Middle Ordovician of the Gorny Altai: litho-biostratigraphy and justification of regional units boundaries

New paleontological and biostratigraphic data were obtained for the five sections, and previously known materials were updated for the three sections of the Middle Ordovician of the Charysh-Inya and Uymen’-Lebed’ facies zones of the Gorny Altai. Regional stratigraphic stages namely Kuibyshevo and Kostinsky horizons are discussed in details. By graptolites the complete stratigraphic volume of the Kuibyshevo regional unit is defined as following zonal succession: (1) sinodentatus/Cardiograptus/Oncograptus, sinodentatus, (2) austrodentatus, (3) dentatus; as well as the Kostinsky regional unit within the lentus/balhaschensis/kirgisicus Zone. For the Kostinsky Horizon, Beds with conodonts Eoplacognathus pseudoplanus were defined in the Batun Section, and in the Baraniy-1 Section, the same age Beds with conodonts Semiacontiodus asymmetricus– Parapanderodus striatus were defined. For the first time in the stratotype of the Kuibyshevo Horizon (Maralikha Section), from the Voskresenka Formation, a benthic fauna assemblage represented by brachiopods was found and identified.