Tempo and mode of morphological evolution in South American subterranean rodents (Hystricomorpha: Ctenomyidae)

Abstract This study analyses evolutionary models and rates of craniomandibular variation in Ctenomys, the most diverse genus of subterranean rodents worldwide. We use geometric morphometrics and comparative phylogenetic methods. We included 61 extant species and 15 extinct ones represented by 587 crania and 514 mandibles. We found that stem Ctenomys exhibited traits suggestive of reduced tooth-digging capacity, and that specialized tooth-digging features emerged during the late Early Pleistocene crown stage. Evolutionary rates for cranial shape remained stable over time, but mandible rates increased within the crown clade. In terms of size, lower evolutionary rates were observed in stem and early crown lineages, with a notable rate increase in a more external crown node associated with smaller sizes probably constrained by intensified subterranean adaptations. Morphological variation in the ventral cranium was best explained by Brownian motion, whereas lateral craniomandibular variation fitted the Ornstein–Uhlenbeck model. The Ornstein–Uhlenbeck model was best for the lateral cranium and the early burst model for the lateral mandible when fossils were included. The OU model also explained evolution in size. Overall, these findings reveal differential adaptive responses in skull regions and underscore the role of selective pressures shaped by subterranean lifestyles in the craniomandibular evolution of Ctenomys.