Morphology and osteo-histology of the weigeltisaurid wing: Implications for aerial locomotion in the world's first gliding reptiles.

The Late Permian Weigeltisauridae are the world's first gliding reptiles, but much remains unknown regarding the anatomy of their patagium (or wing), which, in turn, confounds our understanding of their gliding mechanism and paleobiology. Here, we examine the morphology and osteo-histology of the patagial skeleton of weigeltisaurids using an array of imaging techniques and several thin sections through the wing skeleton of a specimen of Weigeltisaurus from the Late Permian of Germany. We demonstrate that patagials and gastralia share a one-to-one articulation, permitted by the uniquely specialized anatomy of the lateral gastralia. We also show, based on skeletal anatomy, histology, and inferred musculoskeletal relationships, that patagials are likely neomorphic ossifications and are thus not strictly homologous to the gastralia. We provide the first reconstruction of the musculoskeletal anatomy of the weigeltisaurid wing, suggesting that the base of the patagials was likely embedded in the M. obliquus externus group. Similar to the condition in the extant flying lizard Draco, these muscles may have contributed to the unfolding of the patagium, which was likely supported by hooking the manual claws onto the leading edge of the patagium. This would have provided weigeltisaurids with a means to maintain the patagium expanded and under tension while gliding, as well as some measure of control of the dihedral angle of the wing, thereby offering a means to control stability and maneuverability in flight. Wing folding may have been permitted by muscular and tendinous connections between the elements of the patagial skeleton, generating elastic tension toward a folded state, as in Draco. Lastly, the cross sections of the patagials show a bimodal cortical distribution with much thicker cortices along their cross-sectional long axis than short axis. This made the patagials rigid, which likely helped prevent patagial collapse during gliding. This work represents a critical step toward understanding the wing structure and gliding mechanism in weigeltisaurids, paving the way for future morphofunctional or biomechanical studies on the locomotion of the world's first flying vertebrates.