The Ordovician phenomenon of explosive divergence of the Earth's organic realm: Causes and effects on the biosphere evolution

The evolution of biosphere could be presented as the following processes: ( I) emergence of new ecologically specialized groups (guilds), providing a m ore efficient use, transfer, and transformation of matter and energy in eco systems; (2) spatial expansion of life throughout the Earth (gradual transi tion of the biosphere from discrete to continual on exploration of new bion omic zones and biotopes; (3) complication of the trophic structure of ecosy stems (from simple Archean autotrophic-heterotrophic procaryotic systems to the modern global ecosystem); (4) variations in the spatial and energetic indices of biogeochemical cycles. In this context, the Ordovician can be re garded as one of the greatest critical stages in the biosphere evolution. I n the Ordovician, the emergence of new taxa (ecologic guilds) with better t rophic adaptability in benthic associations and settling of pelagic zones i n euphotic sea areas resulted in dramatic changes in marine ecosystems, whi ch predetermined further evolution of marine biotas. The chief evolutionary strategy of Precambrian marine organisms was to improve adaptation to phys icochemical environmental settings by complication of biological organizati on and separation of metabolic and reproductive functions within a body. In the Early Cambrian, main phyla of marine invertebrates emerged, and multis tage trophic realationships between autotrophs and heterotrophs, with divis ion of ecologic functions, began to form. Adaptation to the biotic environm ent became as evolutionarily important as adaptation to abiotic conditions. Starting in the Ordovician, the ecologic mechanisms of organism interactio n became the key factor of the evolutionary strategy in biota associations owing to the gradual stabilization of the abiotic indices in sea basins. New edificator groups first appeared in abundance in the Ordovician and rea ched their acme in the Middle Ordovician: articulate brachiopods and sessil e colonial (tabulates, tetracorals, heliolitoids, and stromatoporoids), agg regated (crinoids), and colonial-aggregated (bryozoans) filter-feeding orga nisms with carcass skeletons. This resulted in a breakdown of biotopes and complication and heterogeneity or food webs. The lowest trophic level was d ominated by ostracodes, first small hydrobiontic universal eaters simultane ously belonging to several trophic levels and capable of a deeper transform ation of organic matter. In the Ordovician, the pelagic zone became a const ant rather than a facultative, as before, habitat for zooplanktonic and nek tonic organisms: graptolites, radiolarians, conodontoforids, nautiloids, me roplankton (mainly larvae of colonial organisms and brachiopods), pelagic t rilobites, ostracodes, and early primitive fishes. In the Ordovician, a spa tial rearrangement of the lowest trophic level - major producers - took pla ce. This had a dramatic effect on the stage and lateral structure of trophi c chains. Until the early Middle Ordovician, bottom cyanobacterial associat ions, or meadows, were widespread in Late Precambrian and Early Paleozoic e picontinental seas and were main photosynthesizing producers. At the Early- Middle Ordovician boundary, the areas of these meadows decreased, and phyto plankton became the main producer. The global ecologic event was accompanie d by the greatest tin the Phanerozoic) burst of the diversity of Ordovician marine biotas followed by rapid stabilization. Later the stability was mai ntained by a phylogenetic succession of ecologically equivalent taxa supple mented by replacement of some ecologic guilds at critical borderlines. Thus, in the Ordovician, marine ecosystems became multistage, their trophic structure became more complex, and a global closed biogeochemical cycle fo rmed for the first time throughout the sea area. The Ordovician global biot ic events matched large-scale geologic events (abrupt climatic changes, max imum range of transgressions and regressions of epicontinental seas, change s in Mg and Ca balance in marine sediments, increase in the content of oxyg en in the Earth's atmosphere and hydrosphere, and formation of the ozone sc reen). It is supposed that the appearance of the ozone screen and increase in the content of oxygen in sea water had a determining impact on the settl ing of the pelagic zone by heterotrophs and formation of coherent (ecologic ally complete) benthic ecosystems. At the initial metastable stage of devel opment of the ozone screen, dramatic fluctuations of biodiversity in bottom and pelagic associations were determined by profound changes in spatial pa rameters of sea shelves, the main habitat of life, which were caused by eus tatic fluctuations of the World Ocean level. The Late Ordovician extinction of marine biotas resulted from an abrupt shrinkage of the shelf habitat ca used by a lowering of the World Ocean, which, in turn, resulted from the fi xation of great volumes of water in continental glaciers after the Ordovici an transgression maximum.