Abstract The biogeography of marine shelf ostracod genera is analysed for two Ordovician time slabs, the earliest Late Ordovician and the terminal Ordovician, that have been considered to reflect comparatively warmer and cooler global climate states, respectively. The earlier time slab is equivalent to the Nemagraptus gracilis graptolite interval (centred about 460 Ma), and defined as the total range of the eponymous species. The Hirnantian time slab comprises the Normalograptus extraordinarius and Normalograptus persculptus graptolite biozones (445.6–443.7 Ma). The ostracod dataset consists of 160 taxa from 24 early Late Ordovician localities and 86 taxa from 10 Hirnantian localities. Ordination and variation partitioning analyses show that patterns in ostracod distribution in the gracilis time slab are largely related to palaeocontinental affinity of the samples and to a lesser degree to palaeolatitude. Some decrease of provincialism can be suggested for the Hirnantian, although the ostracod dataset is limited for this interval.

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