Gradual build-up of powered flight potential among close avian relatives revealed by combining phylogenetic, aerodynamic and anatomical data

Abstract

The evolution of birds from paravian theropod dinosaurs is a classic evolutionary radiation, but a deeper understanding of it, including early flight history, has been frustrated by disagreements over the interrelationships of birds and their closest relatives, the dromaeosaurids and troodontids. We address these discrepancies through a larger, more resolved phylogeny produced by a novel automated pipeline tailored for large morphological datasets. This phylogeny reaffirms the deinonychosaurian grouping (dromaeosaurids+troodontids) as the sister taxon to birds, and corroborates the recovery of Anchiornithidae (Anchiornis and its close kin) as basalmost avialans. Using these phylogenetic results, wing loading and specific lift are calculated as proxies of flight capacity through this transition, using maximum and minimum estimates from available data for vaned feathered taxa. Wing loading and specific lift are then interpreted in the context of the updated phylogeny to provide lower and upper bounds for the evolution of powered paravian flight. The findings show a broad range of paravians with near flight capability, suggesting more widespread experimentation with wing-assisted locomotion than previously appreciated. In this context, we find that fully powered flight originated multiple times from these near flight-capable ancestors, including twice within dromaeosaurids (in Rahonavis and Microraptor).