Postcranial osteology and functional morphology of the Litopterna of Salla, Bolivia (Late Oligocene)

Abstract

ABSTRACT Postcranial remains of litopterns of Salla, Bolivia are described and discussed in terms of their functional morphology. These elements are not associated with dental remains, but can be confidently referred to the family Macraucheniidae or Proterotheriidae. The macraucheniids (cf. Coniopternium spp.) have transversely stable joints indicating their specialization for parasagittal movements at these articulations (e.g., the humeral-antebrachial, cruxastragalar, and astragalonavicular joints). A profound suprapatellar fossa ("patellar pit") of the femur is interpreted as having provided a place in which the patella could have become lodged, thus locking the knee-joint in the extended position. Facets on the cuboid and astragalus and their articulation in life position indicate that cuboastragular contact was present. This contact was slight, however, and may have occurred only during the loaded part of the locomotor cycle. A partial pes of a proterotheriid represents the first unambiguous evidence of this group at Salla and are the first Deseadan postcranial elements confidently referred to the Proterotheriinae. These elements indicate that the animal was tridactyl, but functionally monodactyl to the same degree as all Santacrucian proterotheriid genera except the "hypermonodactyl" Thoatherium. These litopterns and those to which they were compared record a great diversity of tarsal arrangements, such that simple characterizations of litoptern tarsals as being serial and "inadaptive" are unwarranted.