Intrinsic and extrinsic controls on the diversification of the Bivalvia

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’.