MICROBIALLY INDUCED SEDIMENTARY STRUCTURES AS AN ECOLOGICAL NICHE FOR METAZOAN COLONIZATION DURING THE EARLY TRIASSIC

Early Triassic microbially induced sedimentary structures (MISS) are a critical link in understanding the dynamics between changing environmental conditions and its effect on marine communities. The end-Permian mass extinction resulted in vacated ecospace and reduced bioturbation that allowed MISS to expand into the Early Triassic subtidal environments. This propagation led to changes in shallow substrate geochemical conditions that directly impacted macrofaunal communities. The activity and expanse of these microbial communities altered porewater chemistry at the sediment-water interface and potentially affected the bottom water chemistry, leading to anoxic and euxinic conditions in substantially shallower depths than is seen in most modern environments. However, some metazoans had adaptations to these low-oxygen, euxinic environments that allowed them to expand their ecological range during this period of crisis. Two metazoans with such adaptations are the brachiopod Lingula, which has the ability to oxidize hydrogen sulfide in their blood, and the bivalve Claraia, which was well suited to low oxygen environments. Data from southern Idaho and Montana suggests that MISS both inhabited and proliferated in subtidal marine environments during the Griesbachian. Preliminary interpretations are that some metazoans, such as Lingula and Claraia, were in direct contact with these MISS and that these interactions were sustained. These findings indicate that the proliferation of MISS during the Early Triassic provided an ecological niche in which some metazoans had complex and extensive relationships with MISS.