Origin of Tetrapod Forelimb Pronation

All four-limbed vertebrates and their descendants are called tetrapods. The transition from an aquatic to a terrestrial environment has been viewed as one of the most remarkable events in all of tetrapod evolution (Shubin 1995; Boisvert 2005; Daeschler and Jenkins 2006; Shubin, Daeschler and Jenkins 2006).The taxonomic group Tetrapodomorpha, which includes tetrapods and tetrapodomorph fish with limb bone precursors in their fins (Ahlberg and Johansen 1998), documents this transformation. This transition involved major changes in functional morphology as the limbs replaced the tail as the main organs of propulsion.Although posture and locomotion are commonly regarded as two of the most significant sources of complex characters used to separate tetrapod clades (e.g., Charig 1972; Lewis 1983; Gauthier 1984, 1986), the phylogenetic changes in joint planarity and fin-to-limb orientation from fish (Romer and Byrne 1931; Rackoff 1976, 1980), to basal tetrapods (Shubin 1995), and finally to amniotes, are not well studied (Rewcastle 1977;Lewis 1983). This project will focus on a particularly neglected and problematic aspect of this locomotor transition (cf. Boisvert 2005), the evolution of the position and mechanism of radial pronation.