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

Chitinous arm hooks (onychites) of belemnoid coleoids are widely distributed in Mesozoic sediments. Due to their relative abundance and variable morphology compared to the single bullet-shaped, belemnite rostrum, arm hooks came into the focus of micro-palaeontologist 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 as well as 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 specialisation 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.

箭石型蛸亚纲动物的几丁质腕钩（onychites）广泛分布于中生代沉积物中。相较于单一子弹状的箭石鞘，这类腕钩不仅相对富集，且形态多样，因此成为微古生物学家关注的焦点，作为侏罗纪-白垩纪岩石记录中极具潜力的标准化石类群，过往已有诸多针对其功能、生态及系统发育的研究阐释。本研究基于三件保存完好的托阿尔阶双箭石类群Chondroteuthis wunnenbergi的腕冠标本，共计分析了87个微型腕钩的形态特征。Chondroteuthis的腕冠具有独特的单列腕钩结构，而非常见的双列排列。本研究首次将椭圆傅里叶形状分析应用于箭石型蛸亚纲动物的腕附器，成功区分出四种微型腕钩形态型，并实现了各形态型内部形状变异的量化分析。基于保存最完好的腕冠标本，本研究重建了各形态型在腕冠内部以及单根腕上的分布模式。本研究的定量数据验证了既往观测结果：小型腕钩多分布于口部附近及腕部最远端，而大型腕钩则集中于腕冠的中央区域。此外，本研究还发现了显著的腕部分化现象：不同腕足搭载的腕钩形态型并不完全一致。本研究首次报道了箭石类腕冠的功能特化现象，并通过与现代头足类的类比，推测了四种腕钩形态型的潜在功能。本研究分析表明，箭石类腕钩具有高度特化的功能形态，且在个体内部呈现非均匀分布，其形态与分布模式主要服务于猎物捕获过程中的不同功能需求。