Radiolarian diversity changes during the Late Cambrian–Early Ordovician transition as recorded in the Cow Head Group of Newfoundland (Canada)

• Biotic changes amongst radiolarians at the Cambro–Ordovician transition
• Two-step faunal response across this critical interval
• First episode of faunal change = diversity decrease and within-clade extinctions
• 2nd turnover = extinction of 3 genera and within-clade species, diversity increase
• Correlation between faunal change and negative carbon isotope excursions

Upper Cambrian to Lower Ordovician pelagic carbonate rocks of the Cow Head Group in western Newfoundland (Canada) were sampled for the study of their radiolarian fauna. Well-preserved and diverse faunal assemblages were obtained from a number of levels of the Shallow Bay and Green Point formations. Three different assemblages, including a total of five families, eight genera and 20 species, are recognized in three different sections situated on a palaeobathymetric profile. The various recognized assemblages are compared with previous studies conducted on the Cow Head Group; they improve considerably our understanding of the pattern of changes in radiolarian diversity during the Cambrian–Ordovician transition. The Radiolarian biotic record of western Newfoundland unveils a two step faunal change across the Cambrian–Ordovician transitional interval: one situated at the base of the uppermost Cambrian Stage 10 and the second at a poorly constrained interval situated towards the top of the same Stage 10. These biotic changes consist of genus-level selective extinctions, as well as within-clade species faunal turnover (i.e. Echidnina, Paleospiculum), decrease of species abundance, species extinction and within-clade episodes of species diversification (i.e. Protoentactinia, Parechidnina).