VICKARYOUS, M.K.; HALL, B.K.; Dalhousie University, Halifax, NS Canada; Dalhousie University, Halifax, NS Canada: A comparative and developmental study of the crocodylian dermal interclavicle and avian furcula

Common to all major amniote lineages, at least ancestrally, is an unpaired intramembranously derived postcranial element situated within the pectoral/sternal apparatus: the dermal interclavicle (monotremes, lizards and crocodylians), entoplastron (turtles), or furcula (birds). Whereas it is widely held that the turtle entoplastron is homologous with the dermal interclavicle, the avian furcula is considered the homologue of the paired clavicles of mammals. The “furcula-equals-clavicles” homology is supported by a failure of the two elements to coexist in the same organism, a shared topographic relationship to other pectoral/sternal elements, and a common mode of skeletogenesis. The crocodylian dermal interclavicle also shares the above characteristics, notwithstanding an obvious disparity in adult morphology. Our developmental evidence suggests that contrary to some earlier works, the dermal interclavicle is not comparable with the hypocleidium of the furcula. The avian furcula and crocodylian dermal interclavicle are positioned medial to the procoracoids and cranial to the sternum, along the ventral-most aspect of the pectoral apparatus. Developmentally, both the furcula and dermal interclavicle arise early during skeletogenesis as a single pair of condensations that unite and undergo intramembranous ossification (H.H. stage 33 of the domestic fowl and Ferguson stage 20/21 of the American alligator). A systematic comparison of skeletogenesis of the avian furcula, crocodylian dermal interclavicle and mammalian clavicle is discussed, and in this context an alternative scenario involving an interclavicular origin of the furcula is considered. Supported by NSERC and the Jurassic Foundation.