Laser-Stimulated Fluorescence reveals preserved soft tissues which unravel the complexity of avian flight evolution

Abstract

Fossils of exceptionally preserved theropod dinosaurs have pushed back the origins of important anatomies related to avian flight. The unique anatomical configurations they possess indicate a highly diverse road to modern flight capabilities. Whilst less flight proficient lineages went extinct, rare evidence exists of unusual adaptive trends towards improved flight proficiency. For example, our work with LaserStimulated Fluorescence produced a quantitative body outline of the basal bird Anchiornis, from the extinct anchiornithid clade. This revealed a narrow, extended wing form with undifferentiated feathering. Additional data is therefore paramount to understanding the full range of early avian flight development. Using Laser-Stimulated Fluorescence we discovered that the short-tailed early bird Confuciusornis (Pygostylia: Confuciusornithidae) had extensively muscled shoulders in contrast to reduced breast muscles. This evolved along the confuciusornithid lineage through incremental development of the deltopectoral crest of the humerus in the absence of a keeled sternum - opposite to the keel-dominated configuration of modern birds. We provide the first direct soft tissue evidence of an alternative musculoskeletal framework among early pygostylian birds, which led to novel, improved flight proficiency. These findings indicate that the transition to more modern crown birds during the Cretaceous included the elimination of lineages that had differentially improved flight proficiency, suggesting that this process of elimination was more nuanced than previously supposed. Such parallelism evident here presents a valuable opportunity to further explore the role of parallel evolution in shaping the development of avian flight.