EVOLUTIONARY TRAJECTORIES OF SESSILE EPIFAUNA DURING THE NORIAN STAGE (LATE TRIASSIC) IN RESPONSE TO THE EARLY MESOZOIC MARINE REVOLUTION

The Mesozoic Marine Revolution (MMR) was characterized by escalatory adaptations in predators and prey, and related suites of adaptations arose at different intervals in the Mesozoic. Several different groups of benthic or demersal durophagous predators radiated in the Late Triassic and Early Jurassic, which had a dramatic effect on benthic animals in shallow marine communities. Infaunalized and mobile shelly animals became much more common throughout the Late Triassic, but stationary epifaunal animals also developed new adaptations that reflected the shifting pressures they faced above the sediment-water interface. These changes occurred synchronously in many common fossil groups. Cementation appeared independently in several bivalve families, and new ligament attachment modes may have accommodated facultative mobility among other bivalve groups. Many brachiopod taxa increased the degree of crenulation and several novel adaptations arose in crinoids to increase mobility. Bulk sample sequences from Tethyan and Panthalassan shallow marine sedimentary successions reveal that many common genera of sessile epifauna populations decreased in overall size during the Norian stage. The increased predation at the sediment surface in the Late Triassic was closely correlated with new adaptive behaviors and morphologies that increased the ability to resist or avoid durophagy, which led to a cascade of paleoecological changes in the early stages of the MMR.