Meeting Abstract
Brittle stars (Class Ophiuroidea), the most motile group of echinoderms, have evolved a quick, agile and effective form of locomotion in which coordinated arm movements are executed by powerful musculature and complex joint articulations. Little is known regarding the evolutionary steps involved in the development of this form of locomotion from relatively slow-moving ancestors. Ophiuroids originated in the Ordovician, yet the morphology of the modern ophiuroid arm did not emerge until the evolution of the crown clade in the Late Paleozoic. The arm morphology of stem ophiuroids is fundamentally different to that of the crown, particularly with regard to soft tissue morphology and the arrangement of the ossicles hosting the muscle/joint interfaces. The functional importance of these differences has not been tested. In order to examine the derivation of ophiuroid locomotion, the significance of this disparity needs to be assessed. We analyzed the functional implications of stem/crown differences in arm morphology using skeletal morphology and preserved and inferred soft-tissue organization. Novel insights regarding the water vascular system (WVS) in Paleozoic ophiuroids are based on a micro-CT scan of exceptionally preserved (pyritized) WVS structures in a specimen of Protasterina flexuosa from the Upper Ordovician Kope Formation of Kentucky, USA. Muscle architecture of Paleozoic ophiuroids was reconstructed based on the skeletal microstructure of isolated arm ossicles from a stenurid ophiuroid from the Lower Carboniferous (Tournaisian) of Tournai, Belgium imaged using synchrotron tomography at the Advanced Photon Source (Chicago). The differential morphology of the skeletal structures and their functional significance was assessed using 3D modeling. These analyses provide novel insight into the functional capabilities of Paleozoic ophiuroids and greater understanding of the evolution of the ophiuroid lineage.
