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.