Postcranial anatomy of the early notoungulate Allalmeia atalaensis from the Eocene of Argentina

Abstract

We redescribe and interpret the postcranial elements of Allalmeia atalaensis Rusconi, found in the Divisadero Largo Formation, Mendoza, Argentina. Allalmeia is one of the few basal notoungulates with preserved postcranial remains. Rusconi initially documented a skull, several vertebrae and part of the appendicular skeleton but these specimens were subsequently lost. Recently though, distal components of the holotype humeri, the proximal portion of the right femur, several broken diaphyses and most of the left and right pes (MCNAM-PV 507), were relocated. Preparation of this material has revealed new details, especially from the plantar side of the feet. The calcaneum has a distal peroneal process with marked rugosity in the lateral end, a well-developed calcaneal plantar tubercle and a large sustentaculum tali. The naviculars exhibit a well-developed medial tuberosity and slightly contact the calcaneum on the dorsal side, a condition called a ‘reverse alternating tarsus’. The disposition and morphology of the cuneiforms are similar to those of more recent typotheres and the archaic ungulate Tetraclaenodon puercensis. The phalangeal rows are oblique to each other, as in some extant digitigrade mammals. The ungual phalanges are claw-like suggesting that they might have borne claws. Long bones are gracile in comparison with other notoungulates, but with well-developed muscle origins and insertions. The use of body-mass allometric equations and a comparison with extant analogues suggests that Allalmeia was a small, generalized, digitigrade animal. An estimated body mass of approximately 3 kg is consistent with oldfieldthomasiids and archaic ungulates.