Paleoecology of benthic metazoans in the Early Cambrian Maotianshan Shale biota and the Middle Cambrian Burgess Shale biota: evidence for the Cambrian substrate revolution

Abstract

As the depth and intensity of bioturbation increased through the Proterozoic–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 [Seilacher, A., Pflüger, F., 1994. From biomats to benthic agriculture: a biohistoric revolution. In: Krumbein, W.E. (Ed.), Biostabilization of Sediments. Bibliotheks and Informationsystem del Carl von Ossietzky Universitat, Oldenburg, 97–105], and the impact of the subsequent development of the mixed layer on benthic metazoans has been termed the Cambrian substrate revolution [Bottjer, D.J., Hagadorn, J.W., Dornbos, S.Q., 2000. The Cambrian substrate revolution. GSA Today 10 (9), 1–7]. Because the Early Cambrian was a transitional time in this substrate revolution, it is hypothesized that benthic metazoans adapted to typical Proterozoic-style soft substrates co-existed with benthic metazoans adapted to more typical Phanerozoic-style soft substrates. Paleoecological examination of the benthic metazoans of the Early Cambrian Maotianshan Shale (Chengjiang) biota and Middle Cambrian Burgess Shale biota, combined with examination of core samples from the rocks in which the Maotianshan Shale biota is preserved, was performed in order to test this hypothesis. Results of the core analysis reveal that the rocks in which the Maotianshan Shale biota was preserved, and therefore the substrate on which it lived, contains almost no evidence for bioturbation of any kind, while the immediately underlying rocks of similar lithology are dominated by intermediate levels of horizontal bioturbation and contain some evidence for the beginning of mixed layer development. These results suggest that benthic metazoans of the Maotianshan Shale biota lived on a typical Proterozoic-style soft substrate, and that environmental conditions associated with the exceptional preservation of the Maotianshan Shale biota may have also suppressed bioturbation levels. Results of the paleoecological analysis indicate that the Maotianshan Shale and Burgess Shale biotas contain mobile and sessile benthic metazoans adapted to typical Proterozoic-style soft substrates and others adapted to typical Phanerozoic-style soft substrates. In addition, the Maotianshan Shale biota is dominated by benthic suspension feeding genera adapted to typical Proterozoic-style soft substrates, indicating that this biota lived during the early stages of the Cambrian substrate revolution. The Burgess Shale biota, however, contains more benthic suspension feeding genera adapted to typical Phanerozoic-style soft substrates than the Maotianshan Shale biota, which suggests that the soft substrate on which they lived had more advanced levels of mixed layer development. 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 siliciclastic soft substrate environments during the Cambrian.