Using patterns of fin and limb phylogeny to test developmental-evolutionary scenarios

Increasing fossil evidence surrounding the evolutionary origin of vertebrate limbs can be used to reconstruct the assembly of a limb ground-plan common to all tetrapods. The sequence of changes at the fin-to-limb transition can be compared to patterns of fin and limb ontogeny, and further comparisons can be made between phylogenetic changes at pectoral and pelvic levels. Such comparisons inform questions about the evolution of developmental autonomy (modularity). Limb evolution mostly concerns terminal additions and losses; from a developmental standpoint, these probably result either from minor adjustments to limb bud proportions or from the relative timing of gene expression or tissue growth. Evolutionary radiations of large clades are widely assumed to be marked by periods of rapid morphological diversification, raising further questions about the impact of restrictions imposed not only by ecology, but also by development and genetics. The early tetrapod data set is now large enough to allow initial tests of evolutionary inference to be conducted. New results are revealing novel patterns of evolutionary rate-change, encompassing the traditional notion of the fish-to-tetrapod transition and the root of the modern (crown-group) tetrapod radiation.