LOWER CAMBRIAN TRACE FOSSILS OF EASTERN CALIFORNIA: ENGINEERING AN ECOLOGICAL REVOLUTION

Post-Cambrian shallow subtidal siliciclastic rocks tend to contain abundant vertical bioturbation and lack microbially-mediated sedimentary structures. Lower Cambrian shallow subtidal siliciclastic strata, in contrast, typically contain limited vertical disruption of bedding and common microbially-mediated sedimentary structures, features indicative of a substrate made firm by abundant and extensive microbial mats. “Matground” substrates are considered typical of the Neoproterozoic, and their presence in the Early Cambrian suggests that dominance by microbial mats prevented early benthic metazoans from burrowing deeply beneath the sediment-water interface. Seilacher and Pflüger (1994) proposed the “agronomic revolution” to show how benthic metazoans may have overcome the barrier to infaunalization that matgrounds presented by acquiring evolutionary adaptations to vertical burrowing. Bottjer et al. (2000) termed the ecological and evolutionary significance of this transition from two- to three-dimensional exploitation of infaunal ecospace the “Cambrian substrate revolution.” The agronomic and Cambrian substrate revolutions represent an early example of significant metazoan ecosystem engineering, in which organisms played a direct or indirect role in altering the availability of resources for other species.

The main objective of this research was to gain an understanding of the early stages of the Cambrian substrate revolution by examining the Lower Cambrian shallow marine siliciclastic trace fossil record of the White-Inyo Mountains, eastern California. Lower Cambrian siliciclastic strata of the White-Inyo Mountain succession represent a range of depositional environments and contain abundant horizontal bioturbation, predominantly in the form of shallow vermiform burrowing. Preliminary bedding plane data from this section indicate a gradual trend toward increased horizontal bioturbation intensity through time. In addition, x-rays and thin sections of finely-sampled intervals reveal the presence of limited vertical bioturbation. These results suggest that substrate engineering by soft-bodied organisms began to have a significant impact on the ecology of shallow subtidal siliciclastic marine environments in the early stages of the Cambrian substrate revolution.