The evolution of pectoral fin development in gnathostomes

The following investigation focuses on the evolution of development in the paired fins of gnathostomes, in particular among the two major osteichthyan clades, actinopterygians and sarcopterygians. To approach these issues, studies of the comparative morphology of extant and fossil vertebrate taxa are combined with comparative developmental studies of relevant model and non-model organisms. Late Devonian fossil sarcopterygians provide insights in the evolution of morphological novelty in gnathostome paired fins. New specimens of the rhizodontid sarcopterygian, Sauripterus (Sarcopterygii, Osteichthyes) from the Late Devonian (Famennian) Catskill Formation of Pennsylvania possess pectoral fins with an elaborate endochondral skeleton sandwiched within a large dermal skeleton of unjointed lepidotrichia. Such an association between the endochondral and dermal fin skeletons would not have been predicted by some contemporary models of fin developmental evolution. These fossil sarcopterygians demonstrate that the endochondral and dermal skeletons may function together as a unit, yet still remain developmentally independent. Laboratory investigations of the development of basal actinopterygians also lead to the formulation of new hypotheses about the evolution of development. The study of pectoral fin development in the North American paddlefish Polyodon spathula and the white sturgeon Acipenser transmontanus reveals that aspects of both teleost and tetrapod endochondral patterning mechanisms are present in Acipenseriformes. Endochondral skeletal elements considered homologous to teleost radials form via subdivision of a chondrogenic disc while those elements considered homologous to the tetrapod limb (the metapterygium and the metapterygial radials) condense prior to extracellular matrix secretion. Such observations suggest that the distinct ways that teleosts pectoral fins and tetrapod forelimbs develop may not involve the acquisition of novel patterning mechanisms. Instead, these differences may involve correlative loss of patterning mechanisms specific to portions of the primitive gnathostome pectoral bauplan.