Evolution of the deep sea protobranch bivalves

Abstract

The number of protobranch species of the continental shelves of the world comprise between 10 and 15 % of the total number of bivalve species present. This is in contrast to the bivalve fauna of the deep sea which, distant from the lower continental slopes, is dominated by the protobranchs. The protobranchs may comprise more than 70% of the bivalve species in a sample and more than 95 % of the total number of bivalve specimens present. The Subclass Protobranchia has one of the longest recorded geological histories and its continuing success, particularly in the deep sea, is probably due to a suite of physiological characters that enable it to utilize a low and refractory food supply at considerable depths and pressures. Probably as a result of the lack of competition from bivalves of more recent origin as well as the long stability of their environment, the deep sea bivalves show a radiation of form and habit that is analogous to that shown by the more recently evolved lamellibranchs of the continental shelf. The study of the bivalve fauna of the deep sea helps in the understanding of the evolution and ecology of the Mollusca of late Cambrian and early Ordovician periods.