The Lower Palaeozoic palaeobiogeography of Bivalvia
Bivalves first appeared in the Early Cambrian and were virtually cosmopolitan. These very small and insignificant molluscs were probably surface crawlers on the microbial mat floors of the Cambrian sea. They evolved little further in the Mid Cambrian and by the Late Cambrian had apparently disappeared from the fossil record. Their re-appearance in the Early Ordovician coincided with a major diversification in which all the principal bivalve clades evolved, but the class was confined to Gondwana; their habitat was now principally infaunal in siliciclastic sediments. In the Mid Ordovician a few forms reached Baltica and the eastern Laurentian margin, but it was the Late Ordovician before bivalves once again became cosmopolitan. This geographical dispersal allowed bivalves to colonize the low-latitude carbonate platforms and led to the development of diverse epifaunal faunas, although most remained as infaunal forms in siliciclastic sediments. The end-Ordovician regression occasioned by the Hirnantian glaciation caused major extinction of those epifaunal forms restricted to the carbonate platforms. The Silurian faunas were cosmopolitan and the major evolutionary event was caused by the appearance of a Gondwanan cephalopod limestone facies that provided sites for epibyssate praecardiidinid bivalves (=Nepiomorphia) that evolved rapidly and were able to withstand short periods of anoxia.
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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.