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The end-Ordovician was an enigmatic interval in the Phanerozoic, known for massive glaciation potentially at elevated CO 2 levels, biogeochemical cycle disruptions recorded as large isotope anomalies and a devastating extinction event. Ice-sheet volumes claimed to be twice those of the Last Glacial Maximum paradoxically coincided with oceans as warm as today. Here we argue that some of these remarkable claims arise from undersampling of incomplete geological sections that led to apparent temporal correlations within the relatively coarse resolution capability of Palaeozoic biochronostratigraphy. We examine exceptionally complete sedimentary records from two, low and high, palaeolatitude settings. Their correlation framework reveals a Cenozoic-style scenario including three main glacial cycles and higher-order phenomena. This necessitates revision of mechanisms for the endOrdovician events, as the first extinction is tied to an early phase of melting, not to initial cooling, and the largest d 13 C excursion occurs during final deglaciation, not at the glacial apex.

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The purpose of this paper is to propose a chitinozoan zonation of the Ordovician of Quebec and western Newfoundland. On the basis of chitinozoan assemblages previously described from various sections, covering almost the entire Ordovician System, 22 biozones, based on first appearance of index species, are distinguished. As the strata from which the chitinozoan microfaunas were extracted are well dated by other fossils, especially graptolites, it is possible to establish a correlation between the chitinozoan and the established graptolite and conodont zonations of North America and Great Britain.