Cambro-Ordovician microorganisms: acritarchs and endoliths
Sammanfattning: Organic-walled microfossils are abundant and taxonomically diverse in Cambrian-Ordovician strata; some are important for biostratigraphy and for the correlation of geological successions. New assemblages of Cambrian-Ordovician acritarchs from Kolguev Island, Arctic Russia and Middle Cambrian ichnofossils of endoliths from Peary Land, North Greenland are studied. Twenty-seven acritarch species are described in detail and 10 taxa are left under open nomenclature. The diagnosis of one genus is restricted, and two other are emended. New combinations are proposed for three species and one new species is recognised. The studied acritarch assemblages are taxonomically rich and age-diagnostic and used to recognise Upper Cambrian and Tremadoc strata on Kolguev Island. The sedimentologically continuous successions provide for the first time palaeontological evidence of Cambrian strata in the north-eastern sector of Europe. The exact level of the Cambrian-Ordovician boundary was distinguished together with stratigraphic intervals equivalent to the Peltura and Acerocare zones of the Upper Cambrian of Baltica. The newly established relative age of the lowermost sedimentary succession overlying the Timanian unconformity allows verification of the minimum age of the Timanian deformation and the time-span of the hiatus bound to this unconformity. Endoliths occur in the fossil record from the Early Archean and they played an important role in the formation of stromatolites and the process of bioerosion and biodegradation. Endoliths that have actively bored into brachiopod shells or carbonate grains (euendoliths), and some that inhabited the cavities inside brachiopod shells (cryptoendoliths) are described. Borings within the carbonate grains extended with a dentritic pattern, whereas those within the brachiopod shells were formed by a multifilamentous euendolith which produced characteristic longitudinally ridged galleries. The cryptoendolithic morphologies include indeterminate coccoid masses and at least two filamentous forms. However, considerable variation in the dimensions of the currently phosphatised diagenetic crusts of the cryptoendoliths hinders discrimination.
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