Growth rate, extinction and survival amongst late Cenozoic bivalves of the North Atlantic

Late Cenozoic bivalve extinction in the North Atlantic area has been attributed to environmental deterioration. Within scallops and oysters – groups with a high growth rate – certain taxa which grew exceptionally fast became extinct, while others which grew slower survived. Those which grew exceptionally fast would have obtained protection from predators thereby, so their extinction may have been due to the detrimental effect of environmental change on growth rate and ability to avoid predation, rather than environmental change <i>per se</i>. We investigated some glycymeridid and carditid bivalves – groups with a low growth rate – to see whether extinct forms grew faster than extant forms. Extinct <i>Glycymeris subovata</i> grew at about the same rate as the slowest-growing living glycymeridid and much slower than late Cenozoic examples of extant <i>G. americana</i>, which grew at about the same rate as the fastest-growing living glycymeridid. Extinct <i>G. obovata</i> and extinct <i>Cardites squamulosa ampla</i> also grew slower than <i>G. americana</i>. These findings indicate that within bivalve groups with a low growth rate, extinction or survival of taxa through the late Cenozoic was not influenced by whether they were relatively fast or slow growers. By implication, environmental change acted directly to cause extinctions in these groups.