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.

#### Hook metadata

Tabstop-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.txt

#### Supplement Figure 1

Image 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.jpg

#### Raw outline coordinates

Morphometric 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.nts

#### Smoothed outline coordinates

Morphometric 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.nts

#### Harmonic coefficients

Tabstop-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.txt

#### Supplement Figure 2

Image 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.jpg

#### ReadMe file

Plain .txt file. This file contains detailed information about the other supplementary files.