The Cambrian Benthic Regime

The increase in bioturbation levels during the Cambrian radiation created an interval with a distinctive suite of paleoecological, taphonomic, and ichnological characteristics, referred to here as the “Cambrian benthic regime.” Quantitative studies of the substrate adaptations of early epifaunal echinoderms and epifaunal suspension feeders in the middle Cambrian soft-bodied Chengjiang biota reveal a dominance of adaptations to firmer substrates more typical of the Proterozoic (Dornbos, 2006; Dornbos and Chen, 2007). These adaptations peak in occurrence during the early and middle Cambrian. Analysis of the temporal distribution of soft-bodied Burgess Shale-type deposits and phosphatized metazoan embryos reveals a similar pattern, as they are both most common during the early and middle Cambrian (Allison and Briggs, 1993; Donoghue et al., 2006). One explanation for this distribution is that the persistence of firm Proterozoic-style substrates into the Cambrian created many benthic environments in which the oxic-anoxic boundary was shallow within seafloor sediments, allowing exceptional taphonomic pathways to occur more frequently. The presence of firm substrates during the early to middle Cambrian is also indicated by the dominant subtidal trace fossil preservation style. Trace fossils during this time typically exhibit preservation of shallow tiers, open burrow systems, and surficial scratches, while showing limited evidence for mixed layer development (Droser et al., 2002; Jensen et al., 2005). Preservation of all of these ichnological features in subtidal settings provides strong evidence for the prevalence of firm substrates in many early to middle Cambrian environments. These independent lines of paleoecological, taphonomic, and ichnological evidence point to a time during the early to middle Cambrian when subtidal seafloor sediments where often still firm like those of the Proterozoic. The Cambrian benthic regime was thus a unique transitional era in the history of life between benthic realms dominated by layered microbial communities to those dominated by metazoan activity.