Relative to epicontinental seas, shallow, open-ocean settings are underrepresented in the Paleozoic record, and it is possible that higher taxa associated with these environments that appeared to diversify in the post-Paleozoic may have unpreserved Paleozoic histories. As an initial test of this possibility, we evaluated the statistical affinity for open-ocean–facing environments of two elements of the modern fauna that appear to diversify in the middle Mesozoic, the veneroid bivalves and the buccinoid gastropods, and showed that, indeed, Mesozoic genera in both groups had statistical propensities to occur in ocean-facing settings. In contrast, two groups with extensive Paleozoic fossil records had either an affinity for epicontinental seas (the terebratulid brachiopods), or showed no statistical preference for either setting (the mytilid bivalves). We conducted molecular clock analyses to estimate the divergence times of these taxa, and found that our estimates for the origin of terebratulids and mytilids in the Paleozoic were concordant with their known geological records. While our molecular estimate for the origin of the buccinoids was also concordant with their known record, with an estimated origin in the Jurassic, molecular data indicate that the veneroids originated in the Paleozoic, more than 200 m.y. before their first fossil appearance in the Mesozoic. This intriguing result raises the possibility that additional elements of the modern fauna with preferences for open-ocean–facing settings may have had more extensive Paleozoic histories than heretofore appreciated.