SOD BUSTERS ALONG THE OLD CAMBRIAN TRAIL

As the depth and intensity of bioturbation increased through the Proterozooic-Phanerozoic transition, the substrates on which marine benthos lived changed from being relatively firm with a sharp sediment-water interface to having a high water content and blurry sediment-water interface. Microbial mats, once dominant on normal marine Proterozoic seafloors, were relegated to stressed settings lacking intense metazoan activity. This change in substrates has been termed the agronomic revolution, and the impact of this substrate transition, particularly the development of the mixed layer, on benthic metazoans has been termed the Cambrian substrate revolution. Previous work on the Cambrian substrate revolution has focused on the evolutionary response of Cambrian benthic echinoderms and the ecological response of early grazing molluscs to these substrate changes.

Because the Cambrian was a transitional time during these substrate changes, seafloors during this time contained a mix of typical Proterozoic-style substrates that co-existed with more typical Phanerozoic-style soft substrates. Examination of the Lower Cambrian Meishucun Formation in southwest China shows an early phase of these substrate changes, with evidence showing some seafloors were covered with microbial mats, while others show evidence for relatively intense bioturbation. This change in substrate styles might also be reflected by a change in adaptive morphology of benthic metazoans through the Cambrian. This hypothesis was tested through examination of the life styles of benthic organisms from two Cambrian Lagerstätten, the Early Cambrian Maotianshan Shale (Chengjiang, southwest China), and the Middle Cambrian Burgess Shale. Indeed the younger Burgess Shale fauna contains a greater number of benthic suspension feeding genera adapted to typical Phanerozoic-style soft substrates than does the Maotianshan Shale fauna.

This research suggests that the adaptive radiation of benthic metazoans during the “Cambrian explosion” was driven in part by the Cambrian substrate revolution, as benthic metazoans were forced to adapt to the development of the mixed layer in subtidal soft substrate environments during the Cambrian.