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Abstract Siphons in bivalves have been postulated as a key adaptive trait, enabling modes of life inaccessible to asiphonate lineages, that afford better protection from predation and dislodgement, thereby enhancing their taxonomic diversification. To test the impact of siphons on diversity, we compared two bivalve clades with similar shell forms and life positions that differ in the presence/absence of this supposed key trait: the asiphonate Archiheterodonta (origin ~ 420 Myr ago) and the siphonate Veneridae (origin ~ 170 Myr ago). We measured three characters relevant to burrowing (shell length, cross-sectional area, and proportional shell volume) in these two groups, finding that siphonate venerids occupy more modes of life than archiheterodonts because they can live at a greater range of distances from the sediment–water interface, with the thinnest shells occurring in the deepest-burrowing groups. Asiphonate taxa have thicker shells, perhaps as a compensatory adaptation in response to the potential for exposure and attack because they are limited to shallower depths of burial. The lack of siphons may have impeded morphologic and taxonomic diversification in archiheterodonts. In contrast, siphons are consistent with a key adaptive trait in the Veneridae, evidently enabling taxonomic diversification into a greater range of morphologies.