On the Evolution of Feathers from an Aerodynamic and Constructional View Point1

Abstract

The evolution of birds and feathers are examined in terms of the aerodynamic constraints imposed by the arboreal and cursorial models of flight evolution. The cursorial origin of flight is associated with the putative coelurosaurian ancestry of birds. As presently known, coelurosaurs have a center of mass located in the pelvic region and an elongated pubis that is ventrally or anteriorly directed. Both of these characteristics make it difficult to postulate an origin of flight that would involve a gliding phase because the abdomen cannot be flattened into an aerodynamic shape. Moreover, the cursorial model must counteract gravity using the hindlimb and, thus, selection for the power requirement for lift-off would not focus on the forelimb. Therefore, if the hypothesis proposing a coelurosaurian ancestry of birds is to remain viable, it must be via an as yet undiscovered taxon that is compatible with the morphological and aerodynamic constraints imposed by flight evolution.The arboreal model, currently centers around non-dinosaurian taxa and is more parsimonious in that early archosaurs have short pubes that do not preclude an aerodynamic body profile. Moreover, the arboreal proavis uses gravity to create the airflow over the body surfaces and is, thus, energy efficient. Consideration of the initial aerodynamic roles of feathers and feather design are consistent with a precursory gliding phase. Whether avian ancestry lies among coelurosaur theropods or earlier archosaurs, we must remain mindful of the complex aerodynamic dictates of gliding and powered flight and avoid formalistic approaches that co-opt sister taxa, with their known body form, as functional ancestors.