The Hirnantian mass extinction is recognized as the first of the “big three” extinctions and, along with the end-Permian and end-Cretaceous events, is the result of an acceleration in biotic extinctions concomitant with a rise in originations. The Hirnantian mass extinction is characterized by high taxonomic impact and within-community extinctions. The Hirnantian mass extinction is also unusual in that (1) it is associated with glaciation, but there is little evidence elsewhere in the younger Phanerozoic that glaciations have been a cause of mass extinction, and (2) there is limited understanding of how glaciation could directly cause mass extinction, particularly in the marine realm. In this review, we argue that coordinated extinctions occurred at the onset and termination of glaciation and were due to climatically induced changes in relative sea level, ocean redox stratification, and sea-surface temperature gradients. These earth system changes resulted in a reduction in prospective niche space, both in the water column and on the seafloor, which in turn led to increased competition and selection pressures, resulting in extinctions where the carrying capacities of particular ecological niches were exceeded. The long-term ventilation of the oceans broke the link between glaciation and mass extinction.

Abstract Chitinozoans and graptolites are the main components of preserved Ordovician zooplankton. As with much of the modern plankton, the ‘first-order’ species distributions of Ordovician plankton reflect water masses defined on the basis of sea surface temperatures. For ‘time slices’ of less than a few million years, zooplankton distribution patterns can be used to infer latitudinal sea surface temperature gradients, key palaeoceanographical boundaries and the position of Ordovician climate belts. Here, using two Late Ordovician time intervals – the early Sandbian and Hirnantian – we review how zooplankton distribution patterns identify Late Ordovician cooling and reflect the development of severe icehouse conditions. Supplementary material: Additional information on methods and material is available at: http://www.geolsoc.org.uk/SUP18670

Abstract Conodont-bearing limestone clasts in Lower Old Red Sandstone conglomerates in the Lanark and Strathmore basins and the Pentland Hills Inlier, Midland Valley, Scotland, indicate a source in a cryptic arc terrane with a mid-Ordovician (P. serra – P. anserinus Biozone) limestone cover. Simpson coefficients of similarity indicate that the faunas from the limestone clasts are closer to conodont faunas from the Holy Cross Mountains, Poland, and the Wrae Limestone in the Northern Belt of the Southern Uplands, than to those in coeval strata from the Laurentian margin including Girvan. Conodont colour alteration index values indicate separate thermal histories for the limestone clasts and coeval strata in the Girvan Inlier. The cryptic arc was located to north of the Northern Belt of the Southern Uplands during Ashgill time and to south of the Midland Valley in Late Silurian–Early Devonian time and clearly had a complex tectonic history.