Evolutionary palaeoecology of early epifaunal echinoderms: Response to increasing bioturbation levels during the Cambrian radiation

Abstract

Increasing bioturbation depth and intensity during the Cambrian radiation created a transition in dominant subtidal soft substrates from firm Proterozoic-style soft substrates without a well-developed mixed layer to soupier Phanerozoic-style soft substrates with a well-developed mixed layer. Microbial mats were also relegated to settings with inhibited metazoan activity but made resurgences during later ecological crises. Previous work suggests that this substrate change strongly influenced the evolution of Cambrian benthic suspension-feeding echinoderms. In order to test this previous work and reveal longer-term evolutionary patterns, the substrate adaptations of 93 genera in the three classes of benthic suspension-feeding echinoderms with definitive Cambrian origins (edrioasteroids, eocrinoids, and helicoplacoids) were examined throughout their fossil records. This examination reveals that genera adapted to typical Proterozoic-style soft substrates were taxonomically dominant during, and were almost entirely restricted to, the Early and Middle Cambrian. Thereafter, from the Late Cambrian through the Carboniferous, genera adapted to typical Phanerozoic-style soft substrates were taxonomically dominant. This trend supports previous studies and defines a fundamental evolutionary shift in echinoderm substrate interactions in response to increased mixed layer development during the Cambrian radiation. A notable exception to this trend is the Late Devonian edrioasteroid Cooperidiscus, which is morphologically convergent with the Early and Middle Cambrian sediment-attaching edrioasteroid Stromatocystites. This convergence is likely the result of Cooperidiscus adapting as a sediment attacher to typical Proterozoic-style soft substrates that developed during or shortly after the Late Devonian mass extinction. This study indicates for the first time that adaptation to non-actualistic substrates during ecological crises can result in morphological convergence with forms adapted to similar substrates common during the Cambrian radiation.