THE CAMBRIAN SUBSTRATE REVOLUTION: ADAPTING TO CHANGING ECOSYSTEM ENGINEERING REGIMES

The Ediacaran to Paleozoic transition in benthic marine environments is noted for the change in seafloor ecosystem engineering from microbial mat dominance to burrowing metazoan dominance. The effects of this change in ecosystem engineering regime upon benthic organisms have been termed the Cambrian Substrate Revolution. Since this concept was proposed more than twenty years ago much has been learned about the evolutionary and paleoecological effects of this ecosystem engineering change. A variety of studies have detailed how seafloors became more bioturbated from the Ediacaran into the early Paleozoic, with development in places of a mixed layer, while other normal marine benthic settings were still dominated by microbial mats. Adaptations to changes in seafloor rheology from firmer to softer, caused by increased effects of bioturbation, have been analyzed through the lens of the modern understanding of trophic amensalism. This has led to categorizarion of benthic morphologies better adapted to Proterozoic-style substrates versus Phanerozoic-style substrates. Detailed studies of the attachment structures of Cambrian echinoderms show that some attached to microbial mat covered seafloors, while others attached to skeletal fragments on the seafloor. Cambrian molluscs typically lived on seafloor microbial mats and grazed microbes. Sponges commonly rested on relatively firm seafloors, while some groups, such as the chancelloriids, were shallow sediment stickers. Firmground walking, such as that done by Lobopodians, was also a morphological and behavioral adaptation to seafloors with little to no bioturbation. The change from the Cambrian to the Paleozoic Evolutionary Fauna, as well as the change in the Cambrian from dominance by stem to crown group metazoans, significantly reflect adaptation to this turnover in ecosystem engineering regimes. While some trace fossils, such as trilobite scratch marks, provide evidence for the rheology of Cambrian seafloors, measurements on the rheology of modern seafloor mats and poorly bioturbated substrates along with modeling of rheology and different metazoan morphotypes will allow a quantifiable framework to further understand the Cambrian Substrate Revolution.