Evolutionary trends of caviomorph rodents as elucidated using their oldest foot anatomy

Abstract

Extant caviomorph rodents exhibit a wide array of locomotor types and substrate preferences, which are reflected in their foot morphology. It is not known when these patterns were established or how they evolved in this taxonomically and ecologically diverse clade. We studied the anatomy of the earliest preserved feet of caviomorphs represented by the late Oligocene stem octodontoid Platypittamys and the Early Miocene cavioid Neoreomys, stem octodontoid Sciamys and erethizontoid Steiromys. Integrating information provided by extant species, we performed qualitative and geometric morphometric analyses, reconstructing ancestral morphologies to interpret the evolution of the foot of caviomorphs through a time-scaled phylogeny, and infer locomotor behaviours. We identified an important morphological disparity in these early genera that is explained by the presence of different locomotor adaptations and phylogenetic signal at different taxonomic levels. Neoreomys was probably generalized ambulatory, Steiromys possibly scansorial, Platypittamys possibly ambulatory with digging abilities, and Sciamys was probably agile ambulatory–scansorial. Our study indicated that several foot features can distinguish higher taxonomic groups of caviomorphs, at least since the Early Miocene. Early in their history, morphological constraints would have limited the capacity of evolutionary change more markedly in Cavioidea and Chinchillidae. In Cavioidea, at least two evolutionary trends were detected. Among octodontoids, extant octodon tids and Platypittamys showed convergent anatomical patterns. Erethizontoidea retained a foot essentially unchanged since the Early Miocene after acquiring a noteworthy evolu tionary innovation. In sum, the earliest foot morphologies of caviomorphs are very important in identifying evolutionary tendencies and the origin of their postcranial disparity.