Rise and fall of archaeocyath reefs shaped early Cambrian skeletal animal abundance

The Cambrian Explosion is a biodiversification event that involves numerous evolutionary innovations in the biosphere, including the establishment of nearly all marine invertebrate phyla, exploitation of the infaunal realm, and the expansion of skeletal elements as armor. The notable diversification of skeletons has been linked to predator-prey dynamics and the advent of new ecological niches, particularly the development of animal reefs built by archaeocyaths. While some have argued that the appearance and subsequent extinction of archaeocyath reefs was followed by a long-term delay in marine diversity, the influence of archaeocyath reefs on skeletal abundance both during and after their heyday has not been explored. Here, we review existing data and supplement it with new analyses from western Laurentia confirming the influence of archaeocyath reefs on animal abundance. During reef development, the reefs themselves were a locus for skeletal carbonate, but benthic communities adjacent to the reefs were also rich in skeletal taxa. Following the decline of archaeocyath reefs, skeletal abundance declined. The next true increase in animal abundance did not occur until Ordovician time. This work confirms that the influence of early animal reefs was ecologically far-reaching, providing habitat for numerous skeletal organisms. It is likely that the environmental stressors surrounding archaeocyath reef decline also affected other skeletonizing organisms, leading to an interval with fewer shelled animals, dominated by trilobites and echinoderms. This work confirms the broad ecological significance and importance of reef communities in the geologic past, acting as harbingers to the detrimental consequences of our modern reef crisis.