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.