Abstract

The Silurian and Devonian sequence in Tien Shan comprises terrigenous, volcanic, and limestone sections. Continuous carbonate successions are of great importance for determining the boundary between these systems. The Siluro-Devonian boundary is situated in homogenous layers and must be drawn on faunal grounds. On the basis of the Brachiopoda, this boundary is found to lie in the upper part of the Kungakian division in south Tien Shan.

A significant change in the composition of tetracoral and tentaculitid assemblages takes place at the Siluro-Devonian boundary. By the close of the Silurian, among the Tetracoralla, the development of the Petraiidae, Lykophyllidae, Kyphophyllidae, Cystiphyllidae was completed. Representatives of the genera Neobrachyelasma, Phaulactis, Neocystiphyllum, Holmophyllum, Neomphyma died out; representative of the genera Kodonophyllum, Pilophyllum, Entelophyllum, and Microplasma were greatly reduced. By the beginning of the Devonian, Lindstromiidae (Lindstromia, Kionelasma), Digonophyllidae (Pseudodigonophyllum, Pseudomicroplasma), Ptenophyllidae (Ptenophyllum, Acanthophyllum), Stauriidae (Loyolophyllum, Lyrielasma) appeared in south Tien Shan.

By the end of the Silurian, representatives of a typical late-Silurian genus, Paranowakia, disappeared. By the beginning of the Devonian, the genera Poriella and Turkestanella became of great importance. In the Lower Devonian, species of Styliolina and Sogdiana made their appearance.

The study of the Tetracoralla and Tentaculitida supports the drawing of the Siluro-Devonian boundary in the upper part of the Kungakian horizon. The Devonian appears to be represented only by the Lower series, which does not appear to be subdivisible on faunal grounds. Thus the current Soviet divisions into “Gedinnian” and “Coblenzian” could not be recognized.