Data from: ENIGMATIC HOOK-LIKE STRUCTURES IN MESOZOIC AMMONITES (SCAPHITIDAE)

In the last few decades, hook-like structures have been reported in the Mesozoic ammonite family Scaphitidae. Despite their exceptional preservation and debates about their function, no detailed reconstruction of them has yet been made. For the first time, we describe the composition and detailed morphology of these structures in the body chambers of six specimens of the Campanian ammonite Rhaeboceras halli (Meek and Hayden) using high resolution X-ray imaging. The hook-like structures are composed of a thin layer of brushite. The base of the hooks is open on one side forming an internal cavity, now filled with sediments. The tips of the hooks end in one or two cusps or, rarely, none at all. We used geometric morphometrics to capture the morphological disparity of the bicuspidate morphotypes comprising 98% of the hooks. Principal component analysis revealed chirality among the hooks and a cluster analysis (Gaussian mixture) recognized five main morphologies. Contrary to previous interpretation of these structures, we conclude that they are not radular teeth. They are much larger and more variable in size and shape than any known ammonite radulae and completely out of proportion with respect to the size of the jaw. The chirality, the hook-like shape, and the absence of a size relationship between the hooks and the body chambers in which they occur, lead us to propose that these hooks could represent elements of the brachial crown related to copulatory behavior. If so, these would be the first reported remnants of brachial crowns in ammonites.Photographic plate.Photographs of the studied specimens with hook-like structures preserved. A. AMNH 66448, phragmocone (beginning arrowed) and part of the body chamber. B. AMNH 66350, fragment of the body chamber, ventral view. C. AMNH 66351, phragmocone (beginning arrowed) and part of the body chamber. D. AMNH 66433 fragment of the body chamber with lower jaw (aptychus) preserved (arrow). E.AMNH 66434 bodychamber. AMNH 64405 is figured in Figure 2.Appendix_S1.pdfSynchrotron scanning and Conventional CT scanning parametersAppendix_S2.pdfAMNH 64405 scanning parameters (.pca file)Appendix_S3.pcaTIFF z stack of specimen AMNH-64405Appendix_S4.zip.zipXRD report and curveThe main crystalline phase identified is Brushite. The remaining phases exhibit semi-crystalline characteristics and poor peak shapes, therefore considered estimated phases. The pattern consists of two amorphous regions, from 20-30 deg 2-theta and 40-60 deg 2-theta. The first region is charateristic of amorphous compounds that have longer bonds such as organic compounds. The second is characteristic of metallic or inorganic amorphous materials.Appendix_S5.pd