INTERROGATING PALEOENVIRONMENTAL AND EVOLUTIONARY PATTERNS IN BIOTURBATION IN UPPER CAMBRIAN AND LOWER ORDOVICIAN STRATA OF THE GREAT BASIN, USA

The early Paleozoic was a time of unprecedented biological and ecological change, encompassing the dramatic increases in morphological and phylogenetic diversity of the Cambrian Explosion, the subsequent diversity plateau of the mid and later Cambrian, and the ecologic innovations of the Great Ordovician Biodiversification Event. These changes in biodiversity and ecological complexity occurred against a backdrop of dynamic and evolving environmental conditions, including changes in seawater oxygen levels and potential shifts in carbonate chemistry and nutrient availability. Bioturbators—burrowing and sediment-mixing animals—are thought to have been diverse and abundant during this interval, yet previous studies of Cambrian–Ordovician successions have indicated that the ramping up of intensively mixed seafloor sediments was a protracted process. Determination of potential drivers of early Paleozoic bioturbation—and disentanglement of the relative contributions of evolutionary change and environmental variation in shaping these patterns—have been hampered by a paucity of high-resolution records coupled with statistical assessment of sediment-mixing intensities across a broad range of sedimentary facies. To tackle these questions, we collected new bioturbation data from upper Cambrian to Middle Ordovician carbonate strata of the Great Basin, western USA. Preliminary data from the Pogonip Group in the Arrow Canyon Range of Nevada, collected at a decimeter scale, suggest that sediment-mixing intensities remained consistently low throughout this interval; mean bioturbation intensity does not exceed an ii of 3 throughout its extent (where ii range from 1-5), substantially lower than levels of bioturbation characteristic of analogous seafloor settings today. Additionally, we find that bioturbation intensities in these carbonate-dominated successions are higher in mudstones/wackestones relative to flat-pebble conglomerates and packstones/grainstones. Complementary data from correlative strata in other west Laurentian successions will further elucidate bioturbation patterns throughout different facies and depositional environments during this key interval in early animal history.