Revision of Middle Cambrian trilobites and agnostids from Cape Breton Island, Nova Scotia, reveals a Guzhangian faunal interval in the Avalonian cover sequence

The MacLean Brook Group of Cape Breton Island yields a Middle Cambrian trilobite and agnostid fauna that is unlike any other reported from Avalonia. A reassessment of this fauna based on new and archival collections shows that it includes Hypagnostus cf. H. brevifrons (Angelin) and Hypagnostus cf. H. confusus (Westergård), Holasaphus centropyge Matthew, Acrocephalaspis matthewi (Hutchinson), and species of Groenwallia Kobayashi and Bailiaspis Resser. It is assigned to an informal biostratigraphic unit, the H. centropyge Fauna. The composition of the H. centropyge Fauna points to a Guzhangian age, and closest similarities lie with the Erratojincella brachymetopa Zone of the Andrarum Limestone of Sweden (Baltica). This zone is correlative with the lower Lejopyge laevigata Zone of Sweden, which defines the base of the Guzhangian. Although the uppermost Guzhangian Agnostus pisiformis Zone is well documented in Avalonia, previous evidence for the basal part of the stage has been limited. Establishment of a Guzhangian age for the MacLean Brook Group in Cape Breton Island is consistent with the age of the group in Newfoundland and emphasizes the regional lithostratigraphic unity of the cover sequence of the Avalon terrane. It also provides an upper age constraint on the underlying Gregwa Formation volcanics. To completely bracket the Gregwa biostratigraphically, we also revise “ Erratojincella” bretonensis (Hutchinson) from the underlying Dugald Formation, which is consistent with an assignment to the Wuliuan Eccaparadoxides etemenicus Zone.