Boreskov

OP‐22dominated by ostracodes, first small aquatic universal eaters simultaneously belonging toseveral trophic levels and capable of a deeper transformation of organic matter. In theOrdovician, the pelagic zone became a constant rather than facultative, as before, habitatfor zooplanktonic and nektonic organisms: graptolites, radiolarians, conodonts,nautiloids, meroplankton (mainly larvae of colonial organisms and brachiopods), pelagictrilobites, ostracodes, and early primitive fishes. Some spatial rearrangement of thelowest trophic level, major producers, took place in the same period. This had a dramaticeffect on the stage and lateral structure of trophic chains. Until the early Middle Ordovician,the main photosynthesizing producers were bottom cyanobacterial associations, ormeadows (mats), which were widespread in Late Precambrian and Early Paleozoicepicontinental seas. At the Early‐Middle Ordovician boundary, the areas of these meadowsdecreased, and phytoplankton became the main producer. The global ecologic event wasaccompanied by the greatest (in the Phanerozoic) burst of the diversity of Ordovicianmarine biotas followed by rapid stabilization. Later, the stability was maintained by aphylogenetic succession of ecologically equivalent taxa, with some ecologic guilds replacedat critical borderlines. Thus, in the Ordovician, sea ecosystems became multistage, theirtrophic structure became more complex, and a global closed biogeochemical cycleformed for the first time throughout the sea area. The Ordovician global biotic eventsmatched large‐scale geologic events (abrupt climatic changes, maximum range oftransgressions and regressions of epicontinental seas, changes in Mg and Ca balance inmarine sediments, increase in the content of oxygen in the Earth's atmosphere andhydrosphere, and formation of the ozone screen). It is supposed that the appearance ofthe ozone screen and increase in the content of oxygen in sea water had a crucialimpact on the settling of heterotrophs in the pelagic zone and formation of coherent(ecologically complete) benthic ecosystems. At the initial metastable stage of thedevelopment of the ozone screen, eustatic fluctuations of the World ocean levelcaused dramatic biodiversity fluctuations in bottom and pelagic associationsdetermined by profound changes in spatial parameters of sea shelves, the mainhabitat of biota. The Late Ordovician extinction of marine biotas resulted from anabrupt 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 afterthe Ordovician transgression maximum.67