The Cambrian Substrate Revolution and the early evolution of attachment in suspension-feeding echinoderms

Abstract

The Cambrian, characterized by the global appearance of diverse biomineralized metazoans in the fossil record for the first time, represents a pivotal point in the history of life. This period also documents a major change in the nature of the sea floor: Neoproterozoic-type substrates stabilized by microbial mats were replaced by unconsolidated soft substrates with a well-developed mixed layer. The effect of this transition on the ecology and evolution of benthic metazoans is termed the Cambrian Substrate Revolution (CSR), and this is thought to have impacted greatly on early suspension-feeding echinoderms in particular. According to this paradigm, most echinoderms rested directly on non-bioturbated soft substrates as sediment attachers and stickers during the Cambrian Epoch 2. As the substrates became increasingly disturbed by burrowing, forming a progressively thickening mixed layer, echinoderms developed new strategies for attaching to firm and hard substrates. To test this model, we evaluated the mode of attachment of 83 Cambrian suspension-feeding echinoderm species; attachment mode was inferred based on direct evidence in fossil specimens and interpretations of functional morphology. These data were analyzed quantitatively to explore trends in attachment throughout the Cambrian. In contrast to previous studies, the majority of Cambrian clades (eocrinoids, helicoplacoids, helicocystoids, isorophid edrioasteroids, and solutes) are now interpreted as hard (shelly) substrate attachers. Only early edrioasteroids attached directly to firm substrates stabilized by microbially induced sedimentary structures. Contrary to the predictions of the CSR paradigm, our study demonstrates that Cambrian echinoderms were morphologically pre-adapted to the substrate changes that occurred during the Cambrian, allowing them to fully exploit the appearance of hardgrounds in the Furongian. We find no support for the claim that the CSR explains the peak in echinoderm diversity during Cambrian Epochs 2–3, or that it caused the extinction of helicoplacoids.