CHAPTER TWELVE Ecology and Evolution of Cambrian Reefs The history of reef building through the Cambrian records the replacement of predominantly microbial communities by those in which sessile animals participated in construction, so heralding a new reef ecosystem with elaborate trophic webs, complex organism interactions | 12-C1099 8 10 00 2 11 PM Page 254 CHAPTER TWELVE Brian R. Pratt Ben R. Spincer Rachel A. Wood and Andrey Yu. Zhuravlev Ecology and Evolution of Cambrian Reefs The history of reef building through the Cambrian records the replacement of predominantly microbial communities by those in which sessile animals participated in construction so heralding a new reef ecosystem with elaborate trophic webs complex organism interactions increased niche partitioning and high taxonomic diversity. Thus the domical and branching stromatolites of the Proterozoic composed mainly of micron-sized crystals precipitated within or trapped upon laminar biofilms were replaced by highly cavernous sediment-generating structures constructed by the skeletons of sessile filter feeders calcified aggregations of coccoid microbes and mats of cyanobacterial filaments. A new microbial community Epiphyton-Renalcis appeared in the Nemakit-Daldynian and was joined in the Tommotian by archaeo-cyathan sponges which colonized both open surfaces and cavities. Other sessile frame builders such as corals as well as abundant obligate cryptobionts demonstrate increasing complexity of this ecosystem. Metazoan diversity reached its zenith in the early Botoman. Equally dramatic however was the disappearance of this community toward the end of the Early Cambrian. There followed a protracted interval when reefs were almost entirely microbial. Rigid spiculate demosponges began to occupy late Middle Cambrian reefs but a level of complexity comparable to that of the Early Cambrian was not achieved again until the Middle Ordovician when an increase in reef biotic diversity paralleled the radiation of the shelly benthos. FOR BILLION YEARS tropical sea floors within the photic zone which were relatively free from the influence of terrestrial runoff sediment freshwater and nutrients have hosted communities of aggregated sessile organisms. Such communities sometimes mainly microbial at other times mostly skeletal .
