DOI: 10.5061/dryad.2fqz612x9
title: The “elongate chelicera problem”: A virtual approach in an extinct pterygotid sea scorpion from a 3D kinematic point of view

description: 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.
description: The three-dimensional model of <em>Acutiramus</em>
 sp. was created using Blender version 4.0.1. Initially, simple shapes such
 as cubes and spheres were generated. These were then processed in
 <em>Object Mode</em> 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
 <em>Sculpting Mode</em> 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 <em>Object Mode</em> 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.
description: # Acutiramus 3D kinematic models
 [https://doi.org/10.5061/dryad.2fqz612x9](https://doi.org/10.5061/dryad.2fqz612x9) Blender files showing the four permutations 1, 3, 5 and 8 (out of the eight) as an example for each of the four insertion and orientation modes (kinematic settings 1-4) of the right *Acutiramus* chelicera (Settings 1-4). ## Description of the data and file structure **The software: Blender** Blender is open access and available via: [https://www.blender.org.](https://www.blender.org.\\) The single Blender files can simply be opened via double click.  The model appears in the origin of coordinates.  Maybe, depending on the default settings, it is necessary to zoom out or in to find the model. Each individual element of the chelicerae of *Acutiramus* is selectable and can be hidden in Object Mode. **The 3D model of *Acutiramus*** **Kinematic Settings:** To clarify the most likely angle of orientation and movement direction of the chelicerae of *Acutiramus* that could account for their feeding ecology—especially in positioning prey items near the ventral surface and the gnathobasic coxae of the prosomal appendages for additional food processing—we utilized and modeled four different kinematic settings based on the terminology we introduced. Setting 1. euthygnathous chelicerae with a horizontal joint axis, allowing for dorsoventral motion Setting 2. euthygnathous chelicerae with a vertical joint axis, allowing for mediolateral motion Setting 3. klinognathous chelicerae allowing for an inclined dorsoventral motion Setting 4. klinognathous chelicerae allowing for an inclined mediolateral motion **Positions & Permutations:** Each joint in the chelicerae of *Acutiramus* is modeled bicondylar, allowing movement in only one plane (df = 1), which results in just two possible positions: maximum dorsal movement and maximum ventral movement. The full range of motion lies between these two extremes. These positions are labeled as 1 and 2, with position 1 representing an extended basal element (joint 1), an extended chela/fixed ramus (joint 2), and a closed chela/closed free ramus (joint 3). Conversely, position 2 involves a flexed basal element (joint 1), a flexed chela/fixed ramus (joint 2), and an opened chela/free ramus (joint 3). Given that three cheliceral elements (and thus three joints) can move between these two positions (1 and 2), this setup can be mathematically represented as eight potential "permutations with repetition" (2^3) (see Figure 3 and Schmidt et al., 2020, p. 1551, figure 5h). Consequently, there are eight possible maximum movements for each chelicera.