MICROBIAL MATS AND SILICICLASTIC SUBSTRATES: INTERPLAY WITH EVOLVING METAZOANS

The first several billion years of life on Earth was microbial, and Precambrian carbonates are well known for sedimentary structures such as stromatolites where growth was largely mediated by microbial activity. Until relatively recently, sedimentary structures mediated by microbial communities living on subaqueous siliciclastic sediments were relatively unstudied. Such structures are particularly evident in siliciclastic rocks deposited in submarine environments from 500-600 million years ago during the Neoproterozoic-Phanerozoic transition. This was a time on Earth when metazoans were beginning to evolve the ability to burrow, but before bioturbation was so extensive that it could destroy the possibility for formation of submarine microbial sedimentary structures. These structures can have a variety of surface macroscopic morphologies that have led to evocative names such as “wrinkle structures” and “elephant skin”.

Along with bioturbators this was also the time when metazoans with other benthic lifestyles were evolving. It is likely that during the “Cambrian explosion” many early metazoans were adapted to conditions of these Neoproterozoic-style microbially-bound seafloors. During this time bioturbation increased differentially between subtidal settings so that seafloors were commonly a varying mosaic of Neoproterozoic-style substrates and bioturbated Phanerozoic-style substrates. Thus in Early Cambrian faunas benthic metazoans adapted to Neoproterozoic-style soft substrates co-existed with benthic metazoans adapted to more typical Phanerozoic-style soft substrates. The evolutionary and ecological effects of this change in substrate style on non-actively-burrowing benthic metazoans have been termed the “Cambrian substrate revolution”. Such effects have included extinction and adaptation as well as environmental restriction/migration, and have involved a variety of taxa including molluscs and echinoderms and also very possibly lobopods and priapulids. Cambrian benthic faunas contain fossils of many organisms that seemingly have strange morphologies. Rather than being early evolutionary experiments, many of these organisms were actually just well-adapted for survival on non-actualistic microbial-mat-bound soft substrates typical of Neoproterozoic seafloors.