Palaeoatmosphere facilitates a gliding transition to powered flight in Eocene bats

Abstract Bats are the only flying mammals but their transition to flight remains poorly understood and controversial. We applied aerodynamic modeling to reconstruct flight in the oldest complete fossil bat, the archaic Onychonycteris finneyi from the early Eocene of North America, under paleo-atmospheric conditions. Results indicate that Onychonycteris was capable of both gliding and powered flight either in a standard normodense aerial medium or in a hyperdense atmosphere estimated for the Eocene from two independent paleogeochemical proxies. Aerodynamic continuity across a morphological gradient is demonstrated by modeled intermediate forms with increasing aspect ratio (AR) produced by gradual digital elongation, such as indicated by chiropteran developmental data. Here a performance gradient emerged of decreasing sink rate with increasing AR that eventually allowed available muscle power to achieve level flight using flapping, a process greatly facilitated under elevated atmospheric densities. This gradient strongly supports a gliding (trees-down) transition to powered flight in bats.