High flyer or high fashion? A comparison of flight potential among small bodied paravians

Abstract

The origin of flight in birds and its relationship to bird origins itself has achieved something of a renaissance in recent years, driven by the discovery of a suite of small-bodied taxa with large pennaceous feathers. As some of these specimens date back to the Middle Jurassic and predate the earliest known birds, understanding how these potential aerofoil surfaces were used is of great importance to answering the question: which came first, the bird or the wing? Here we seek to address this question by directly comparing key members of three of the major clades of paravians: anchiornithines, Microraptor and Archaeopteryx across their known size classes to see how they differ in terms of major flight-related parameters (wing loading; disc loading; specific lift; glide speed; takeoff potential). Using specimens with snout to vent length (SVL) ranging from around 150 mm to 400 mm and mass ranging from approximately 130 g to 2 kg, we investigated patterns of inter- and intraspecific changes in flight potential. We find that anchiornithines show much higher wing- and discloading values and correspondingly high required minimum glide and takeoff speeds, along with lower specific lift and flapping running outputs suggesting little to no flight capability in this clade. In contrast, we see good support for flight potential, either gliding or powered flight, for all size classes of both Microraptor and Archaeopteryx, though there are differing patterns of how this shifts ontogenetically. These findings, coupled with previous findings of a lack of wing-based locomotion in early-diverging troodontids or dromaeosaurids, suggest that flight was not a synapomorphy of Paraves. This supports the contention that flight capacity and potentially powered flight itself arose convergently in at least two distinct paravian lineages, first in birds minimally 150 million years ago, then in the microraptorines 20–30 million years later. Our work indicates that the origin of flight among paravians was likely a more complex phenomenon than previously appreciated.