EVOLUTIONARY PALEOECOLOGY OF EARLY BENTHIC ECHINODERMS: CONVERGENCE OF CAMBRIAN AND LATE DEVONIAN FORMS

As bioturbation depth and intensity increased during the Proterozoic-Phanerozoic transition, the soft substrates on which benthic metazoans lived changed from typically firm Proterozoic-style soft substrates with a sharp sediment water interface and low water content to typically softer Phanerozoic-style soft substrates with a blurry sediment water interface, high water content, and well-developed mixed layer. Microbial mats, once dominant on normal marine Proterozoic seafloors, were relegated to stressed settings with inhibited metazoan activity. Initial work indicates that the ecological impact of this substrate change influenced the evolution of Cambrian benthic suspension-feeding echinoderms. This study tests these initial results by examining the substrate adaptations of the three classes of benthic suspension-feeding echinoderms with Cambrian origins, the edrioasteroids, eocrinoids, and helicoplacoids, at the genus and stage level throughout their evolutionary histories. Based on the primary literature, the adaptive morphologies of 97 genera were interpreted and their adaptation styles plotted at the stage level from the Lower Cambrian through the Pennsylvanian. Results show that genera adapted to typical Proterozoic-style soft substrates, sediment attachers, sediment resters, and shallow sediment stickers, taxonomically dominate, and are almost entirely restricted to, the Early and Middle Cambrian. Thereafter, genera adapted to typical Phanerozoic-style soft substrates, primarily hard substrate attachers, are taxonomically dominant. The one exception is the Late Devonian edrioasteroid Cooperidiscus, which is morphologically convergent with the sediment-resting Early to Middle Cambrian edrioasteroid Stromatocystites. This convergence suggests that Cooperidiscus was adapted to firm substrates created by the ecological stress, and resulting lower bioturbation levels, of the Late Devonian. This study demonstrates that the unusual morphologies of many Cambrian echinoderms are due to their adaptation to non-actualistic substrate conditions, and that the later development of similar non-actualistic substrates during ecological crises can result in the evolution of metazoans morphologically convergent with those of the earliest Phanerozoic.