Data from: Grasping the shape of belemnoid arm hooks - a quantitative approach

Chitinous arm hooks (onychites) of belemnoid coleoid cephalopods are
widely distributed in Mesozoic sediments. Due to their relative abundance
and variable morphology compared with the single, bullet-shaped, belemnite
rostrum, arm hooks came into the focus of micropaleontologists as a
promising index fossil group for the Jurassic–Cretaceous rock record and
have been the target of functional, ecological, and phylogenetic
interpretations in the past. Based on three well-preserved arm crowns of
the Toarcian diplobelid Chondroteuthis wunnenbergi, we analyzed the shape
of a total of 87 micro-hooks. The arm crown of Chondroteuthis is unique in
having uniserial rather than biserial hooks. The first application of
elliptic Fourier shape analysis to the arm weapons of belemnoid coleoids
allows for the distinction of four micro-hook morphotypes and the
quantification of shape variation within these morphotypes. Based on the
best-preserved arm crown, we reconstructed the distribution of morphotypes
within the arm crown and along a single arm. Our quantitative data support
former observations that smaller hooks were found close to the mouth and
at the most distal arm parts, while the largest hooks were found in the
central part of the arm crown. Furthermore, we found a distinct arm
differentiation, as not every arm was equipped with the same hook
morphotype. Here, we report the functional specialization of the belemnoid
arm crown for the first time and speculate about the potential function of
the four morphotypes based on comparisons with modern cephalopods. Our
analyses suggest a highly adapted functional morphology and
intra-individual distribution of belemnoid hooks serving distinct purposes
mainly during prey capture.