Early Ordovician community evolution with eustatic change through the middle Beekmantown Group, northeast Laurentia

The Beekmantown Group records the important early interval of the Ordovician Radiation. This Upper Cambrian–Middle Ordovician, carbonate-dominated, tropical succession was deposited near the eastern passive margin of the Laurentian platform. This depositional setting remained remarkably stable although the craton was flooded repeatedly with eustatic rises and unconformity-bound, macroscale sedimentary cycles were deposited as successive geological formations. The individual depositional cycles (i.e., formations) show a nearly identical vertical succession with a type 1 sequence boundary, a basal conglomerate, transgressive sandstones, locally a subtidal shale-dominated unit that marks the deepest facies, and a highstand carbonate facies with thrombolite buildups in its middle part. The thrombolitic buildups of each depositional cycle contain a mollusc-dominated macrofauna that changed remarkably from cycle to cycle. In the limestones of the Upper Cambrian Ritchie and Rathbunville School members, the macrofauna is very rare and of low diversity. By comparison, the absolute abundance of macrofossils is high throughout the Lower Ordovician thrombolitic limestones. The genus-level diversity of brachiopods, trilobites, gastropods, and cephalopods increased moderately during the three Lower Ordovician depositional sequences. Dramatic changes in cephalopod disparity, body size, and biomass indicate significant paleoecological changes at the top of the ecosystem food chains, and are an indication of community evolution and intrinsic evolutionary processes. Increased coiling and ornamentation in cephalopods and an increasing number of large gastropod genera with thick shells indicate an escalation among predators. We interpret these changes as evidence for a rise in competition within ecological guilds by a continuing increase in internal differentiation of the food web. Increased organismal interaction and the differentiation of the food web (i.e., community evolution) are regarded as a major driving mechanism early in the Ordovician Radiation.