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R Soc Open Sci
2020 Jun 10;76:200191. doi: 10.1098/rsos.200191.
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Arm waving in stylophoran echinoderms: three-dimensional mobility analysis illuminates cornute locomotion.
Clark EG
,
Hutchinson JR
,
Bishop PJ
,
Briggs DEG
.
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The locomotion strategies of fossil invertebrates are typically interpreted on the basis of morphological descriptions. However, it has been shown that homologous structures with disparate morphologies in extant invertebrates do not necessarily correlate with differences in their locomotory capability. Here, we present a new methodology for analysing locomotion in fossil invertebrates with a rigid skeleton through an investigation of a cornute stylophoran, an extinct fossil echinoderm with enigmatic morphology that has made its mode of locomotion difficult to reconstruct. We determined the range of motion of a stylophoran arm based on digitized three-dimensional morphology of an early Ordovician form, Phyllocystis crassimarginata. Our analysis showed that efficient arm-forward epifaunal locomotion based on dorsoventral movements, as previously hypothesized for cornute stylophorans, was not possible for this taxon; locomotion driven primarily by lateral movement of the proximal aulacophore was more likely. Three-dimensional digital modelling provides an objective and rigorous methodology for illuminating the movement capabilities and locomotion strategies of fossil invertebrates.
Figure 1. Stylophoran Phyllocystis crassimarginata. (a) Morphology of the aulacophore. The segments of the proximal aulacophore consist of two tectals (not illustrated) and two inferolaterals, creating a hollow tube [5]. The segments of the distal aulacophore are composed of two cover plates (not preserved here) and one larger ossicle. The stylocone is a massive ossicle connecting the proximal and distal portions of the aulacophore. (b) In situ position (left; both faces of fossil) and estimated maximal (max.) ranges of motion (ROM) versus in situ and straightened poses (right).
Figure 2. Stereo images of the proximal aulacophore (a) and stylocone (b) of Phyllocystis crassimarginata (UCBL-FSL 712515). Tri-coloured axes represent the coordinate system of the joint centre (see Material and methods). The centre of each axis is positioned at the joint centre and demarcates three rotational degrees of freedom: dorsoventral (red, x-axis), mediolateral (green, y-axis) and internal/external (blue, z-axis). Orientation of the coordinate system is based on [16]. Visualized using Autodesk Maya.
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