Cephalopods have their earliest occurrence in Late Cambrian shallow-water carbonates on the North China Platform and rapidly dispersed across the globe within the latest Cambrian. Latest Cambrian and initial Ordovician cephalopod occurrences are restricted to the palaeotropical realm. The Ordovician records a unique morphological diversification and expansion of cephalopod habits and habitats which is expressed in a massive morphological diversification and unique palaeogeographical patterns of dispersal. The Ordovician cephalopod diversification was a complex process of appearance and disappearance of higher groups with a specific palaeogeographical signature and a clear selective component. A general Ordovician trend showed decreasing evolutionary turnover rates, increasing number of widespread genera, decreasing proportion of endemic genera, and decreasing beta-diversity. This is interpreted as a result of an increasing ecosystem stability during this time interval.
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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.