Locomotor habits and phenotypic evolution of the appendicular skeleton in the oryzomyalian radiation in the Neotropics (Sigmodontinae, Cricetidae, Rodentia)

Sigmodontine rodents colonized South America in Late Miocene or earlier, leading to the clade Oryzomyalia, which rapidly radiated in distinct lineages and occupied almost all continental habitats, in a pattern classically interpreted as an adaptive radiation. Nevertheless, no evidence of strong influence of niche diversification on the evolution of cranial and mandibular morphology, or of deceleration in rates of phenotypic evolution in these structures over time following niche saturation, as expected according to the Ecological Opportunity model, has been detected. Here, we investigated morphometric variation among 59 oryzomyalian species using phylogenetically informed comparative analyses for testing (1) if the diversification of locomotor habits played an important role in shaping the morphology of the appendicular skeleton, and (2) if the disparification of appendicular skeleton showed high rates at the early diversification of Oryzomyalia and then has deaccelerated. Results showed that the different locomotor habits are associated with different shapes in both the forelimb and hindlimb, and selection of evolutionary models suggested that each locomotor habit was associated with their own adaptive optima. Moreover, the most extreme and specialized phenotypes, found in the semifossorial species Geoxus valdivianus, Blarinomys breviceps, and Paynomys macronyx, seem to have appeared after events of acceleration in the rates of morphological evolution. On the other hand, no evidence of a reduction in the rate of evolution over time was detected. The results suggest that the acquisition of different locomotor habits in oryzomyalians was associated with morphological specializations in the appendicular skeleton guided by natural selection and that, especially in the case of the evolution of fossoriality, there was a marked change in evolutionary regimes, generating highly modified phenotypes after acceleration of the pace of morphological changes. Despite the strong association between diversification of locomotor niches and evolution of the appendicular skeleton, the diversification of oryzomyalians does not seem to have experienced niche saturation, as noted in some other adaptive radiation events on Neotropics.