SHIFTING PATTERNS OF FUNCTIONAL INTEGRATION DURING THE EVOLUTION OF FLIGHT IN THEROPODS

The transition to flight from theropod dinosaurs to birds during the Jurassic and Cretaceous was one of the most significant innovations in locomotion in evolutionary history; it allowed birds to exploit new ecological niches and survive the Cretaceous-Paleogene extinction event- one of the worst mass extinctions in Earth’s history. Flight fundamentally changed the function of the limbs, shifting the primary mode of locomotion from the hind limbs of non-avian theropods to the forelimbs of birds. The biomechanical mechanisms for hind limb locomotion also changed during the evolution of flight, functionally decoupling the tail from the hind limb and allowing it to be used as a rudder during flight.

Previous research on limb element proportions hypothesized that non-avian theropod dinosaurs had one functional module of locomotion made up of the hind limbs, hips, and tail while birds transitioned to three locomotor modules: the forelimbs, the hind limbs and hips, and the tail.

In this study, we tested whether the patterns of phenotypic integration changed and whether the number of functional modules involved in locomotion transitioned from one to three modules during the evolution of avian flight. In contrast to previous work that focused on limb element length and element proportions, we used geometric morphometrics and a suite of complimentary analyses including common principal components analyses, two-block partial least squares, and cluster analyses to identify and compare the evolutionary integration patterns of both fossil and recent hind limbs, hips, and forelimbs of 48 species of non-avian theropods and 42 species of birds.

We found that the patterns of evolutionary integration are different in birds and non-avian theropods. This supports the hypothesis that locomotor modules changed during the evolution of flight in theropods. However, we propose that rather than three avian modules of locomotion, the tail and forelimbs act in concert during flight and thus form a single functional module. We also found that non-avian theropods exhibited tighter evolutionary correlations than birds in both the fore- and hind limbs. We hypothesize that this may be due to the diverse functions of the forelimbs (e.g. gliding, flapping, soaring, etc.) and hind limbs (e.g. prey capture, swimming, perching, etc.) of birds.