BRYOZOAN CARBONATES THROUGH TIME AND SPACE

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-formin g abundances of bryozoans be used to recognize nontropical sediments i n the geologic past? We have addressed this question by assembling a d atabase of 176 Ordovician-Pleistocene bryozoan-rich deposits and plott ing them on paleogeographical maps to obtain their latitudes of format ion. The results reveal a striking difference between post-Paleozoic a nd 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 di stribution, 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-Pal eozoic corresponds with a major taxonomic turnover in bryozoan faunas. Biotic interactions, especially with predators, may have led to the e xclusion of sediment-producing bryozoans from the post-Paleozoic tropi cs, These evolutionary changes have had a more profound effect on carb onate platforms than has been previously appreciated.