ORIGIN OF PTEROSAURIA AND STEPWISE EVOLUTION OF THE PTEROSAURIAN FLIGHT APPARATUS

Pterosaurian origins and the evolution of pterosaurian flight remain two of the greatest mysteries in the study of past life. Much of the difficulty in meaningfully examining these events derives from the unique and morphologically disparate bauplan of nearly all pterosaurs, limiting phylogenetic and ecomorphological comparison. However, the earliest known pterosaurs from the Triassic and earliest Jurassic represent exceptions to this hyper-derived bauplan and allow for the placement of pterosaur evolution back into the context of the rest of the vertebrate record. Here we reexamine the known record of early pterosaurs and perform phylogenetic and morphometric analyses to comprehensively investigate the relationships of Pterosauria and ecomorphological transformation associated with pterosaurian evolution. The anatomy of early pterosaurs reveals a suite of skeletal features unique to avemetatarsalian archosaurs, and Pterosauria is consistently recovered as the sister group to Dinosauromorpha within Ornithodira, with only distant affinities to other proposed pterosaur relatives like “prolacertiform-grade” archosauromorphs and lepidosaurs. Morphometric analyses reveal early archosauriforms, basal non-pterosaurian ornithodirans, early pterosaurs, and later derived pterosaurs show greatest respective morphological similarity to subcursorial, cursorial, arboreal-leaping, and flight-specialized extant taxa. Several features considered to be unique to the pterosaurian flight apparatus are found to arise within archosauriforms and are further developed in the earliest ornithodirans as specializations for quadrupedal cursorial locomotion. This stepwise assembly of the derived pterosaur bauplan continued into early Pterosauria with adaptations tied to takeoff and flapping flight occurring initially as specializations for arboreal gap-crossing and maneuvering behaviors, respectively, before being gradually expanded and exapted for use in sustained flight in non-arboreal, “archetypal” pterosaurs of the later Mesozoic.