The crinoid Marsupites in the Upper Cretaceous Nanaimo Group, British Columbia: Resolution of the Santonian-Campanian boundary in the North Pacific Province

The strongly endemic nature of Late Cretaceous molluscan faunas of the North Pacific region creates challenges for correlation of stratigraphic sections in basins surrounding the proto-Pacific Ocean with European standard successions. A particularly problematic stage boundary in the Pacific region is the Santonian-Campanian boundary, which is poorly defined in the northwest and northeast Pacific regions on the basis of local ammonite and inoceramid bivalve assemblages or poorly constrained magnetostratigraphy. Examples of the crinoid genus Marsupites Mantell in Miller 1821, considered a marker of the Santonian-Campanian boundary at many places around the globe, have been collected recently from a number of localities of the Upper Cretaceous Nanaimo Group succession of southwestern British Columbia, including the species M. testudinarius and M. lamberti. Co-occurring specimens of Marsupites testudinarius in the Nanaimo Group include both smooth and coarsely ornamented forms which do not show stratigraphic segregation, suggesting that M. testudinarius is a morphologically-variable species which includes forms that were attributed to different species by previous workers. The presence of M. testudinarius in stratigraphic sections of the Nanaimo Group allows the precise placement of the Santonian-Campanian boundary interval in the Nanaimo Group succession, and correlation of this boundary with the European standard succession. As well, the occurrence of the crinoid allows for revision of the biostratigraphic zonation of the upper Santonian-lower Campanian interval in the Nanaimo Group. Characteristic molluscan assemblages which are found in association with M. testudinarius in the Nanaimo Group are known to occur elsewhere along the North American Pacific slope (e.g., northern California, USA), as well as in the northwest Pacific region (Japan, Sakhalin, Koryakia), and thus provide a marker for the Santonian-Campanian boundary in those areas where Marsupites has not yet been found.