Acta Palaeontologica Polonica

New evidence on graptolite succession across the Ordovician-Silurian Boundary in the Asian Part of the USSR

T.N. Koren’, R.F. Sobolevskaya, N.F. Mikhajlova and D.T. Tsai

Acta Palaeontologica Polonica 24 (1), 1979: 125-136

Continuous sections of the Upper Ordovician-Lower Silurian terrigenic-carbonate deposits in the Basin of the Kolyma River, Southern Kazakhstan and Tien Shan display a seell'ence of the following biostratigraphic units: the supernus Zone with the longispinus and pacificus subzones, the extraordinarius, persculptus and acuminatus Zones. Graptolites from the above sections are associated with diverse benthic fauna, namely: brachiopods, trilobites and corals. The persculptus Zone of Kazakhstan and its analogues in the basin of the Kolyma River are distinguished by the occurrence of the Dalmanitina-Hirnantia assemblage. The supernus extraordinarius and persculptus/acuminatus boundaries are the most distinct correlative levels as far as graptolites are concerned. The former is marked by full disappearance of typically Ashgillian graptoloids, while the latter displays distinct renewal of diplograptid fauna due to the simultaneous appearance of various morphologic features in several new lineages. The extraordinarius and persculptus Zones are distinguished by the development of impoverished diplograptid assemblages. TIley contain the few new clements, while species of the genus Climacograptus have broad Ashgillian-Llandovery biozones. The bottom of the acuminatus Zone coincides with the disappearance of brachiopod and trilobite associations which were traditionally believed to be Ordovician.

Key words: Graptolites, stratigraphic boundaries, Ordovician, Silurian, Asia.

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