On the evolution of feathers from an aerodynamic and constructional view point

The evolution of birds and feathers are examined in terms of the aerodynami c constraints imposed by the arboreal and cursorial models of flight evolut ion, The cursorial origin of flight is associated with the putative coeluro saurian ancestry of birds. As presently known, coelurosaurs have a center o f mass located in the pelvic region and an elongated pubis that is ventrall y or anteriorly directed. Both of these characteristics make it difficult t o postulate an origin of flight that would involve a gliding phase because the abdomen cannot be flattened into an aerodynamic shape. Moreover, the cu rsorial model must counteract gravity using the hindlimb and, thus, selecti on for the power requirement for lift-off would not fetus on the forelimb, Therefore, if the hypothesis proposing a coelurosaurian ancestry of birds i s to remain viable, it must be via an as yet undiscovered taxon that is com patible with the morphological and aerodynamic constraints imposed by fligh t evolution. The arboreal model, currently centers around non-dinosaurian taxa and is mo re parsimonious in that early archosaurs have short pubes that do not precl ude an aerodynamic body profile. Moreover, the arboreal proavis uses gravit y to create the airflow over the body surfaces and is, thus, energy efficie nt. Consideration of the initial aerodynamic roles of feathers and feather design are consistent with a precursory gliding phase. Whether avian ancest ry lies among coelurosaur theropods or earlier archosaurs, we must remain m indful of the complex aerodynamic dictates of gliding and powered flight an d avoid formalistic approaches that co-opt sister taxa, with their known bo dy form, as functional ancestors.