Paleobiology of Argyrolagus (Marsupialia, Argyrolagidae): an astonishing case of bipedalism among South American mammals

Abstract

Argyrolagus constitutes, both for its craniodental and postcranial anatomy, one of the most notably specialized South American Neogene metatherians. Differentiating it from any other South American mammal, bipedal jumping has been proposed for Argyrolagus, even though this hypothesis was not supported by morphofunctional studies. Here, we describe the postcranium of A. scaglai (from the Pliocene of Argentina), perform a functional analysis, and interpret it against a varied background of locomotor adaptations of extant mammals. The configuration of joints, the degree of development and location of muscular insertions were mainly analyzed, and functional indices were evaluated. This study indicates that Argyrolagus had stabilized glenohumeral and humeroulnar joints, a great development of the arm retractors, flexors-extensors of the digits, pronator, and supinator muscles, low restrictive humeroradial joint, powerful extensor muscles of the hip, knee, and ankle, good development of the iliac muscle, and restrictive hind limb joints. Joint configurations are interpreted to be optimal to resist the impacts during jumping, avoiding dislocation, compatible with digging activity. A compromise between the capacities to dig and manipulate objects is inferred. It is concluded that Argyrolagus had bipedal jumping locomotion as well as good capacity to dig, constituting an astonishing case of convergence with the small bipedal rodents and small Australian macropodids. We suggest that bipedal jumping in Miocene and Pliocene argyrolagids should not be necessarily related to a particular arid environment. Finally, we evaluate the importance of postcranial features to understand the phylogenetic relationships of Argyrolagidae in a comprehensive phylogeny of Metatheria.