Molecular analyses of 13 species of the marine bivalve family Thyasiridae, using sequences from 18S rRNA and 28S rRNA genes, showed that the family is monophyletic despite the anatomical disparity and inclusion of both chemosymbiotic and asymbiotic species. This new analysis also confirmed that the three families (Thyasiridae, Lucinidae and Ungulinidae), previously included in the Lucinoidea, were not closely related. Four species of Ungulinidae grouped within a clade containing Veneridae, Arcticidae and Mactridae. In relation to a range of other heterodont bivalves, Thyasiridae occupied a near basal position, apart from a clade comprising Carditidae/Astartidae/Crassatellidae. The earliest thyasirid recognized in the fossil record is a species from the Lower Cretaceous of England. Within the Thyasiridae, some groups can be identified but relations between these are weakly supported. Amongst the taxa analysed, those with symbiotic bacteria and two ctenidial demibranchs belong to at least three groups, while there is some support for a clade of asymbiotic taxa with single demibranchs. In recognition of the monophyletic status of the Thyasiridae, distinct from all other heterodont bivalves, we elevate the rank to superfamily Thyasiroidea.