Graptolite faunas exhibited strong biogeographical differentiation during the Early Palaeozoic, particularly in the Ordovician. Skevington recognized two major faunal provinces, the high to mid palaeolatitude ‘Atlantic Province’ and the low-palaeolatitude ‘Pacific Province’. Subsequent workers have generally accepted this pattern of graptolite distribution, but the controls on this pattern have been the subject of considerable debate. Two competing models have emerged: a surface water temperature model and a depth stratification model. It is likely that the some of the physical and chemical oceanic factors that vary with latitude may also vary in a similar way along an onshore to offshore transect. Hence, it may be that both depth and surface temperature play an important role in biogeographical differentiation. Biogeography also played a critical role in the evolutionary history of graptoloids. Important examples include the origination of axonophorans in deep, offshore environments from isograptid and pseudisograptid ancestors and their subsequent migration into shallow water regions; the replacement of the Diplograptina by Neograptina in the low palaeolatitudes during the Late Ordovician extinction event; and the origination of expansograptids in the ‘Atlantic’ Province as shallow water endemics followed by their worldwide dispersal into the oceanic biofacies.
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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.