Abstract
SummaryThe bizarre scansoriopterygid theropods Yi and Ambopteryx had skin stretched between elongate fingers that form a potential membranous wing. This wing is thought to have been used in aerial locomotion, but this has never been tested. Using laser-stimulated fluorescence imaging, we re-evaluate their anatomy and perform aerodynamic calculations covering flight potential, other wing-based behaviors, and gliding capabilities. We find that Yi and Ambopteryx were likely arboreal, highly unlikely to have any form of powered flight, and had significant deficiencies in flapping-based locomotion and limited gliding abilities. Our results show that Scansoriopterygidae are not models for the early evolution of bird flight, and their structurally distinct wings differed greatly from contemporaneous paravians, supporting multiple independent origins of flight. We propose that Scansoriopterygidae represents a unique but failed flight architecture of non-avialan theropods and that the evolutionary race to capture vertebrate aerial morphospace in the Middle to Late Jurassic was dynamic and complex.
Highlights
The findings presented here, including the discovery of integumentary features and the presence of feathers lining the purported patagium, permits a detailed anatomical and aerodynamic analysis that informs our inferences of other members of this still poorly known clade
We find that the most likely reconstruction for Yi and Ambopteryx follows a mixed bat and maniraptoran model, indicating that the membrane may have been an adapted and exaggerated expression of the pro- and postpatagium found in other theropods (Kaye et al, 2015; Falk et al, 2016; Wang et al, 2017)
We infer the styliform element to be relatively fixed to the posterior edge of the carpus and that the membranous, feathered patagium associated with the manus articulates along its handward edge
Summary
The origin of birds is one of the most studied vertebrate macroevolutionary transitions (Makovicky and Zanno, 2011; Zelenitsky et al, 2011; Brusatte et al, 2014, 2015; Xu et al, 2014b; Dececchi et al, 2016). The discovery of more than one membrane-winged and potentially volant taxon within this poorly known clade is intriguing, especially since other members show both arboreal and terrestrial adaptations (Zhang et al, 2002, 2008), implying high ecological diversity within this group. Their unique wing construction has implications for understanding the developmental plasticity of the theropod forelimb and evolutionary variation of theropods as they tested the stringent physical constraints of flight (Dudley et al, 2007; Xu et al, 2014b)
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