Abstract

Muscle scars on internal molds of the Early Cambrian bivalve Pojetaia runnegari help confirm P. A. Jell's suggestion that it is closely related to Ordovician and younger palaeotaxodonts. Characters which differentiate Pojetaia from Ordovician palaeotaxodonts and which have been used to unite it with the only other known Cambrian bivalve, Fordilla, probably result from its very small size (ca. 1 mm). The bivalve order Fordilloida, proposed initially for Fordilla and subsequently expanded to include Pojetaia, is no longer useful, since the latter genus can be referred to the family Praenuculidae of the palaeotaxodont order Nuculoida, and the former genus seems to be a modiomorphid mytiloid. Both Pojetaia and Fordilla have long stratigraphic ranges; Pojetaia from late Tommotian to the end of the Early Cambrian, and Fordilla from about early Atdabanian to late Early Cambrian. Pojetaia and Fordilla are suitable ancestors for most of the known higher taxa of bivalves.--Modified journal abstract.

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