Aerodynamics Show Membranous-Winged Theropods Were a Poor Gliding Dead-End

The bizarre Scansoriopterygidae theropods Yi and Ambopteryx are the only dinosaurs thought to have had skin stretched between elongate fingers that form a potential membranous wing. While these structures have been suggested to have been used in aerial locomotion, this has never been shown to be the case. Demonstrating this function is of great importance because if these wings permitted flight it would support the growing view that theropod dinosaurs evolved flight multiple times in distinctive ways before the origin of modern birds. Here using Laser-Stimulated Fluorescence imaging we re-evaluate the anatomy of Yi and integrate this new information into several different equation-based aerodynamic calculations covering flight potential, methods of take-off (terrestrial-based running, arboreal launch), and powered versus gliding flight capabilities. For the first time we test whether Yi and Ambopteryx had flight capabilities and if so how efficient and effective it was. We also evaluate wing-based behaviours including wing-assisted incline running (WAIR). We find that Yi and Ambopteryx were highly unlikely to have any form of powered flight and had limited gliding abilities compared to similar-sized living and fossil gliders. We find that both taxa could not take-off from a terrestrial setting, supporting the idea that they were likely arboreal. Compared to similar-sized early-diverging fossil paravians, like Archaeopteryx and Microraptor, Scansoriopterygidae show significant deficiencies and differences in flapping-based locomotion models that indicate a much less competent flight stroke with limited aerial abilities. Combining these findings, our results show that the Scansoriopterygidae are not models for the early evolution of bird flight and that their structurally distinct and inefficient wings differed greatly from contemporaneous paravians including the earliest birds. Instead, we propose that Scansoriopterygidae represent a unique but failed flight architecture of non-avialan theropods. These findings conform with emerging data supporting multiple independent origins of flight in modern lineages. They show that the battle to capture the vertebrate aerial morphospace in the Middle to Late Jurassic was a dynamic and complex one not restricted to pterosaurs and only one theropod lineage.