Decoupled diversity and disparity after faunistic turnover in caviomorph rodents

Caviomorph rodents diversified widely in the Americas. Within this group, the sister clades Octodontoidea (spiny rats and allies) and Chinchilloidea (chinchillas and allies) illustrate strikingly imbalanced evolution, with 195 extant species in the former and only six in the latter. Fossil evidence, however, documents greater past diversity and disparity in Chinchilloidea, including the largest known rodents. Here, we integrate data from extant and extinct species to investigate how evolutionary dynamics shaped these contrasting trajectories. Using a phylogenetic framework, we reconstructed patterns of body mass and craniodental evolution. The ancestral body mass was small, but Chinchilloidea expanded into a broader size range, showing significantly higher rates of body mass evolution than Octodontoidea. Subsequent Neogene and Quaternary extinctions erased much of this variation, reversing a ~30 million-year trend of greater body mass disparity. Craniodental disparity, however, followed a different trajectory: initially higher in Chinchilloidea, it later became greater in Octodontoidea after the Miocene. Importantly, craniodental disparity remained relatively stable in both clades despite major diversification and extinction events. These findings highlight the decoupling of taxonomic diversity, body mass and craniodental morphology, underscoring the complexity of evolutionary dynamics even for sister clades that evolved on the same continent.