DIGITAL MODELING OF THE FUNCTIONAL EVOLUTION OF THE OPHIUROID ARM

Brittle stars (Class Ophiuroidea, Phylum Echinodermata) have evolved a unique rapid locomotion strategy which involves the use of coordinated whip-like arm motions effected by powerful musculature and hundreds of coordinated skeletal elements. This strategy differs drastically from the slow, tube-foot based locomotion of closely related living echinoderms and, presumably, their common ancestor with ophiuroids. It has been difficult to infer how modern ophiuroid locomotion evolved. Stem-group ophiuroids have skeletal morphologies distinct from their relatives in the crown complicating interpretations of their locomotion strategy and the group’s functional history. The use of 3D imaging and virtual modeling has revealed the movement capabilities of three stem-group ophiuroid taxa for the first time and allowed these locomotion strategies to be compared to those of extant taxa using 3D behavioral data from live ophiuroids. We observed several key mechanical differences between stem- and crown-group ophiuroids regarding, for example, articulations between skeletal elements and the range of motion of joint interfaces, which indicate significant disparities in their arm function. Through the incorporation of digital technology for biomechanical analyses, our results provide novel insights into the functional evolution of the ophiuroid arm.