The  growing field of palaeontological CT scanning has only recently begun to be  applied to fossil chondrichthyan fishes (i.e., sharks, rays, and chimaeroids).  In recent years, CT scan-based studies have provided new information on  chondrichthyan cranial morphology, particularly regarding internal features such  as the interior surface of the braincase and the inner ear.

Many of these  features have turned out to be significant in shedding new light on patterns of  chondrichthyan evolution. Hybodonts, the sister group of modern sharks, are of  particular interest in what they can reveal about the evolutionary history of  their living relatives.

The inner ear of modern sharks (neoselachians) is highly  adapted toward low-frequency semi-directional sound detection (LFSDP). New  investigations of two fossil hybodonts (Tribodus limae and Egertonodus basanus) using  high-resolution CT scanning confirms that the structure of the inner ear in  these sharks was also adapted for LFSDP. However, this adaptation is absent in  earlier chondrichthyans (e.g., symmoriiforms, ctenacanths, Pucapampella), suggesting that it arose  only after the divergence of the hybodont/neoselachian lineage from these  earlier groups. Other features of evolutionary interest include.the loss of the  cranial fissures and elaboration of the vagal and glossopharyngeal nerve canals;  development of a medial capsular wall; and changes in patterns of cranial  arterial circulation.

In facilitating identification of key features such as  positions of nerve and blood vessel pathways and foramina, CT scanning and  digital reconstruction techniques may also pave the way for future developmental  studies (such as reconstructing the positions and growth patterns of the  embryonic cranial cartilages).

Contact: Greg Edgecombe  g.edgecombe@nhm.ac.uk