EVALUATING STENUROID (ASTEROZOA) DIVERSITY IN THE LATE PALEOZOIC BASED ON LATERAL ARM PLATE MICROFOSSILS

Stenuroidea is a clade of asterozoan echinoderms that have variously been interpreted as stem ophiuroids or a class unto themselves. They have a poor fossil record that has generally been interpreted to be most abundant and diverse during early and middle Paleozoic, with a scant fossil record in younger rocks. Recent surveys of Middle Mississippian through Lower Permian for ophiuroid lateral arm plate (LAP) microfossils have uncovered a rich, late Paleozoic stenuroid fossil record in North America, represented by four genera and numerous species. By far the most commonly encountered stenuroid is Umerophiura which is represented in most units by one or two species and, at one locality, is the most abundant plate type in the fauna. This taxon is characterized by paddle-shaped LAPs with a long articulation peg, a fringe of fixed spines along the distal margin and lateral multiple rows of simple lateral spine bases along the edge of the underside of the plate. Umerophiura shows systematic proximal to distal variation of LAPs, with the plates bearing a lower height to length ratio along the arm. Species can be differentiated by a combination of characters such as external surface ornamentation, shape of the main plate body, size and shape of the vertebral articulation peg and size, shape and distribution of fixed spines along the distal margin. A second stenuroid taxon, Eriniceaster, is characterized by more scythe-shaped LAPs with simple spine articulations positioned along the distal margin of the exterior surface. This taxon is rarely encountered but has been found in two Viséan localities. A third taxon, Sturtzaster, bears conical, paddle-shaped LAPs with a long articulation peg and a single row of simple spine articulations. It has only been found at one latest Tournaisian locality. Preliminary results suggest that the Paleozoic hangover ophiuroids recently described from Triassic deposits, in fact, belong to Sturtzaster. Lastly, Antiquaster LAPs are characterized by elongate, boot-shaped LAPs with a wide and thin adradial margin and an elongate peg leading to two terminal spine bases abradially. These plates are found sporadically from the Viséan through Gzhelian. Our results show that, rather than becoming rare after the Devonian Biodiversity Crisis, stenuroids remain common and fairly diverse components of Late Paleozoic faunas and even extend into the Triassic.