The Cambrian Substrate Revolution and Early Evolution of the Phyla

Evidence from Precambrian carbonate and siliciclastic sedimentary structures indicates that in marine settings before the Cambrian conditions of seafloor environments were largely controlled by microbes and the mats which they form. During the Ediacaran-Cambrian transition, a vertical component to marine bioturbation evolved, as well as overall increased seafloor bioturbation. The “Cambrian substrate revolution (CSR)” encompasses the evolutionary and ecological effects that occurred due to these substrate changes. The continued evolution of bioturbating organisms caused the development of a significant variety of new microenvironments, which led to the formation of new ecospace and evolutionary opportunities for other benthic organisms. Numerous studies have evaluated the “weird” morphology of early seafloor animals and how they adapted to an increasingly bioturbated substrate. Many early animals adapted to seafloors with strong microbial mat development are stem groups of the phyla we recognize today, and thus have morphological features absent in modern representatives. Fossils of crown groups of modern phyla first began to appear in the Cambrian and subsequently dominated Phanerozoic bioturbated seafloor environments. The CSR is thus a primary component of the evolution of stem and crown groups of the phyla during the Cambrian explosion.