Meeting Abstract
Among cartilaginous fishes (Chondrichthyes), Holocephali is the sister group to the Elasmobranchii (sharks, rays). Crown group holocephalans lack teeth, instead having dental plates in the upper and lower jaws. In the Chimaeridae and Rhinochimaeridae, adult plates include extensive trabecular dentine supporting crushing tritoral pads along with unusual series of dentine ovoids along the lateral plate margins. Instead of ovoids, juveniles have elongate rods of dentine. In Chimaera, dentine includes the unusual calcium phosphate mineral whitlockite, containing magnesium (Mg). We examined the dentition of the rhinochimaerid Harriotta, to determine whether whitlockite occurs more broadly within the Holocephali, and to investigate dental plate development in juveniles and adults, where tissues are continually renewed deep to the oral surface. Here, rods, ovoids and tritoral pads develop within patterned and organized spaces in the trabecular dentine framework. Patterning is reminiscent of other chondrichthyan dentitions, but in adults, mineralizing tissue in ovoids and tritors is initially a granular, disorganised cluster of crystals differing in shape and composition (ß-tri calcium phosphate) from hydroxyapatite crystals in other mineralized vertebrate tissues. Elemental analysis shows there is relatively more Mg in these early-forming tissues, decreasing as mineral density increases towards the oral surface. We propose that Mg in dental plates characterizes at least the Chimaeridae and Rhinochimaeridae, and is related to the Jurassic origins of the Chimaeriformes in Mg-rich, aragonitic seas.
