The Ecology of the Cambrian Radiation

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 archaeocyathan 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 3.5 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 metazoan—acquired topographic relief and typically were the sites of synsedimentary cementation, thereby forming reefs.

Here we summarize the composition of Cambrian reefs, outline its community

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