A chitinozoan morphological lineage and its importance in Lower Silurian stratigraphy

Margachitina margaritana is a distinctive, chain-forming chitinozoan taxon of the uppermost Telychian Stage (Llandovery Series) to lower Homerian Stage (Wenlock Series). M. margaritana is shown to be the end member of a morphological lineage that developed from Calpichitina densa via the intermediate Margachitina banwyensis sp. nov. A local M. banwyensis Biozone, which correlates with the upper lapworthi graptolite Biozone, is proposed for the Banwy River section. The morphological lineage described provides a tool for the enhanced subdivision of the uppermost Llandovery Series, which may prove global in extent. This research suggests also that the base of the Wenlock Series may correlate with a level below the centrifugus graptolite Biozone and it also brings into question the systematic status of Calpichitina and Margachitina, although both genera are retained herein.