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