The Taxonomic, Palaeoecologic and Biogeographic Importance of the Late Ordovician Laurentian Ostracod Fauna of the Girvan District, SW Scotland

Ostracods form a numerically important component of certain Late Ordovician faunas of the Girvan district, southwest Scotland. Fifty-two ostracod taxa are identified, including twenty-three discussed in open nomenclature, and seventeen species that are considered as nomen dubia. The fauna reveals new and important data bearing on the biostratigraphic, palaeogeographic and palaeoecologic significance of the Girvan Lower Palaeozoic succession. Much of the Ordovician ostracod fauna of the Girvan district preserved in clastic deposits represents transported material. However, those ostracods from the early Katian Craighead Limestone Formation define shallow marine and open marine faunas respectively. The Girvan ostracods include many short-ranging species which are useful for inter-regional correlation of the Late Ordovician succession of Scotland and North America; these include Hippula ventrospinosa, Balticella deckeri, Eurychilina sunbloodensis and Baltonotella parsispinosa which characterize Sandbian age strata, and Kinnekullea comma which is typical for the Katian (anceps graptolite Biozone). Biogeographically most of the Girvan fauna is typically of North American aspect at the species level but from the early Katian onwards shows generic links with Baltica and Avalonia. The spatial and temporal distribution patterns of the Girvan Late Ordovician ostracod fauna are assessed in the context of the entire Late Ordovician Laurentian ostracod assemblage particularly for the Sandbian interval, where ostracod diversity was highest at species and genus level. Multivariate analysis demonstrates that geography had the strongest control on the global distribution of Ordovician benthic ostracods. Within Laurentia Sandbian ostracods show striking inter-basinal endemism at species level, ranging from 25% endemic in the Foxe Basin of the Franklin district, Canada to 75% in the Michigan basin. This pattern of endemism is common in other benthic groups such as brachiopods, and may indicate rapid speciation in microhabitats. Despite the strong overprint of local endemism, pan-continental faunal differences are noticed between the midcontinent and marginal Laurentian ostracod assemblages that are confirmed by multivariate analysis, and these allow the demarcation of a ‘Midcontinent’ and a southern ‘Marginal’ province for the Sandbian. Ostracod assemblages of the two provinces appear to have been controlled by physical parameters such as temperature and salinity which are related to water depth and latitude.