A GLOBAL CHITINOZOA BIOZONATION FOR THE SILURIAN

A global Chitinozoan biozonation for the Silurian Period is proposed. Each biozone is an interval range defined by the first occurrence of a n index species, selected from well-defined species with a relatively short biostratigraphic range. To be included in the scheme, index spec ies must have been recorded from at least two major Silurian palaeocon tinents where usable Chitinozoa assemblages have been studied, that is , Avalonia-Baltica (which had already undergone collision by the Silur ian), Laurentia, Gondwana and Southern China. Seventeen biozones are i dentified with seven in the Llandovery: the fragilis, postrobusta, ele cta, maennili, alargada, dolioliformis and longicollis biozones; four in the Wenlock: the margaritana, cingulata, pachycephala and lycoperdo ides biozones; three in the Ludlow: the elongata, philipi and barrande i biozones; and three in the Pridoli: the kosovensis, elegans and supe rba biozones. Chronostratigraphic calibration is partly provided by re ference to the range of the appropriate Chitinozoa index species in th e global stratotype sections and points (GSSP) of the Silurian series, e.g. in Bohemia (Czech Republic) for the Pridoli and the Welsh Borderl and in England for the Ludlow and part of the Wenlock. When this infor mation was not available, independent biostratigraphic control was pro vided by calibration with graptolite biozones or in a few cases, conod ont or trilobite biozonal schemes. The index and most characteristic s pecies of each biozone are illustrated and their total stratigraphic r ange provided.