Arm waving in stylophoran echinoderms: three-dimensional mobility analysis illuminates cornute locomotion

Area of Science:

• Paleontology
• Biomechanics
• Evolutionary Biology

Background:

• Interpreting fossil invertebrate locomotion often relies on morphology, but this can be misleading.
• Homologous structures in extant invertebrates don't always indicate similar locomotion capabilities.

Purpose of the Study:

• To develop and apply a new methodology for analyzing locomotion in fossil invertebrates with rigid skeletons.
• To reconstruct the locomotion strategy of the cornute stylophoran Phyllocystis crassimarginata.

Main Methods:

• Digitized three-dimensional (3D) morphology of Phyllocystis crassimarginata.
• Analysis of the range of motion of the stylophoran arm.
• 3D digital modeling to simulate movement capabilities.

Main Results:

• Previously hypothesized arm-forward epifaunal locomotion via dorsoventral movements was not possible for this taxon.
• Locomotion was likely driven primarily by lateral movement of the proximal aulacophore.
• 3D digital modeling proved an objective and rigorous method for analysis.

Conclusions:

• The study challenges previous locomotion hypotheses for cornute stylophorans.
• 3D digital modeling offers a powerful tool for understanding extinct invertebrate movement.
• This methodology can be applied to other fossil invertebrates with rigid skeletons.