Early diversification of avian limb morphology and the role of modularity in the locomotor evolution of crown birds.

The article investigates the evolution of avian forelimb and hind limb morphology in relation to locomotor behavior across 1,241 extant bird species, testing the hypothesis that separate locomotor modules (forelimb and hind limb) evolved independently and contributed to diverse avian locomotor strategies. Using morphometric data and phylogenetic comparative methods, the study finds that hind limb morphological disparity correlates positively with locomotor disparity, while forelimb disparity is more influenced by clade age and species richness. Forelimb disparity accumulated rapidly early in avian evolution, whereas hind limb disparity diversified later within clades. Additionally, evolutionary covariation is stronger within each limb module than between forelimbs and hind limbs, supporting the idea of modular evolution facilitating morphological and behavioral diversity in birds.