The onset of the Ordovician evolutionary radiation of benthic animals in the Baltic Region: Explosive diversity of attachment structures of stalked echinoderms, substrate revolution and the role of cyanobacterial communities

The study of the Ordovician holdfasts of the Baltic Paleobasin revealed an explosive increase in the morphological diversity of echinoderms of the Baltic Paleobasin, up to 12 major morphological types in the Volkhov Regional Stage (Dapingian), reflecting an explosive growth in the taxonomic and morphological diversity of stalked echinoderms in general, but primarily crinoids. A sudden increase in the size and diversity of holdfasts at that time suggests a considerable increase in calcite production by stalked echinoderms and their adaptation to a wide distribution of a new type of substrate — hardgrounds and firm coarse-grained bioclastic substrates. The possibility of wide distribution of such substrates was due to a considerable increase, of at least an order of magnitude, in the carbonate productivity of benthic communities, whereas the rapid distribution of a new type of substrate and an explosive increase in the diversity and abundance of their inhabitants in the Baltic Basin was related to the appearance of positive feedbacks between the living and non-living components of this ecosystem. The existence of this positive feedback was due to cyanobacterial communities, the development of a biofilm on the substrate, and mineralization of an extracellular polymer substance, which led to the formation of hardgrounds and consolidation of the surface of the bioclastic substrates. The appearance in the Middle Ordovician of positive feedback between the expansion of the bioclastic substrates and an increase in carbonate production by their inhabitants, resulted in the Ordovician substrate revolution and explosive diversification of Ordovician benthic animals. Cyanobacterial films episodically appearing on the substrate, were an essential factor in the formation of this feedback.