HOOKS OR TEETH? THE STUDY OF MYSTERIOUS STRUCTURES IN RHAEBOCERAS HALLI

Hook-like structures are known in several species of Scaphitidae from the Campanian and Maastrichtian of the U.S. Western Interior. These bicuspidate structures were first described by Landman & Waage (1993) in Hoploscaphites, without any conclusive results on their function. Similar structures were found in 11 macroconchs of Rhaeboceras halli from the Bearpaw Shale (Kennedy et al. 2002) and identified as radular teeth. Based on the overall morphology of the hooks, Landman & Klofak (2004) doubted this function. Kruta et al. 2012 reached the same conclusions after discovering radular teeth (unlinked to these hook-like structures) between the jaws of Rhaeboceras halli. As a result, the function of these hook-like structures remains unknown, especially because comparable structures have never been observed in extant or fossil cephalopods. In order to provide new insights, accurate morphological studies are essential.

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.