Article: The origins and aerodynamics of flight in extinct vertebrates [Palaeontology Review]

Active (flapping, powered) flight has evolved in only three groups of vertebrates: pterosaurs (late Triassic), birds (late Jurassic), and bats (early Tertiary). Gliding has arisen many times in vertebrates, is a separate adaptation from flying, and does not appear to be a prerequisite for active flight. Skeletal features that distinguish flyers from gliders include modifications of the pectoral and forelimb apparati, elongation of the distal part of the wing skeleton for thrust, and certain physiological modifications that often leave clues in the skeleton. Soaring evolved in birds and pterosaurs secondarily, after powered flight was well established in both groups: it is a necessary result of phyletic size increase outstripping the ability to meet power requirements for sustained flapping.The origin of flight can be approached through a combination of phylogenetic, functional, and aerodynamic evidence. A basic question is whether flight evolved in the trees or on the ground. Of the three groups of active flyers, two (pterosaurs and birds) show no trace of gliding antecedents and appear to have evolved flight directly from the ground. Bats show many morphological and phylogenetic indications of an arboreal, gliding ancestry and are very different in all such respects from pterosaurs and birds. The theory of an arboreal origin of flight in birds so far lacks support from phylogenetic and functional-morphologic evidence; arguments In favour of this theory have invoked hypothetical selective advantages of features that either cannot be tested or apply equally to a terrestrial origin. Most of these features were already present in the coelurosaurian dinosaur ancestors of birds. Pterosaurs were structurally and functionally convergent on birds in many locomotory respects, and show prima-facie evidence of a cursorial, non-gliding origin of flight.Aerodynamic considerations of extinct vertebrates have mainly focused on two animals: Archaeopteryx (the first known bird) and Pteranodon (a specialized Cretaceous pterosaur). Functional inferences from skeletal evidence imply that Archaeopteryx was capable of flapping flight, though most 'modern' avian flight features were not developed; it does not seem well built for gliding. Pteranodon (a soarer, not a glider), like many large birds, was capable of active flight but probably only used it to take off, gain altitude, and avert danger. All pterosaurs were strong, active fliers and only large size constrained this ability. Aerodynamics of Pteranodon have commanded much productive interest, but nearly all models have been based either on (1) a morphologic analogy to bats, which is structurally incorrect, or (2) an aerodynamic analogy to certain low-speed aircraft or hang-gliders, which is both structurally and aerodynamically incorrect. Reappraisal of the anatomy and aerodynamic parameters indicate that Pteranodon's flight range was higher and that it was more active and manoeuverable than previous studies have suggested, and so more comparable to modern soaring birds. Studies of flight in extinct organisms cannot rely solely on engineering models or presumed selective advantages or pressures; they must take into consideration all aspects of phylogeny, function, and aerodynamics.