TESTING HYPOTHESES REGARDING SUPER-PRECOCIAL DEVELOPMENT IN ENANTIORNITHES

Since their origin, birds have been enormously successful, quickly diversifying into numerous lineages and encompassing significant ecological diversity. While their non-avian dinosaurian relatives appear to be predominantly precocial, developmental patterns in extant bird taxa vary along a spectrum from altricial to precocial, accompanied by marked differences in their early physiology, anatomy, and behavior. At one end of the spectrum, altricial hatchlings enter the world highly vulnerable: naked, blind, and entirely reliant on their parents. At the other end, precocial birds hatch covered in natal down with well-developed hindlimbs that allow them to forage independently, relying on their parents primarily for protection. An exception are the super-precocial Megapodidae, the so-called mound-builders, which can fly after emerging from the mound and receive no post-hatching parental care. The most abundant and successful clade of Mesozoic birds is the Enantiornithes, a group of predominantly arboreal birds that dominated terrestrial avifaunas throughout the Cretaceous. All available information suggests these birds were developmentally highly precocial, hatching fully independent and flight capable. This evidence includes highly ossified and or fledged embryos and hatchlings with fully developed remiges. To test the hypothesis that enantiornithine hatchlings were flight capable, we quantify growth trajectories in enantiornithines and compare them to ontogenetic changes in extant birds across the altricial-precocial developmental spectrum. This data supports previous interpretations that enantiornithines were precocial.