Tarsal morphology and locomotor adaptation of some late middle Eocene caviomorph rodents from Peruvian Amazonia reveal early ecological diversity

In rodents, and other vertebrates in general, the morphology of tarsal bones, especially the astragalus and calcaneus, has been shown to be tightly linked to locomotor movements. As a result, it has been used to infer locomotor behaviors in extinct species. Recent expeditions in Peruvian Amazonia have led to the discovery of the oldest caviomorph rodent fossils in South America, including two calcanei and one astragalus. The morphologies of these three tarsal bones are described in detail and compared with other extant and extinct caviomorphs. In order to assess and infer the locomotor behaviors of these rodents, linear measurements were taken on these tarsal bones and analyzed via multivariate analyses based on a previously assembled large data set. Both qualitative and quantitative analyses consistently suggest that the osteological adaptations of the astragalus enhance movements for climbing, those of one calcaneus rather enhance movements indicating terrestrial and partly fossorial lifestyle, whereas the other calcaneus may have belonged to a generalist form with a tendency toward a semiaquatic lifestyle. These results fit well with the associated paleoenvironments and hint at ecological diversity early in caviomorph history.