MAISEY, J.G.; American Museum of Natural History: CT scanning and the evolution of the elasmobranch braincase

The archetypal status of the elasmobranch braincase is challenged by findings from CT scan analyses of both Recent and fossil shark neurocrania. Inner ear morphology in modern elasmobranchs is highly derived, and many of the same specializations are recognized in extinct hybodonts, although more primitive taxa such as xenacanths and symmoriids lack these features. An osteichthyan-like cranial fissure is primitively present in elasmobranchs, and the braincase of the cladistically primitive early Devonian chondrichthyan Pucapampella is anatomically quite similar to that of a primitive actinopterygian, Ligulalepis (also early Devonian), despite lacking a series of dermal bones. The platybasic arrangement of most elasmobranch braincases is probably primitive; the tropibasic arrangement (in which the interorbital septum is very narrow) is rarely developed, and characterizes chimaeroids and extinct symmoriid sharks. This seems to have come about by enlargement and posterior migration of the eyeballs, as in actinopterygians and some arthrodires.