PROPOSAL FOR ADOPTION OF THE BASE OF THE UNDULOGRAPTUS-AUSTRODENTATUSBIOZONE AS A GLOBAL ORDOVICIAN STAGE AND SERIES BOUNDARY LEVEL

The base of the Undulograptus austodentatus Biozone appears to be a sy nchronous event that is widely recognizable within graptolitic facies around the world. It occurs within an interval in which graptolite spe cies ranges are now well known and in which there is a rapid turnover in the composition of graptolite faunas. This turnover reflects the ra pid evolutionary radiation of the Diplograptacea simultaneously with t he appearance of several distinctive pseudisograptid and glossograptid species. These events provide the basis for the recognition of two th in but widely applicable subzones: a lower Arienigraptus zhejiangensis Subzone and an upper U. sinicusSubzone. The occurrence of the lower b oundary of the U. austrodentatus Biozone within a succession of first appearances also permits accurate and reliable identification of the b oundary as well as assessment of stratigraphic completeness across the boundary interval in correlated sections. Diverse graptolite faunas o f late Yapeenian and early Darriwilian age occur in association with t he Histiodella altifrons Biozone of the North American midcontinent co nodont zonation and the Paroistodus originalis and Microzarkodina parv a biozones of the North Atlantic conodont zonation. They also occur in association with the shelly-fossil zonations developed for several di fferent continents. These features of the base of the U. austrodentatu s Biozone make it a suitable level for use as the boundary level for a global stage. Its stratigraphic position within the Ordovician System relative to other likely global stages as well as its coincidence wit h one of the major events in graptolite evolutionary history suggest t hat this level also maybe a suitable level for the base of a global Mi ddle Ordovician Series.