The calcareous skeletons of modern bryozoans are major contributors to sediments only outside the tropics, and many temperate limestones are dominated by bryozoans, often in conjunction with mollusks in the so-called “bryomol” grain association. Can the presence of rock-forming abundances of bryozoans be used to recognize nontropical sediments in the geologic past? We have addressed this question by assembling a database of 176 Ordovician-Pleistocene bryozoan-rich deposits and plotting them on paleogeographical maps to obtain their latitudes of formation. The results reveal a striking difference between post-Paleozoic and Paleozoic latitudinal distributions. Whereas the great majority (94%) of post-Paleozoic bryozoan-rich deposits formed outside the tropics, Paleozoic bryozoan-rich deposits have a more variable latitudinal distribution, most (68%) having formed within the tropics. The presence of rock-forming abundances of bryozoans in Paleozoic sediments should not, therefore, be used to infer nontropical carbonate deposition. The change in latitudinal distribution between the Paleozoic and post-Paleozoic corresponds with a major taxonomic turnover in bryozoan faunas. Biotic interactions, especially with predators, may have led to the exclusion of sediment-producing bryozoans from the post-Paleozoic tropics. These evolutionary changes have had a more profound effect on carbonate platforms than has been previously appreciated.

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