FOSSIL INVERTEBRATES/Bryozoans 319 Figure Diagram of changing family diversities in the bryozoan orders Tentative phylogenetic relationships are indicated by dashed lines a few modern species have restricted bathymetric distributions, many have depth ranges exceeding 100 m High diversities of bryozoans can be found in all major climatic zones, but tropical species tend to be inconspicuous, typically occurring as small encrusting colonies in cryptic habitats such as the undersides of stones and shells Bryozoans are a major contributor to modern cool-water carbonates, such as those forming on the southern Australian continental shelf In the geological record bryozoans are often found in association with carbonate facies The best collecting opportunities are in unconsolidated fine-grained clastics, where surface picking, bulk sampling, and gathering of shells that may be encrusted with bryozoans can yield diverse faunas In some instances, bryozoans are present in rock-forming abundance, usually comprising branch fragments of parautochthonous erect colonies (Figure 7) and less often small bioherms of robust in-situ colonies Bryozoan limestones in the Cenozoic are typically clean-washed cross-bedded calcarenites or calcirudites Mud mounds, including the Waulsortian reefs of the Carboniferous, often contain bryozoans It has been suggested that the bryozoans assisted in mud sedimentation by acting as baffles and through the production of faecal pellets Bryozoan-rich deposits show a striking change in latitudinal distribution through time, from a panlatitudinal pattern in the Palaeozoic to an extratropical distribution in the post-Palaeozoic The apparent displacement from the tropics of bryozoans contributing significant amounts of carbonate sediment may be related to the end-Permian mass extinction event, which had a severe effect on stenolaemate Figure Bryozoan limestone from the Eocene of North Carolina, USA A poorly cemented facies of the Castle Hayne Limestone consisting mainly of broken branches of tree like cyclostome and cheilostome colonies Scale bar 10 mm orders, and/or to increasing levels of colony destruction by grazing predators and exclusion by fast-growing corals and calcareous algae in the post-Palaeozoic The continuous presence of bryozoans through thick sequences deposited over long intervals is unusual Instead, they tend to be patchily distributed, which is one of the factors that has hindered their use in biostratigraphy Another limitation arises from typically long species durations; e.g for cheilostomes currently living in the Mediterranean Sea and represented in the fossil record, average species longevity is about 15 Ma