Pentadactyl ground state of the manus of Alligator mississippiensis and insights into the evolution of digital reduction in Archosauria

The three-fingered state of the avian manus poses intriguing questions about the evolution of digit reduction. Although digit reduction in most tetrapods appears to be the product of straightforward digit loss, avian digit reduction may have occurred with a dissociation of digit position from digit identity. The three digits of birds have the ancestral identities of I, II, and III but develop from an early pentadactyl ground state from digital anlage 2, 3, and 4. A series of hypotheses have been proposed in an attempt to explain this disparity, including a recent suggestion that the anteriormost condensation visible in the avian limb bud is in fact a vestigial structure from a hexadactyl ancestral ground state. We investigated this proposal by presenting sets of compatible evolutionary developmental trajectories starting from a hexadactyl state to test hypotheses of digit reduction. The development of skeletogenic mesenchymal condensations in a crocodylian, the closest extant relative to birds, is used to identify any extra precartiloginous digital vestiges. A developmental series of Alligator mississippiensis forelimb buds reveal only five digital anlagen, supports a pentadactyl ground state for the archosaurian manus, and rejects portions of the evolutionary developmental trajectories proposed. This condition lends further support to the contribution of a homeotic transformation during digit reduction in avian ancestry to account for the dissociation between digital identity and developmental position.