ECOSYSTEM ENGINEERS DURING THE CAMBRIAN EXPLOSION: TRACE FOSSIL RECORD FROM THE LOWER CAMBRIAN OF EASTERN CALIFORNIA

Early Cambrian shallow subtidal marine environments were characterized by firm, typically microbially-bound substrates that restricted the burrowing activities of benthic organisms. These conditions are reflected by low ichnofabric indices, high bedding plane bioturbation indices, and common suspect microbial features in Lower Cambrian siliciclastic strata of the White-Inyo Mountains, eastern California. Upon evolving widespread adaptations for vertical burrowing, benthic organisms engineered a dramatic shift toward soft, “mixed-layer” substrates, forcing microbial mats into marginal environments and thereby expanding the infaunal habitat for three-dimensional exploitation by the rapidly diversifying Cambrian marine fauna. The evolutionary and ecological impact of this change in substrate conditions on other benthic metazoans has been termed the “Cambrian substrate revolution” by Bottjer et al. (2000). The Cambrian substrate revolution represents one of the earliest examples 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 goal of this research was to begin to identify which particular organisms were the primary substrate engineers in the Early Cambrian. Siliciclastic strata of the Lower Cambrian succession in the White-Inyo Mountains contain abundant horizontal bioturbation, predominantly in the form of shallow vermiform burrowing. Vermiform burrows, many assigned to Planolites, occur in several size classes, some with a minor vertical component and others intersecting suspect microbial features such as wrinkle structures. On most bedding planes with high bioturbation indices (i.e. 4 or 5), densely overlapping vermiform burrows leave few undisturbed patches exposed. Dominance of the ichnofauna by vermiform burrows occurs in all depositional environments examined, from nearshore subtidal to storm-dominated shelf. These data indicate that vermiform organisms were the principle substrate engineers in shallow subtidal siliciclastic environments represented by Lower Cambrian strata of the White-Inyo Mountains.