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Russian Geology and Geophysics

2001 year, number 4


A. V. Kanygin
Keywords: Ordovician, biosphere, ecosystems, evolution, ecology, biogeochemical cycles, oxygen, ozone screen, eustatic fluctuations
Pages: 631-667


The evolution of biosphere included 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) areal expansion of life throughout the Earth (gradual transition of the biosphere from discrete to continual on exploration of new bionomic zones and biotopes; (3) complication of the trophic structure of ecosystems (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 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 marine 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 realationships 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 the abiotic indices 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 stromatoporates), aggregated (crinoids), and colonial-aggregated (bryozoans) filter-feeding organisms with carcass skeletons. This resulted in a breakdown of biotopes and complication and heterogeneity of food webs. The lowest trophic level was dominated by ostracods, first small hydrobiontic 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 a facultative, as before, habitat for zooplanktonic and nektonic organisms: graptolites, radiolarians, conodontoforids, nautiloids, meroplankton (mainly larvae of colonial organisms and brachiopods), pelagic trilobites, ostracods, and early primitive fishes. In the Ordovician, a spatial rearrangement of the lowest trophic level - major producers - took place. This had a dramatic effect on the stage and lateral structure of trophic chains. Until the early Middle Ordovician, bottom cyanobacterial associations, or meadows, were widespread in Late Precambrian and Early Paleozoic epicontinental seas and were main photosynthesizing producers. 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 supplemented 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 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 the 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 determining impact on the settling of the pelagic zone by heterotrophs and formation of coherent (ecologically complete) benthic ecosystems. At the initial metastable stage of development of the ozone screen, dramatic fluctuations of biodiversity in bottom and pelagic associations were determined by profound changes in spatial parameters of sea shelves, the main habitat of life, which were caused by eustatic fluctuations of the World Ocean level. 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 great volumes of water in continental glaciers after the Ordovician transgression maximum.