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.