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

High disparity among avian forelimb and hind limb segments in crown birds relative to non-avialan theropod dinosaurs, potentially driven by the origin of separate forelimb and hind limb locomotor modules, has been linked to the evolution of diverse avian locomotor behaviors. However, this hypothesized relationship has rarely been quantitatively investigated in a phylogenetic framework. We assessed the relationship between the evolution of limb morphology and locomotor behavior by comparing a numerical proxy for locomotor disparity to morphospace sizes derived from a dataset of 1,241 extant species. We then estimated how limb disparity accumulated during the crown avian radiation. Lastly, we tested whether limb segments evolved independently between each limb module using phylogenetically informed regressions. Hind limb disparity increased significantly with locomotor disparity after accounting for clade age and species richness. We found that forelimb disparity accumulated rapidly early in avian evolution, whereas hind limb disparity accumulated later, in more recent divergences. We recovered little support for strong correlations between forelimb and hind limb morphology. We posit that these findings support independent evolution of locomotor modules that enabled the striking morphological and behavioral disparity of extant birds.

Wings versus legs in the avian bauplan: development and evolution of alternative locomotor strategies.

Wings have long been regarded as a hallmark of evolutionary innovation, allowing insects, birds, and bats to radiate into aerial environments. For many groups, our intuitive and colloquial perspective is that wings function for aerial activities, and legs for terrestrial, in a relatively independent manner. However, insects and birds often engage their wings and legs cooperatively. In addition, the degree of autonomy between wings and legs may be constrained by tradeoffs, between allocating resources to wings versus legs during development, or between wing versus leg investment and performance (because legs must be carried as baggage by wings during flight and vice versa). Such tradeoffs would profoundly affect the development and evolution of locomotor strategies, and many related aspects of animal ecology. Here, we provide the first evaluation of wing versus leg investment, performance and relative use, in birds-both across species, and during ontogeny in three precocial species with different ecologies. Our results suggest that tradeoffs between wing and leg modules help shape ontogenetic and evolutionary trajectories, but can be offset by recruiting modules cooperatively. These findings offer a new paradigm for exploring locomotor strategies of flying organisms and their extinct precursors, and thereby elucidating some of the most spectacular diversity in animal history.