Internal and external flight-related anatomy of early theropod fliers revealed by Laser-Stimulated Fluorescence fills knowledge gaps in functional morphology and flight capability

Abstract

Exceptional coelurosaurian theropod dinosaur fossils provide astonishing insight into the evolution of theropod flight. Comparative morphology and different modelling approaches (such as physical models and aerodynamic equations) have documented a range of early flying strategies (including thermal soaring) as well as the functional abilities of coelurosaurs that appeared just short of entering the aerial realm. Laser-Stimulated Fluorescence (LSF) is a rapid, non-destructive geochemical imaging technique that has important applications in early theropod flight studies because it can visualise otherwise hidden flight-related anatomies exposed as fine geochemical differences in these fossils. LSF has so far revealed near complete soft tissue body outlines including of the wings and legs as well as propatagial feather follicle patterns in a small sample of select early theropod flyers and near-flyers. Here we present LSF data from ~600 specimens of the short-tailed early bird Confuciusornis (Pygostylia: Confuciusornithidae) that for the first time reveals the soft tissue outline of the pectoral girdle and tail as well as subsurface details of the original propatagial muscle complex. We detect the form of the ancestral propatagial ligament, extensively muscled shoulders anchored to the well-known, unusually expanded deltopectoral crest of the humerus as well as reduced breast muscles associated with its sternumless ventrum. Our results directly record a novel avian flight architecture that pre-existed the ventrally-placed flight muscles of the modern avian flight apparatus and that was also relatively long-lived among Confuciusornithidae. Our results underscore the youth of the modern condition and the antiquity of the role of dorsally-positioned muscles in the avian flight stroke.