The “elongate chelicera problem”: A virtual approach in an extinct pterygotid sea scorpion from a 3D kinematic point of view

Chelicerae, distinctive feeding appendages in chelicerates, such as spiders, scorpions, or horseshoe crabs, can be classified based on their orientation relative to the body axis simplified as either orthognathous (parallel) or labidognathous (inclined), exhibiting considerable diversity across various taxa. Among extinct chelicerates, sea scorpions belonging to the Pterygotidae represent the only chelicerates possessing markedly elongated chelicerae relative to body length. Despite various hypotheses regarding the potential ecological functions and feeding movements of these structures, no comprehensive 3D kinematic investigation has been conducted yet to test these ideas. In this study, we generated a comprehensive 3D model of the pterygotid Acutiramus, making the elongated right chelicera movable by equipping it with virtual joint axes for conducting Range of Motion analyses. Due to the absence in the fossil record of a clear indication of the chelicerae orientation and their potential lateral or ventral movements (vertical or horizontal insertion of joint axis 1), we explored the Range of Motion analyses under four distinct kinematic settings with two orientation modes (euthygnathous, klinogathous) analogous to the terminology of the terrestrial relatives. The most plausible kinematic setting involved euthygnathous chelicerae being folded ventrally over a horizontal joint axis. This configuration positioned the chelicera closest to the oral opening. Concerning the maximum excursion angle, our analysis revealed that the chela could open up to 70°, while it could be retracted against the basal element to a maximum of 145°. The maximum excursion in the proxi mal joint varied between 55° and 120° based on the insertion and orientation. Our findings underscore the utility of applying 3D kinematics to fossilized arthropods for addressing inquiries on functional ecology such as prey capture and handling, enabling insights into their possible behavioral patterns. Pterygotidae likely captured and processed their prey using the chelicerae, subsequently transporting it to the oral  opening with the assistance of other prosomal appendages.

Methods
The three-dimensional model of Acutiramus sp. was created using Blender version 4.0.1. Initially, simple shapes such as cubes and spheres were generated. These were then processed in Object Mode in Blender using the "Catmull-Clark" modifier, transforming the initial six faces into a total of 6,144 faces. This allowed for detailed modeling and distortion of the individual faces, and ultimately the entire object, in Sculpting Mode using specific brushes such as "Grab," "Snake Hook," and "Elastic Deform." If necessary, the modifiers "Decimate" (to reduce the number of faces) or "Multiresolution" (to increase the number of faces) were applied in Object Mode to adjust the density. This process was carried out for all major body parts of the sea scorpion. The completed individual components were then scaled to the appropriate size according to the scales found in the literature and assembled into a cohesive object. 

In Autodesk Maya, the kinematic articulation of the individual legs was achieved by creating hypothetical joint axes in the form of simple cylinders, which were inserted into the articulation points of each individual joint, in accordance with the methodology and kinematic settings to be analyzed.

螯肢（Chelicerae）是螯肢动物（chelicerates，如蜘蛛、蝎子或鲎）特有的摄食附肢，可根据其相对于体轴的朝向简化分为直螯型（orthognathous，与体轴平行）或前螯型（labidognathous，与体轴倾斜），在不同类群中展现出显著的多样性。已灭绝的螯肢动物中，属于翼肢鲎科（Pterygotidae）的海蝎是唯一一类螯肢长度相较于体长显著延长的类群。尽管学界针对这类附肢的潜在生态功能与摄食运动提出了多种假说，但目前尚未有全面的三维运动学研究对这些假说进行验证。
本研究针对翼肢鲎科的尖肢鲎属（Acutiramus）构建了完整的三维模型，通过为其右侧延长的螯肢设置虚拟关节轴，实现了螯肢的可动性，以开展运动幅度（Range of Motion）分析。由于化石记录中未明确留存螯肢的朝向信息，也无法确定其是否存在侧向或腹向运动（即关节轴1的垂直或水平植入方式），因此本研究参考陆生螯肢动物的术语体系，设置了两种朝向模式：真直螯型（euthygnathous）与斜直螯型（klinogathous），共四种不同的运动学配置开展运动幅度分析。
最符合生物学合理性的运动学配置为：真直螯型螯肢沿水平关节轴向腹侧折叠，此时螯肢的位置最接近口部开口。关于最大活动角度，分析结果显示螯爪可张开至70°，同时可向基部结构收回到最大145°。根据关节轴的植入方式与朝向不同，近端关节的最大活动幅度介于55°至120°之间。本研究结果证实了将三维运动学分析应用于节肢动物化石的实用性，可用于解答诸如猎物捕获与处理等功能生态学问题，从而深入理解这类古生物可能的行为模式。翼肢鲎科大概率通过螯肢捕获并处理猎物，随后借助其他前体部附肢将猎物送入口部开口。

方法
本研究使用Blender 4.0.1版本构建了尖肢鲎未定种（Acutiramus sp.）的三维模型。首先创建立方体、球体等基础几何体，随后在Blender的对象模式下通过Catmull-Clark修改器对初始的6个面进行细分，最终生成总计6144个面。该操作支持后续在雕刻模式下使用“抓取（Grab）”“蛇形钩（Snake Hook）”与“弹性变形（Elastic Deform）”等笔刷对单个面乃至整个模型进行精细化建模与形变调整。若有需要，还可在对象模式下应用“精简（Decimate）”修改器（用于减少面数）或“多分辨率（Multiresolution）”修改器（用于增加面数）调整模型面密度。上述建模流程覆盖了该海蝎的所有主要身体部位。完成的独立部件将依据文献记载的尺寸比例进行缩放，最终组装为完整的整体模型。
在Autodesk Maya中，本研究参考预设的分析方法与运动学配置，通过在每个关节的连接点植入简单圆柱体形式的虚拟关节轴，实现了各附肢的运动铰接。