DAVIS, M.C.: The evolution of paired fin development: Insights from the basal actinopterygian Polyodon

The paired appendages of chondrosteans offer a good morphological and phylogenetic intermediate between derived actinopterygian (Danio) and derived sarcopterygian (Mus, Gallus) models. In particular, the paddlefish Polyodon exhibits a broad proximodistal area of overlap between the endochondral and dermal fin skeletons that is similar to the condition seen in some extinct sarcopterygians, namely rhizodontids and osteolepids. This overlap must be explained in light of some current models of fin development which predict little or no overlap between the two skeletal compartments. These models are based on the observation that the proliferation and patterning of the endochondral radials is maintained by signals from the apical ectodermal ridge (AER). However, they are also based on the assumption that the transformation of the AER into the apical ectodermal fold (AEF) is correlated with cessation of endochondral proliferation in the fin bud and the onset of lepidotrichia formation in the fin fold. While the predictions of these models are congruent with the condition seen in most neopterygians, such as Danio, they do not explain the intermediate morphologies of Polyodon and some fossil sarcopterygians. Models of appendage development based on a small number of model systems cannot account for the morphological and ontogenetic diversity seen in basal actinopterygian and sarcopterygian fins.