Selection, constraints, and contingency drive mosaic patterns of evolution during the carangarian adaptive radiation.

Abstract

Adaptive radiations generate remarkable morphological diversity, yet traits often diversify at different rates and modes, producing mosaic patterns of evolution. Modularity, the semiautonomous organization of traits, can shape how selection and constraint interact during these radiations, but its macroevolutionary consequences remain unclear. We examined skull modularity during the post-Cretaceous radiation of carangarian fishes, a diverse marine clade that diversified along the benthic-pelagic axis after the end-Cretaceous mass extinction. Using three-dimensional morphometric data from 188 species integrated with phylogenetic comparative analyses, we found that the carangarian skull is highly modular and exhibits pronounced mosaic evolution. Oral jaw and "uro-cranium" modules evolved fastest and attained the greatest morphological disparity, whereas hyoid arch-derived modules evolved more slowly and remained comparatively constrained. Repeated transitions into pelagic habitats promoted often convergent oral jaw diversification across independent lineages, while elevated rates and disparity in the uro-cranium were concentrated within benthic flatfishes. These findings show how ecological opportunity and developmental architecture generate mosaic evolution during vertebrate radiations.