INTRODUCTION

At the present day the Class Bivalvia comprises some 8000 species, sub-divided into approximately 700 genera, 107 families, and 41 superfamilies (Morton, 1996, and references therein). It represents one of the most diverse invertebrate groups within the marine realm and is often portrayed as the end-product of a long-term, adaptive radiation (e.g. Stanley, 1977). Over the last 250 Ma, in particular, there has been a steady but inexorable rise in the number of bivalve taxa (Fig. 10.1), and today they have come to occupy a very broad spectrum of benthic habitats and trophic categories (e.g. Bottjer, 1985). This is what Morton (1996, p. 348) has referred to as the ‘expanding success of the Bivalvia’.

Although the possible effects of the ‘pull of the Recent’ cannot be ignored (Hallam & Miller, 1988), it would appear that there was a particularly steep rise in the number of bivalve taxa through the latest Mesozoic and Cenozoic eras (Fig. 10.1). Other benthic groups, such as the echinoids, gastropods, decapod crustaceans and fishes, show this pattern too (e.g. Vermeij, 1977), and it becomes a matter of some importance to determine why this should be so. Geologists are now almost certain that, over the greater part of the last 90 Ma, marine climates have been deteriorating (Pickering, this volume); glacial climates may now be traced back over 40 Ma in Antarctica (see below).