Meeting Abstract
Birds have wide variation in flight capability, from flightless kiwi birds to hyper-aerial swifts. Their dinosaurian ancestors also show tremendous disparity in limb proportions and locomotor modes, spanning massive quadrupedal sauropods, carnivorous bipeds, and small bipedal herbivores. Key questions about locomotor macroevolution in dinosaurs include whether locomotor innovations were pulsed or gradual, and to what extent increases in locomotor disparity correlate with changes in patterns of integration both between and within limbs. We conducted disparity-through-time and integration analyses on a limb proportion database of 822 species of non-avian dinosaurs, fossil birds, and extant birds spanning 230 million years of evolutionary history. We found a pulse-like pattern with an early increase in relative subclade disparity of limb proportions coincident with the origin of major non-avian dinosaur clades during the Triassic and with the Early Cretaceous radiation of birds. There was a subsequent increase in disparity concurrent with the origin of crown Aves, followed by a large drop in disparity just prior to the end-Cretaceous extinction. Further, we found that shifts between locomotor modes were accompanied by a restructuring of within and between limb integration patterns. Flightless bipeds had moderate integration between limbs and high integration within limbs. In contrast, volant species had high integration within both limbs, but low between-limb integration. These results suggest that dinosaur and bird limb innovations evolved in pulses and that these pulses were strongly correlated with changes in limb integration regimes.
