Exploring low-oxygen conditions in the Cambrian through trilobites of the Wheeler Shale
John, Douglas Lawrence
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The Middle Cambrian Wheeler Shale of Utah is host to one of the most famous trilobite assemblages in North America. These trilobites are exceptionally well-preserved, and many bear an unusually thick encrustation of cone-in-cone calcite. The conditions under which these trilobites lived and were preserved has been scrutinized in many previous studies; however, more detailed study has led only to more outstanding questions about the paleoenvironment in the Cambrian. The environment in the Wheeler Shale has been interpreted as dysoxic in the benthos, based on sedimentology and ichnofabric in the sediment, and on the occurrence of the benthic ptychopariid Elrathia kingii in low-diversity, high-abundance assemblages that are characteristic of such conditions in modern oceans. As a consequence of specialization for extreme low-oxygen conditions, Elrathia was proposed to be possibly symbiotic with sulfur-oxidizing bacteria, based on their morphological similarity to olenid trilobites. Olenids were previously proposed as symbiotic low-oxygen specialists, based on their morphology which would be well-suited to harboring symbionts. However, the accuracy of a symbiotic interpretation and the presence of low-oxygen conditions that would predicate such symbiosis have not been directly examined. This study addresses these questions by examining the morphological basis for interpreting symbiosis in trilobites, and comparing putatively symbiotic forms to the spectrum of morphologies among Cambrian trilobites. I search for direct evidence of sulfur-oxidizing bacterial symbionts through molecular biomarkers associated with trilobite exoskeletons. I perform several geochemical analyses that constrain the extent and position of low-oxygen conditions above and below the sediment-water interface. I found that symbiosis was not well supported in trilobites, and morphology may be related more to taxonomic relationships than to environmental constraints such as reduced oxygen. While biomarker evidence of bacterial symbionts was not found, fossils from the Wheeler Shale revealed a suite of other biomarkers indicating more about Anthropocene terrestrial changes, indicating fossils may be a storehouse for information about modern ecosystem dynamics. Geochemistry suggests that benthic conditions were not extremely dysoxic, and the distribution of Elrathia may not be related to specialization for extreme low oxygen.