The diversification of the acanthomorph fishes: Ecomorphological perspectives on an evolutionary radiation

Acanthomorph fishes make up nearly one third of all living vertebrate species. Paleontological datasets imply that acanthomorphs underwent a prolific taxonomic radiation in the aftermath of the end-Cretaceous extinction. A clear understanding of the effects of this event upon marine fishes forms necessary context for investigating the subsequent diversification of acanthomorphs. Patterns of extinction were investigated with respect to two ecologically and functionally relevant aspects of anatomy: jaw closing mechanical advantage and body size. Raw genus values indicate selectivity against taxa with large body sizes and low jaw closing mechanical advantages, with body size being the most important predictor of extinction risk. Analyses that correct for phylogenetic non-independence show no relationship between size and vulnerability, but do recover a significant link between biomechanically 'fast' jaws and elevated extinction risk.;Patterns of morphological evolution among acanthomorphs both prior to and after the end-Cretaceous extinction event were investigated using a geometric morphometric dataset representing over 600 extinct species. Landmarks were selected to capture gross aspects of cranial and postcranial morphology. Acanthomorphs show low levels of morphological disparity during the Cretaceous, an increase in disparity between the end of the Cretaceous and the Early Eocene, and roughly consistent patterns of morphospace occupation throughout the remainder of the Cenozoic. A similar pattern is apparent within and between fossil acanthomorph faunas: individual fossil assemblages become more diverse and distinct from one another after the end-Cretaceous. Morphological diversity accumulates more quickly for skulls than bodies. This pattern appears consistent with the predictions of some models of adaptive radiation, which propose that morphological divergence early in diversification events should be concentrated along trophic lines. Fitting statistically explicit models of trait evolution to these datasets provides reveals divergent evolutionary patterns underlying these contrasting trajectories of diversification.;Many modern acanthomorph groups display very specialized bodyplans. In many cases, living outgroups provide few clues about the sequence of character evolution leading to these apomorphic clades. Fossil data presented here document the origin of the unusual anatomy of flatfishes, and show that the cranial asymmetry characterizing all living members of this group arose in stepwise fashion.