Exploring the Pathways and Mechanisms of Evolutionary Transformation of Paired Appendages in Gnathostomes

The homology of paired appendages in gnathostomes—namely, fish fins and the limbs of terrestrial vertebrates—is widely recognized. At the same time, due to a number of significant morphological differences, establishing the evolutionary continuity and homology of individual structural elements of fins and limbs, and consequently reconstructing the scenario of evolutionary transformations that led to the emergence of terrestrial limbs, has been a topic of debate for decades. Recently, a substantial amount of new data has been obtained not only in the traditional fields addressing this issue—paleontology and comparative morphology—but also in more recent genetic studies. Of particular interest are studies of representatives of chondrichthyans, whose fins are considered as a baseline model for gnathostome paired limbs, as well as lungfishes, the closest relatives of terrestrial vertebrates. The data on the developmental features and gene architecture of fins in these key phylogenetic groups, which we aim to review in this article, in our view, significantly complement the overall picture of the evolutionary history of vertebrate paired limbs.