Meeting Abstract
Male chondrichthyans bear a pair of pelvic claspers, but male chimaeras (Hydrolagus colliei) have five of these devices, including a pre-pelvic clasper on each half of the girdle, and a fifth clasper projecting from the forehead. Examples of such cranial claspers are present in all living and many fossil chimaeroids, at least as far back as the Mesozoic. Cranial clasper function is uncertain, but it appears likely that the structure is used for grasping the female during copulation (station holding?); to the best of our knowledge, this function and behaviour has not been properly documented. We suggest these claspers represent a classic evolutionary novelty, as well as an extra midline appendage unique to chimaeroids that has persisted throughout much of the phylogenetic history of the clade. However, these structures have received little scientific attention, and for this reason we investigated a cranial clasper from Hydrolagus colliei using micro-computed X-ray tomography and soft tissue thin sectioning and staining. Cranial claspers are already known to consist of a cartilage-supported shaft with a pad of denticles at the distal tip. Unexpectedly, we found that the denticles are strikingly tooth-like in terms of their individual morphology, and collectively in their patterned arrangement. Each denticle base underlies the base of its proximal neighbour, thus forming what appears to be a successive series. Taken together, these denticles resemble a tightly packed suite of tooth whorls, and we infer that these teeth developed in a coordinated, spatiotemporally restricted manner. This discovery prompts new consideration of cranial clasper biology, and whether these structures manifest a classic example of evolutionary developmental ‘bricolage.’
