Palaeobiogeography of Early Palaeozoic vertebrates
The oldest known Palaeozoic vertebrate record currently is Early Cambrian in age. The first taxa with mineralized exoskeletons are at least Ordovician in age, followed by a sporadic fossil record with Talimaa’s Gap of c. 3 myr in the Rhuddanian (earliest Silurian). Ordovician and Silurian vertebrate faunas are dominated by ‘agnathans’. Early Palaeozoic vertebrates occupied a wide range of environments: nearshore marine to restricted marine in the Ordovician, and on the marine epicontinental shelves of the Silurian. Silurian vertebrates are useful biostratigraphical indicators, as well as good markers of palaeocontinental margins. Two main palaeobiogeographical units are renamed for the Ordovician: a Gondwana Realm and a Laurentia–Siberia–Baltica Realm. Vertebrate fossil localities are more numerous in the Silurian; therefore a series of palaeobiogeographical provinces and realms are defined on Laurentia, Baltica, Avalonia, Siberia, South China and East Gondwana. More discoveries of Silurian vertebrate-bearing localities should certainly help to define additional provinces, in particular along the northern margins of Gondwana and in SE Asia.
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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.