SOFT TISSUE PRESERVATION IN PALEOZOIC OPHIUROIDS: NOVEL INSIGHTS THROUGH 3D IMAGING

Ophiuroids, commonly known as brittle stars, have evolved a complex and effective form of locomotion. They execute periodic coordinated arm motions with powerful musculature and complex joint articulations to facilitate quick and agile movement, making them the most motile class of echinoderms. However, little is known about the evolutionary steps that led to this unique form of locomotion during their divergence from comparatively slow-moving relatives. Although many stem ophiuroid taxa have been identified from as early as the Ordovician, the arm morphology of modern ophiuroids did not evolve until the Late Paleozoic. The morphology of stem ophiuroids is fundamentally disparate compared to the crown, and the functional capabilities of Paleozoic arm morphologies are essentially unknown. It is imperative to analyze how fossil ophiuroid taxa moved in order to elucidate the evolutionary steps that produced the derived locomotion strategy of modern ophiuroids. In order to do this, the first step is to determine the anatomy of the primary soft tissue involved in echinoderm locomotion in fossilized taxa. Here we present novel insights into Paleozoic ophiuroid soft tissue morphology, both of the water vascular system and muscle, based on micro-CT scans and synchrotron tomography of fossil specimens. The water vascular system of a pyritized specimen of Protasterina flexuosa from the Kope Formation of the Upper Ordovician of Kentucky, USA was revealed via a micro-CT scan. This is the first report of 3D preservation of the internal structure of the water vascular system in a fossil ophiuroid. Musculature of the ophiuroid arm was determined based on the stereom structure of isolated ambulacral ossicles from an unidentified stenurid ophiuroid from the Tournaisian, Lower Carboniferous of Tournai, Belgium imaged using synchrotron tomography at the Advanced Photon Source (Chicago). These insights further our understanding of the Paleozoic ophiuroid bauplan, allowing greater understanding of the evolution of the ophiuroid lineage both in terms of anatomy and functional morphology.