The Mitrofanovo and Tastuba Reefs are composed of organic limestones containing large amounts of diverse fossil animals (foraminifers, brachiopods, bryozoans, gastropods, crinoids, bivalves, trilobites). In
the Tastuba Reef, the records of trilobites are restricted
to the lower member of massive, cavernous limestones. In the Mitrofanovo Reef, they occur throughout the thickness of the sediments.