP Viajes y Estadías 2016 to SFV and NT, and CIC 1827/15 (Comisión de Investigaciones Científicas de la provincia de Buenos Aires) to MSB. This is a contribution to the projects UNLP 11/N750 (Universidad Nacional de La Plata) and PICT 2013-0386 and 0389 (Agencia Nacional de Promoción Científica y Técnica). Data sharing is not applicable to this article as no datasets were generated or analyzed during the current study.

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Toledo, N., De Iuliis, G., Vizcaíno, S.F. et al. The Concept of a Pedolateral Pes Revisited: The Giant Sloths Megatherium and Eremotherium (Xenarthra, Folivora, Megatheriinae) as a Case Study. J Mammal Evol 25, 525–537 (2018). https://doi.org/10.1007/s10914-017-9410-0

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Keywords

  • Xenarthra
  • Folivora
  • extinct sloth
  • foot anatomy
  • foot inversion