HOOKS OR TEETH? THE STUDY OF MYSTERIOUS STRUCTURES IN RHAEBOCERAS HALLI
The purpose of this work is to formally describe the morphological diversity of the hooks in order to identify and define the morphogroups. The hooks are preserved in the ammonite body chamber and their morphology was revealed by propagation phase contrast synchrotron X-ray microtomography (at ESRF) a non-destructive 3D analysis technique. The hooks were reconstructed in 3D and measurements were taken and analysed (cluster analyses) in order to describe and compare different morphologies. At least 50 hooks are present per specimen and more than 8 different morphogroups are identifiable. Hooks that are disposed in a circular pattern differ in their shape and size. Left and right hooks could be identified based on the position of the longest cusps. Most of the specimens have two cusps except some rare occurrences (e.g., rounded or tricuspid hooks). Although the different morphogroups differ greatly in size (e.g., the biggest is up to ten times longer than the smallest) no intermediate sizes were observed that could be related to growth patterns. Results clearly confirm that these structures are not radular teeth. This preliminary work will be extended to the 25 specimens in the AMNH collections in order to provide the quantitative and qualitative data necessary for comparative studies.