Definition of the Pragian/Emsian Stage boundary

The boundary for the Pragian-Emsian Stage Global Stratotype Section and Point (GSSP) in the Lower Devonian has been ratified by the ICS and the IUGS and is placed in a section at the Zinzil'ban Gorge in the Kitab State Geological Reserve, 170 km SSE of Samarkand, Uzbekistan, near the base of the Zinzil'ban Beds. The position of the boundary was selected by the Subcommission on Devonian Stratigraphy in 1989 to coincide with the level at which the phylogenetic succesor to the conodont Polygnathus pireneae appeared, that is the species then named Polygnathus dehiscens but in this report, because of uncertainties regarding the holotype of Polygnathus dehiscens, the name Polygnathus kitabicus is used because the type material for the latter came from the GSSP, although it is thought by some to be a junior synonym of Po. dehiscens. The evolutionary series associated with these changes has already been published. The GSSP is located very soon after a global sea level rise and the onset of changed conditions. Rich faunas of other fossil groups occur in the Zinzil'ban Formation. Several species of monograptids and several genera of brachiopods occur above the GSSP and new subspecies of dacryoconarid genera and of brachiopods occur above the GSSP. Also appearing above the GSSP level is the tabulate coral Favosites regularissimus which is the name-giver for the Regularissimus Zone widely traced in Asia. This account documents the record. Since the acceptance of this GSSP has concluded decisions on all the Devonian system, series and stage boundaries, comments are made on the problems that remain, especially those resulting from the fact that most of the GSSP's have been defined in pelagic facies, leaving outstanding their correlation into neritic and terrestrial facies where auxiliary stratotypes are required to provide a better international chronostratigraphic framework for future international scientific work on the Devonian System.