Microbially induced sedimentary structures as an ecological niche in subtidal early Triassic environments of Eastern Panthalassa

Early Triassic microbially induced sedimentary structures (MISS) are a critical link in understanding the dynamics between changing environmental conditions and their effect on marine communities. The Permo-Triassic mass extinction (PTME) resulted in vacated ecospace and reduced bioturbation that allowed MISS to expand into Early Triassic subtidal environments. Data from southern Idaho and Montana indicate that MISS inhabited and proliferated in subtidal marine environments during the Griesbachian. This propagation led to changes in shallow substrate geochemical conditions that directly affected macrofaunal communities. The proliferation of microbial mats would have created anoxic and euxinic porewaters and made vertical bioturbation physiologically difficult. Geochemical data shows the bottom water signals associated with MISS were oxic to suboxic, but the porewaters below the microbial mat surface could have been anoxic even if the overlying water column was oxygenated. However, some metazoans adapted to these low-oxygen, shallow euxinic environments, allowing them to expand their ecological range during this period of crisis. Two metazoans with such adaptations were the lingulid brachiopods, which have the ability to oxidize hydrogen sulfide in their blood, and the bivalve Claraia, which was well suited to low oxygen environments. These specific disaster taxa, defined as opportunists that occupy vacated ecospaces during recovery periods but are subsequently forced into marginal settings when the recovery gains momentum, were found in direct contact with these MISS. These findings indicate that the subtidal proliferation of MISS during the Early Triassic provided an ecological niche in which specific metazoans had complex and extensive relationships.