Abstract

Eodiscoids form a significant component of the Lower Cambrian (Branchian) trilobite fauna of the St. Mary’s and lower Jigging Cove members of the Brigus Formation exposed along Conception and Trinity bays, northern Avalon Peninsula, Newfoundland. Two species of Serrodiscus Richter & Richter, four species of Hebediscus Whitehouse, and rare sclerites of Meniscuchus? Öpik and Calodiscus Howell are documented; type material of M.? helena (Walcott) is reillustrated. Hebediscus attleborensis (Shaler & Foerste) is restricted to the types, and H. williamsi is new. A parsimony analysis indicates that Serrodiscus is paraphyletic but supports monophyly of Acidiscus Rasetti, Bolboparia Rasetti and Stigmadiscus Rasetti. Above a sparsely fossiliferous interval in the lowest part of the Brigus, three eodiscoid faunas can be identified in the upper St. Mary’s and lower Jigging Cove members, in ascending order, the Hebediscus planus, H. williamsi and Calodiscus cf. C. lobatus faunas. The lower two faunas correspond to part of the traditional “Callavia Zone” of previous workers.