The “elongate chelicera problem”: A virtual approach in an extinct pterygotid sea scorpion from a 3D kinematic point of view
Data files
Sep 11, 2024 version files 837.88 MB
Abstract
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.
README: Acutiramus 3D kinematic models
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.
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.
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.