- Titre du projet
- Axial skeleton of Mesozoic hexanchiform sharks
- Descriptif du projet
- Living hexanchiform sharks are characterized inter alia by reduced calcification patterns in the axial skeleton. The caudal vertebrae in the sharpnose sevengill shark (Heptranchias perlo) still display calcified double cones and secondary lamae, whereas those of the trunk are more or less completely uncalcified. In the bluntnose sixgill shark (Hexanchus), conversely, all vertebral centra are completely uncalcified. It has been proposed that this pattern represents a line of degeneration and that Hexanchus represents the most degenerated hexanchiform. Reduction of calcified vertebral structures might represent an adaptation to deep-water environments in both hexanchiforms. However, the processes underlying reduction of vertebral centrum calcification as well as its timing remain ambiguous. Calcification patterns of fossil hexanchiform sharks thus will provide a better understanding of these processes. Skeletal material of hexanchiform sharks, however, is very scarce. The oldest holomorphic species, Notidanoides muensteri from the Late Jurassic of Germany still displays well calcified vertebral centra. Hexanchus gracilis from the plattenkalks of Lebanon is the only Cretaceous hexanchiform known from skeletal material displaying calcified vertebral centra, which, however, are seemingly less calcified. Thus, this shark is important for understanding the pathways and probably also the start of reduction of axial skeleton in hexanchiform sharks.
Travaux réalisés par la plateforme AST-RX
