Bivalves and gastropods, prominent members of the Modern Evolutionary Fauna, are traditionally noted for sharing remarkably similar global diversity trajectories and environmental distributions throughout the Phanerozoic. By comparing their fossil occurrences at several scales within a finely resolved geographic, environmental, and temporal framework, it is possible to evaluate whether such similarities are caused primarily by intrinsic macroevolutionary factors or extrinsic ecological factors. Using a database of 7779 global gastropod and bivalve genus occurrences, we investigate the geographical and environmental attributes of bivalves and gastropods during the Ordovician Period at scales ranging from global, to a comparison among five paleocontinents, to an intracontinental comparison of four regions within Laurentia. Although both classes shared statistically indistinguishable global diversity trajectories and broadly similar environmental distributions during the Ordovician, their environmental distributions differed in several significant features. Furthermore, the diversity trajectories and environmental distributions of these classes differed significantly among paleocontinents and among regions within Laurentia. Bivalves were consistently most diverse in deeper water, siliciclastic-rich settings in higher-latitude paleocontinents whereas gastropods were consistently most diverse in shallower, carbonate-rich settings in more-equatorial paleocontinents. Notably, these environmental differences were robust to changing physical parameters within paleocontinents, with each class consistently tracking its preferred environmental setting. These results suggest that environmental factors played significant, albeit distinct, roles in the Ordovician diversifications of gastropods and bivalves. However, their similar global diversity trajectories suggest that shared, intrinsic macroevolutionary attributes also may have played an important role in the evolution of these classes during the Ordovician Radiation.

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