THE ORDOVICIAN EXPLOSIVE DIVERGENCE OF THE EARTH’S ORGANIC REALM: CAUSES AND EFFECTS OF THE BIOSPHERE EVOLUTION Available to Purchase

Evolution of the biosphere can be presented as the following processes: (1) emergence of new ecologically specialized groups (guilds), providing a more efficient use, transfer, and transformation of matter and energy in ecosystems; (2) spatial expansion of life throughout the Earth (gradual transition from a discrete to continual biosphere on exploration of new bionomic zones and biotopes; (3) complication of the trophic structure of ecosystems (from simple Archean autotrophic-heterotrophic prokaryotic systems to modern global ecosystems); (4) variations in the spatial and power parameters of biogeochemical cycles. In this context, the Ordovician can be regarded as one of the greatest critical stages in the biosphere evolution. In the Ordovician, the emergence of new taxa (ecologic guilds) with better trophic adaptability in benthic associations and settling of pelagic zones in euphotic sea areas resulted in dramatic changes in sea ecosystems, which predetermined further evolution of marine biotas. The chief evolutionary strategy of Precambrian marine organisms was to improve adaptation to physicochemical environmental settings by complication of biological organization and separation of metabolic and reproductive functions within a body. In the Early Cambrian, main phyla of marine invertebrates emerged, and multistage trophic relationships between autotrophs and heterotrophs, with division of ecologic functions, began to form. Adaptation to the biotic environment became as evolutionarily important as adaptation to abiotic conditions. Starting in the Ordovician, the ecologic mechanisms of organism interaction became the key factor of the evolutionary strategy in biota associations owing to the gradual stabilization of abiotic parameters in sea basins. New edificator groups first appeared in abundance in the Ordovician and reached their acme in the Middle Ordovician: articulate brachiopods and sessile colonial (tabulates, tetracorals, heliolitoids, and stromatoporoids), aggregated (crinoids), and colonial-aggregated (bryozoans) filter-feeding organisms with carcass skeletons. This resulted in the breakdown of biotopes and complication and heterogeneity of food webs. The lowest trophic level was dominated by ostracodes, first small aquatic universal eaters simultaneously belonging to several trophic levels and capable of a deeper transformation of organic matter. In the Ordovician, the pelagic zone became a constant rather than facultative, as before, habitat for zooplanktonic and nektonic organisms: graptolites, radiolarians, conodonts, nautiloids, meroplankton (mainly larvae of colonial organisms and brachiopods), pelagic trilobites, ostracodes, and early primitive fishes. Some spatial rearrangement of the lowest trophic level, major producers, took place in the same period. This had a dramatic effect on the stage and lateral structure of trophic chains. Until the early Middle Ordovician, the main photosynthesizing producers were bottom cyanobacterial associations, or meadows (mats), which were widespread in Late Precambrian and Early Paleozoic epicontinental seas. At the Early-Middle Ordovician boundary, the areas of these meadows decreased, and phytoplankton became the main producer. The global ecologic event was accompanied by the greatest (in the Phanerozoic) burst of the diversity of Ordovician marine biotas followed by rapid stabilization. Later, the stability was maintained by a phylogenetic succession of ecologically equivalent taxa, with some ecologic guilds replaced at critical borderlines. Thus, in the Ordovician, sea ecosystems became multistage, their trophic structure became more complex, and a global closed biogeochemical cycle formed for the first time throughout the sea area. The Ordovician global biotic events matched large-scale geologic events (abrupt climatic changes, maximum range of transgressions and regressions of epicontinental seas, changes in Mg and Ca balance in marine sediments, increase in the content of oxygen in the Earth’s atmosphere and hydrosphere, and formation of the ozone screen). It is supposed that the appearance of the ozone screen and increase in the content of oxygen in sea water had a crucial impact on the settling of heterotrophs in the pelagic zone and formation of coherent (ecologically complete) benthic ecosystems. At the initial metastable stage of the development of the ozone screen, eustatic fluctuations of the World ocean level caused dramatic biodiversity fluctuations in bottom and pelagic associations determined by profound changes in spatial parameters of sea shelves, the main habitat of biota. The Late Ordovician extinction of marine biotas resulted from an abrupt shrinkage of the shelf habitat caused by a lowering of the World ocean, which, in turn, resulted from the fixation of huge volumes of water in continental glaciers after the Ordovician transgression maximum.