Latitudinal distribution of bryozoan-rich sediments in the Ordovician

Most bryozoans have calcareous skeletons that locally contribute large amounts of carbonate sediment to the sea floor. Whereas Recent bryozoans are diverse in shelf seas pan-globally, it is only in mid to high latitudes that they are potential limestone producers; tropical bryozoans invariably have too small a biomass relative to other carbonate producers (corals, algae and molluscs) to be important sources of sediment. During the Palaeozoic, however, bryozoan-rich deposits were formed at all palaeolatitudes, including the tropics. Extending the work of Taylor & Allison (1998), we have compiled data on 42 occurrences of bryozoan-rich deposits of Ordovician age to determine whether the Palaeozoic distributional pattern extends back to their earliest appearance in the fossil record. Estimated palaeolatitudes of deposition ranged from 10-75, but the majority (71%) were found to be tropical, i.e. < 23.5 degrees. Of the 14 reefal occurrences, 11 (79%) were formed in tropical palaeolatitudes. No significant trend in depositional palaeolatitude could be detected with time through the Ordovician. The most persuasive explanation for the broader palaeolatitudinal distribution of bryozoan-rich deposits (including reefs) in the Ordovician than at the present day is that durophagous predators were ecologically unimportant, allowing large erect, sediment-producing bryozoan colonies to grow in the tropics where today they are vulnerable to grazing fishes, decapods and echinoderms.