DOI: 10.5061/dryad.5q5m1
title: Data from: Grasping the shape of belemnoid arm hooks - a quantitative approach

description: 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.
description: Hook
 metadataTabstop-delimited .txt
 file. This file contains basic information about the micro-hooks
 investigated in this study. The columns contain the following: (1)
 Belemnite specimen: The number of the specimen, 1-3. (2) Hook name,
 morphological analysis: The label of the hook, corresponding to the labels
 used throughout the article and in the other data files. (3) Hook running
 number, fossil: The running number of the hook according to the
 catalogization on the rock sample. This information is only of interest if
 one wants to work with the sample specimens and needs to find a particular
 hook. (4) Hook cross sectional area [mm2], replication 1: The
 cross-sectional area of the hook (in square-millimetres) as measured
 during the first replication of the outline extraction. (5) Hook cross
 sectional area [mm2], replication 2: The cross-sectional area of the hook
 (in square-millimetres) as measured during the second replication of the
 outline extraction. (6) Hook distance to mouth [mm]: The measured distance
 of the hook to the presumed mouth of the belemnoid (in millimetres).
 Values for this parameter have only been measured for Specimen 1, because
 it was the only one which was preserved well enough to estimate the
 position of the mouth, and also there the value could not be measured for
 all hooks. Missing values are encoded as NA.Hook_Metadata.txtSupplement Figure 1Image in .jpg format. This file contains a visualization and describing caption of several stages of the outline analysis of the micro-hooks performed in this study. It includes: (1) The choice of the number of outline points extracted. (2) The choice of the amount of smoothing applied to the raw outline coordinates. (3) The choice of the number of harmonics that were calculated during the normalized elliptic Fourier analysis.OutlineProcedures.jpgRaw outline coordinatesMorphometric data file in .nts format. This file contains 70 x- and y-coordinates of the raw outlines of all 87 micro-hooks investigated in this study, across two replications of outline extraction (marked as ".R1" and ".R2" in the row labels). The data are provided as .nts file standard. The first line contains the metadata. The second line contains the row labels. The remaining lines contain the outline coordinates. Each line represents one individual hook. The coordinates are given in pairs of x and y. So the first value in the third line is the x-coordinate of the first outline point of hook number 1, the second value in the third line is the corresponding y-coordinate, the third value in the same line is the x-coordinate of the second outline point, the fourth value is the corresponding y-coordinate, and so forth.RawOutline.ntsSmoothed outline coordinatesMorphometric data file in .nts format. This file contains 70 x- and y-coordinates of the smoothed outlines of all 87 micro-hooks investigated in this study, across two replications of outline extraction (marked as ".R1" and ".R2" in the row labels). The data are provided as .nts file standard. The first line contains the metadata. The second line contains the row labels. The remaining lines contain the outline coordinates. Each line represents one individual hook. The coordinates are given in pairs of x and y. So the first value in the third line is the x-coordinate of the first outline point of hook number 1, the second value in the third line is the corresponding y-coordinate, the third value in the same line is the x-coordinate of the second outline point, the fourth value is the corresponding y-coordinate, and so forth.SmoothedOutline.ntsHarmonic coefficientsTabstop-delimited .txt file. This file contains the first ten harmonic coefficients for 87 micro-hooks investigated in this study across the two replicated outline extractions. The coefficients were derived from a normalized elliptic Fourier analysis with conserved starting point (tip of the hook) and normalization of the outline orientation on the basis of the longest axis of the ellipse described by the first harmonic. The first column contains the specimen label, O 1 to O 87. The second column contains the number of the replication, 1 or 2. The remaining columns contain the harmonic coefficients. The first to tenth harmonic coefficient are grouped by their components, and each component is numbered in the column header to indicate to which harmonic it belongs: (1) Columns 3-12 contain component A of the first ten harmonics. (2) Columns 13-22 contain component B of the first ten harmonics. (3) Columns 23-32 contain component C of the first ten harmonics. (4) Columns 33-42 contain component D of the first ten harmonics. (5) Columns 43-44 contain the zeroth harmonic ao and co.HarmonicCoefficients.txtSupplement Figure 2Image in .jpg format. This file contains a visualization of the error that is introduced into the elliptic Fourier analysis by manually tracing the micro-hooks as black-and-white image.ReplicationError.jpgReadMe filePlain .txt file. This file contains detailed information about the other supplementary files.