Convergence, divergence, and novelty in the ungulate-like hindlimbs of South American litopterns

Abstract

South America’s Cenozoic isolation led to the evolution several types of ungulate-like mammals. One of these, Litopterna, included two main families: Proterotheriidae, with relatively small (~15–70 kg), functionally monodactyl species, and Macraucheniidae, with larger (> 100 kg), tridactyl species. Litopterns resemble perissodactyls and artiodactyls in some respects but have shorter distal limbs than expected based on the metatarsal:femur ratio. We explore potential explanations for this pattern by comparing the functional morphology and proportions of litoptern hindlimbs to those of 153 extinct and 82 extant mammals of nine orders. We find that the litoptern tarsus resembles that of rodents and artiodactyls, having a secondary fulcrum in the transverse tarsal joint, and that the distal tibia resembles cursorial rodents (e.g., Dolichotis) in having a small medial malleolus. Proterotheriids likely used a half-bound or rotary gait, rather than the horse transverse gallop, and had relatively long proximal phalanges, which represents a strategy for monodactyl cursoriality distinct from horses (equids). In contrast, macraucheniids increased in size, evolved weight-bearing adaptations, and lost tarsal traits associated with mobility. Fossil trackway studies have suggested that macraucheniids used a walking pace. Our data indicate that macraucheniids had relatively long legs, providing a link between this unusual gait and the expected morphology. Litoptern tarsal specializations are present in remains from the Paleogene of Itaboraí, Brazil, which suggests that adaptations in small, bounding, early litopterns may have been preadaptive for cursoriality evolving in proterotheriids and macraucheniids.