Dependence of the generic composition of the Ataxophragmiid Order (foraminifers) on dwelling conditions

Studies on the West-Siberian genera of theAtaxophragmiid Order indicated the dependence of their generic composition on dwelling conditions. The obvious confinement of thesetaxa to transgressive stages of the basin was revealed. The primitively arranged forms prevailed during regressions. Agglutinated tests ofmore advanced foraminifers of the Ataxophragmiid Order are considered, which have test walls of diverse chemical composition. Fromthe carbonate facies, the tests were encountered whose agglutinate was composed of calcite grains fastened with carbonaceous cement.Within terrigenous non-calcareous or siliceous clays, tests often possess quartz-siliceous walls. When analysed in detail, tests of thegenera established previously as common proved to be distinct because of the diverse chemical wall composition. The authordistinguished three pairs of the twin genera from the Ataxophragmiid Order: Verneuilina dOrbigny, 1840 - Verneuilinoides Loeblichet Tappan, 1949; Gaudryina dOrbigny, 1840 - Gaudryinopsis Podobina, 1975; Marssonella Cushman, 1933 - ArenogaudryinaPodobina, 1975. These twin genera are illustrated on the palaeontological plate. The twin genera Gaudryina dOrbigny, 1840 andGaudryinopsis Podobina, 1975 have been studied, which are known from the Mesozoic  Palaeogene deposits of Western Siberia andconfined to the peaks of transgressions. These deposits, excluding the Gankinskaya Suite, are composed of terrigenous, non-calcareousclays with embedded forms of the Gaudryinopsis Podobina, 1975 Genus possessing quartz agglutinate in the structure of test walls. Inthe Gankinskaya Suite composed of calcareous siltstones and clays, the Gaudryina dOrbigny, 1840 Genus has been encountered; itstest walls contain calcite grains as agglutinate. These two genera were also found to be morphologically distinct. In the Mesozoic -Palaeogene section, the Gaudryina and Gaudryinopsis genera are often established as the index-species of the distinguishedforaminiferal zones. This information is schematically demonstrated in the table where the described genera are assigned to the peaks oftransgressions. The maximum transgression has been determined in the Middle Eocene (the Middle Lyulinvosrakaya Subsuite), and theMiddle Eocene Gaudryinopsis subbotinae Zone has been established from these deposits by the author. The zonal species occurs widelythroughout Western Siberia, and the deposits of this zone are supposed to correspond to the Middle Eocene (second) rock complex ofthe Lomonosov Ridge section (the Arctic Ocean). This is evidenced by the lithology of the rocks, which are composed of the biosiliceousmaterial and are similar to deposits of the Middle Lyulinvosrakaya Subsuite of Western Siberia. They are also supposed toenclose the West-Siberian index species Gaudryinopsis subbotinae Podobina or its vicariant. The assignment of different species of theGaudryinopsis Genus and other genera of the Ataxophragmiida Order to deposits corresponding to the maximum transgressionsenhances their stratigraphic and correlational significance, enabling to adjust the age of the most disputable stratigraphic units.

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