Appearance and evolution of marine benthic communities in the Early Palaeozoic

The development of marine benthic communities in the Early Palaeozoic occurred mainly in the shallow water epicontinental seas. It included those stages of the Cambrian and Ordovician evolutionary radiations that were dominated by a high rate of morphogenesis, when new food and territory resources were not limited. This provided many opportunities for coadaptation of emerging organisms. At the time of the Cambrian radiation, the body plans of all animals were formed, while in the Ordovician, the maximum rank of emerging taxa did not exceed the level of class. The beginning of each radiation was explosive. Vendian benthic communities developed in cold seas and in the shallowest areas of warm seas, where organic matter from the surface layers was available at the bottom because of the absence of a thermocline. The Cambrian radiation began with the appearance of pelagic suspension feeders, because of which much of the primary production could penetrate the thermocline and settle at the bottom. This allowed the occupation of warmer seas and greater depths. At the same time, the productivity of the pelagic region sharply increased because of the emergence of positive feedback between the producers and consumers in the water, leading to increased water transparency and elongation of trophic chains. Arthropods, the first suspension feeders, were the launch group of the Cambrian radiation. Cambrian benthic suspension feeders could seize only the smallest particles, mostly bacteria, and dissolved organic matter. This food resource was contained in the thin bottom water layer. Therefore, the food grasping structures of all the Cambrian suspension feeders were near the bottom, without forming tiers. The Ordovician evolutionary radiation began with the launch of the Pelmatozoan echinoderms, which were the first benthic suspension feeders to begin feeding on plankton. The exploitation of this resource led to the creation of a 1-m tier above the bottom and an increase in their calcite productivity. Positive feedback emerged between the grounds and the community of its inhabitants and considerably changed the composition and diversity of grounds, which sharply increased the diversity of benthos. The appearance of positive feedback between different components of ecosystems resulted in explosive evolution in both the Cambrian and Ordovician.