Three-dimensional digital reconstructions of birds and dinosaur species successively more closely related to them have allowed John Hutchinson and colleagues to establish how and when the 'dinosaurian' body plan transformed into the typical 'avian' body plan. Birds have adopted a uniquely crouched hindlimb posture. Reconstructions of 17 archosaur species — including whole skeletons and bodies of Chinese fossil birds, Velociraptor and Archaeopteryx — indicate a gradual, stepwise acquisition of more-crouched limb postures across much of theropod evolution, with more rapid change in the maniraptorans (true birds and their immediate deinonychosaur relatives). Skeletal changes imply that pectoral limb modifications were important in shifting the mechanical balance of the body and hence the transformation of two key behaviours of birds — bipedalism and flight.
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