BRYOZOAN SCLEROBIONTS ACROSS MASS EXTINCTIONS: PREDICTED EFFECTS OF MAJOR ENVIRONMENTAL CHANGES

Bryozoans have been key components of hard substrate communities since the Ordovician. Although under-researched, they provide the best opportunity for studying the effects of known and hypothesized environmental changes associated with mass extinctions and their aftermaths on a group of colonial sclerobionts. These changes include phytoplankton crashes, climatic excursions, anoxia, and high CO₂ levels, as well as declines in the availability and size of organic substrates, and in competitor and predator diversities and numbers. For example, a brief increase in the proportion of cheilostome to cyclostome bryozoans in the early Danian of Denmark is consistent with a phytoplankton crash because cyclostomes have generally lower trophic demands. However, new collections from the south-eastern USA and The Netherlands have fail to replicate this pattern. Food and substrate resource shortages following mass extinctions might be expected to favour runner- and spot-like over sheet-like encrusting colonies but there is as yet no evidence to support this inference. Zooid size within bryozoan species varies inversely with temperature and a recent study has applied this relationship to track temperature changes leading up to the KT boundary in Denmark. The potentially devastating effects of high CO₂ levels on bryozoans with calcareous skeletons during the end Permian event is supported by the existence of numerous ghost lineages of non-calcified ctenostomes that apparently survived this event compared with the huge extinctions suffered by the calcified stenolaemates.