ONSHORE INITIATION OF THE AGRONOMIC REVOLUTION: QUANTITATIVE EVIDENCE FROM LOWER CAMBRIAN STRATA IN THE GREAT BASIN

The advent of macroscopic, motile organisms during the Ediacaran-Cambrian transition permanently changed the character of marine substrates. As communities of benthic metazoans evolved and expanded, bioturbation became more extensive, complex, and physically disruptive. Over time, biogenic sediment disruption dramatically increased sediment water content, facilitating metazoan habitation of infaunal ecospace and promoting the development of a modern mixed layer. Seilacher and Pflüger (1994) termed this early example of metazoan ecosystem engineering the agronomic revolution, and Bottjer and colleagues (2000) recognized the significant ecological and evolutionary effects of this transition as the Cambrian substrate revolution.

Quantitative data from correlative shallow marine Lower Cambrian strata in California and Nevada suggest that the agronomic-substrate revolution commenced in nearshore marine environments before progressing offshore. This pattern may be analogous to the onshore origination of higher taxa in shelf communities (e.g. Jablonski et al., 1983). Digital images of bioturbated bedding planes were analyzed using the intersection grid method (Marenco and Bottjer, 2010) to produce quantitative (percentage) estimates of bioturbation intensity. Results indicate that bioturbation intensity is tied to facies in these successions, with bedding planes from nearshore facies typically exhibiting higher bioturbation intensities than their offshore equivalents. In addition, data from the more proximal Death Valley succession indicate a trend toward higher bioturbation intensities through time. These results demonstrate the utility of quantitative bioturbation data for elucidating the evolutionary history of early shallow marine benthic communities.