Abstract

Recent and fossil faunas indicate that Polysyringia outnumbered Protobranchia and Septibranchia continuously since the Middle Ordovician; although the oldest Septibranchia are Triassic, Septibranchs were rare through Middle Cretaceous, but now are abundant in deep water and cold shallow water along with protobranchs. Polysyringians are the most numerous pelecypods in warm water, but are poorly represented in deep or cold waters. Evidence indicates extinction of many polysyringians at the end of the Cretaceous, without corresponding extinction in the other two groups, although polysyringians remained the most numerous.

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