Abstract Commonality of patterns and processes, identified from geological proxy data, occurs in the sequence of events leading to Cenozoic and Ordovician glaciations. Both glaciations were set against a backdrop of long-term declining p CO 2 likely initiated by changes in plate configuration that resulted in increased weathering and nutrient cycling into the oceans. Rapid expansion of ice volume was triggered by the redirection of warm, circumequatorial currents into high latitudes to provide a source of warm moist air and high levels of snowfall. Once ice-sheets were large enough to survive successive precession and obliquity maxima, eccentricity pacing of ice-margin processes embedded in obliquity and the monsoonal climate system embedded in precession largely controlled their size. Changes in family and generic diversity in conodonts, ostracods and graptolites reflect a longer glaciation scenario for the Ordovician. The rising diversity trajectory to the Llanvirn was terminated in the Caradoc. This was followed by a slight rise in diversity in all groups into the Ashgill as taxa adapted to the new environmental conditions. A decline in diversity, of varying severity, into the Llandovery reflects the impact of mass extinction. Commonality in the sequence of events and pattern of environmental change leads to the rejection of the Ordovician glaciation being unique in Earth history.

Abstract Constrained seriation of euconodont generic presence-absence matrices for four time slices between the late Llanvirn and late Llandovery provides a qualitative method for defining shelf and oceanic biofacies, reconstructing biofacies architectures and analysing biodiversity within a regional context. We propose many North Atlantic Province taxa had a pelagic mode of life and ranged widely across the Iapetus Ocean. Oceanic biofacies are considered to reflect water mass structure. Changes in vertical distribution of one such biofacies (including Amorphognathus and Spinodus ) suggest adaptation to cold, nutrient-rich, oxygen-poor upwelling water. Biofacies distributions suggest that upwelling occurred along the Avalonian margin throughout the Ashgill, but was only initiated along the Laurentian margin immediately prior to the Hirnantian glacial maximum. Clade diversities and trajectories differ between biofacies and latitudes, reflecting different causal mechanisms. In Laurentia, diversity fell in the early Ashgill, coincident with the onset of ocean cooling. Diversity declined in Avalonia when the microcontinent drifted into tropical latitudes. The stability of euconodont biofacies architecture during the Late Ordovician indicates that global cooling and plate reorganization had a low palaeoecological impact despite decreases in alpha and beta diversity.