Platystrophia (Orthida, Brachiopoda) from the Arenig and lowermost Llanvirn of northwestern Russia

Six species of the genus Platystrophia (Plectorthidae, Orthida) are described from the Arenig and lowermost Llanvirn of the Baltic-Ladoga Klint, northwestern Russia. One taxon represents an earlier known species, one a new species, and four forms are described in open nomenclature. The oldest species of the genus in the Ordovician of Baltoscandia are Platystrophia sp. a Rubel, 1961 and Platystrophia? sp. 1 (in this paper) from the Volkhov Stage (upper part of the Baltoniodus triangularis-B. navis conodont Zone) of the St. Petersburg region. The lack of granulation on the shell makes the generic attribution of both species tentative. The distribution of the genus Platystrophia in the upper Arenig-lower Llanvirn of Russia, Wales, North America, South America, and China is discussed. The earliest known species of the genus is Platystrophia sp. from the Niquivilia brachiopod Zone (lower part of the B. triangularis-B. navis Zone) of the Argentine Precordillera. This species has no granulation on the well-preserved shell; therefore its generic position should be confirmed by further study.