Testing Silurian palaeogeography using ‘European’ ostracod faunas
The distributions of representative European and eastern North American species of the three major ostracod groups present in the Silurian independently support the standard palaeogeographical reconstructions of the ‘North Atlantic’ region for that time period. A range of assumed benthic, biostratigraphically key palaeocope and podocope ostracods show a provincial distributional pattern that characterizes a ‘European’ ostracod faunal province which reflects the amalgamated palaeoplates of Avalonia and Baltica. By contrast, representatives of the low-diversity, late Silurian, supposedly pelagic myodocope ostracod fauna from the UK, France, the Czech Republic, Sardinia and other locations globally seemingly have much wider dispersal capacity that includes, for example, trans-(Rheic)ocean locations on the Avalonia, Baltica, Armorica (West Gondwana) and Perunica palaeoplates. That there was an apparent major barrier to the migration of supposed benthic but not pelagic ostracod faunas strongly supports the notion of the presence of the Rheic Ocean and adjacent palaeocontinents. Comparable provincial and trans-oceanic distributional patterns are evident amongst Recent benthic and pelagic ostracods. The ostracod faunas of Avalonia/Baltica seemingly remained essentially distinctive, at least at species level, from those of the remainder of the Laurentian plate in the Silurian, but the reason for this is uncertain.
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The Early Palaeozoic was a critical interval in the evolution of marine life on our planet. Through a window of some 120 million years, the Cambrian Explosion, Great Ordovician Biodiversification Event, End Ordovician Extinction and the subsequent Silurian Recovery established a steep trajectory of increasing marine biodiversity that started in the Late Proterozoic and continued into the Devonian. Biogeography is a key property of virtually all organisms; their distributional ranges, mapped out on a mosaic of changing palaeogeography, have played important roles in modulating the diversity and evolution of marine life. This Memoir first introduces the content, some of the concepts involved in describing and interpreting palaeobiogeography, and the changing Early Palaeozoic geography is illustrated through a series of time slices. The subsequent 26 chapters, compiled by some 130 authors from over 20 countries, describe and analyse distributional and in many cases diversity data for all the major biotic groups plotted on current palaeogeographic maps. Nearly a quarter of a century after the publication of the ‘Green Book’ (Geological Society, London, Memoir 12, edited by McKerrow and Scotese), improved stratigraphic and taxonomic data together with more accurate, digitized palaeogeographic maps, have confirmed the central role of palaeobiogeography in understanding the evolution of Early Palaeozoic ecosystems and their biotas.