The evolution of the composition of Tabulata and Heliolitoidea corals in the continuous Upper Silurian–Lower Devonian carbonate sequence of the Andygen River in Southern Fergana, as well as in adjacent areas (the Isfara River, etc.), is presented. This series, with a thickness of about 2200 m, was formed on a vast carbonate platform of the southern margin of the Turkestan Ocean. Bioclastic limestones partly can be replaced by open-marine graptolite facies. After the closure of the ocean, the platform became part of the Osh-Uratyube thrust unit within the Southern Tien Shan collision complex. Its stratigraphy was studied in detail in the 1930s–1980s. The boundaries used for chronological correlation do not reflect any geological events of regional significance. The units accepted now as horizons (Dal’yan, Isfara, Kunzhak, etc.) were initially identified as complex zones (beds) with brachiopods. For example, systematic evolution and morphologic changes of tabulate corals, generally quite gradual, may reflect other boundaries, creating difficulties, in defining the Isfara horizon. Progress has been achieved now in the general correlation of these deposits based on the widespread pelagic fossils (graptolites, some conodonts, and tentaculites). This has refined the age and stratigraphic significance of fossil benthos, including corals. However, as a result, there is an unnecessary duplication of international units with regional horizons. It seems preferable to use beds with geographic or paleontological names instead, which are understood as complex zones, do not imply strict isochronism of boundaries, and do not require stratotypes.
Yu. S. Biske (Thu,) studied this question.