Position of the Lower/Middle Devonian Boundary from the Viewpoint of Crinoid Evolution

Abstract The position of the conodont partitus Zone as the Lower/Middle Devonian boundary in Devonian sections within the USSR (Kazakhstan, Urals, South Tien Shan, Salair, Northeast and so on) is not always confirmed by stratigraphic analysis, or by phases and patterns of historical evolution of benthic faunal communities (brachiopods, crinoids). In some regions the base of the partitus Zone is within a common and everywhere well outlined correlation level, the Megastrophia uralensis-Zdimir pseudobaschkiricus Zone. The following is necessary for drawing the Lower/Middle Devonian boundary: 1) to take account of characteristic contents of Lower and Middle Devonian rocks on the basis of assemblages of faunal groups, proceeding from their stratigraphic range and evolutionary stages in the early and middle Paleozoic; and 2) on this basis elucidate the most distinct biostratigraphic limits in the succession of faunal communities in the Lower and Middle Devonian. Against this background the general evolution of the Lower and Middle Devonian crinoids has distinctive independent characteristics. Throughout the USSR there are four main time associations of crinoids in the Lower and Middle Devonian. They succeed each other in the stratigraphic succession and are characterized by successive evolution. The most distinctive change in taxonomic composition, both at species and generic level, is everywhere recorded at the bases of beds with abundant cupressocrinitids, hexacrinitids, floricrinids and pestericrinids. In regional schemes for the Lower and Middle Devonian of the USSR this biostratigraphic boundary corresponds to the base of the Megastrophia uralensis — Zdimir pseudobaschkiricus Zone, i.e., the base of Middle Devonian beds.