Gravity-defying Behaviors: Identifying Models for Protoaves1

SYNOPSIS. Most current phylogenetic hypotheses based upon cladistic methodology assert that birds are the direct descendants of derived maniraptoran theropod dinosaurs, and that the origin of avian flight necessarily developed within a terrestrial context (i.e., from the ‘‘ground up’’). Most theoretical aerodynamic and energetic models or chronologically appropriate fossil data do not support these hypotheses for the evolution of powered flight. The more traditional model for the origin of flight derives birds from among small arboreal early Mesozoic archosaurs (‘‘thecodonts’’). According to this model, protoavian ancestors developed flight in the trees via a series of intermediate stages, such as leaping, parachuting, gliding, and flapping. This model benefits from the assemblage of living and extinct arboreal vertebrates that engage in analogous non-powered aerial activities using elevation as a source of gravitational energy. Recent reports of ‘‘feathered theropods’’ notwithstanding, the evolution of birds from any known group of maniraptoran theropods remains equivocal.