THE CAMBRIAN SUBSTRATE REVOLUTION AND EVOLUTION OF STEM AND CROWN GROUPS 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. This increase in bioturbation strongly altered biochemical and diagenetic reactions and led to the redistribution of sedimentary particles and pore water at the sediment-water interface. The “Cambrian substrate revolution” (CSR) encompasses the evolutionary and ecological effects that occurred due to these substrate changes. The continued evolution of bioturbating organisms gradually destroyed the wide extent of subtidal microbial mat environments and associated niches, but also 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 stem group members of the phyla were adapted to benthic environments characterized by microbial mats and minimal vertical bioturbation. Recently proposed examples of animals with “weird” morphologies adapted to these seafloor conditions include Dickinsonia, a putative stem group placozoan that would have employed external digestion of microbial mats, and Odontogriphus and Wiwaxia, proposed stem group molluscs adapted to grazing on microbial mats. Other stem group members of the phyla with “weird” morphologies were adapted to these early bioturbated seafloors, such as the stylophorans and solutes, echinoderm stem groups with a proposed “flatfish” morphology. Many of these stem group representatives of the phyla became extinct or were restricted in their environmental distribution as bioturbation further increased. 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.