From fins to limbs to fins: limb evolution in fossil marine reptiles.

Limb osteology and ontogenetic patterns of limb ossification are reviewed for extinct lineages of aquatically adapted diapsid reptiles. Phylogenies including these fossil taxa show that paddle-like limbs were independently derived, and that the varied limb morphologies were produced by evolutionary modifications to different aspects of the limb skeleton. Ancient marine reptiles modify the limb by reducing the relative size of the epipodials, modifying the perichondral and periosteal surface of elements distal to the propodials, and evolving extremes of hyperphalangy and hyperdactyly. Developmental genetic models illuminate gene systems that may have controlled limb evolution in these animals.