Benthic and Pelagic Marine Ecology Following the Triassic/Jurassic Mass Extinction

The Triassic/Jurassic mass extinction (201.3 Mya) is the most severe biotic crisis in the history of the Modern Fauna, the marine invertebrates that dominate modern oceans (e.g., gastropods, bivalves). The ecological consequence in marine habitats is an outstanding question of particular interest because similar mechanisms of environmental change are revolutionizing marine ecosystems today. This dissertation investigates these consequences by examining evidence of ecological complexity within benthic (sea floor-dwelling) and pelagic (water column-dwelling) fossil faunas. First, analysis is presented of sedimentology and benthic fossils within post-extinction strata (~ 2 Ma record) in North and South America, with comparison to other records from across the globe. Second, a framework is established for evaluating the ecology of pelagic faunas, specifically ammonoids, in general and with respect to extinction aftermath, contrasted between the Triassic/Jurassic and Permian/Triassic events.;Benthic ecology is examined in the Gabbs Valley Range of Nevada, USA and in the Central Andes of Peru, by determining the environments of deposition and the contribution of biology to sedimentation. First order investigations of the sedimentology and fossil abundance in Nevada, presented in Chapter 2, show that the reappearance of abundant rock-forming metazoan biocalcifiers occurs about two million years after the extinction, despite continual and increasing intensity of carbonate facies accumulation. This demonstrates decoupling of carbonate saturation and biocalcifier production associated with previously unrecognized long-term ecological collapse of marine shelf carbonate ramp systems. The post-extinction interval of non-metazoan-mediated carbonates is examined in Chapter 3. This analysis shows that widespread concretions formed early near the sediment/water interface, completely replacing otherwise dominantly biosiliceous clasts, and formed a profound carbonate sink. Microfacies techniques are used in conjunction with body fossil observations to determine the nature of metazoan ecology during the extinction aftermath in Chapter 4. In both Nevada and Peru, Upper Triassic carbonate systems are replaced by siliceous fossil sponge-dominated ecosystems in the Lower Jurassic. The "sponge takeover" was likely facilitated by a unique confluence of circumstances: extinction-driven changes in benthic ecology coupled with increased global silica flux (a limiting nutrient for sponges) from weathering of the massive Central Atlantic Magmatic Province (CAMP). The sensitivity of global silica cycling to changes in weathering flux is calculated to learn more about this sponge interval in Chapter 5. Specifically, it is shown that the Central Atlantic Magmatic Province could have provided enough silica for sponges to have expanded across tropical carbonate shelves in less than one million years.;Pelagic ecology is evaluated using fossil ammonoids. Chapter 6 presents a review of recent developments in ammonoid paleobiology. Relatively direct ecological evidence springs from body fossils: hard and soft part preservation; development; isotopes; and hydrodynamics. Indirect evidence on ammonoid ecology stems from analyses of shell shape distributions through time and space: in long-term "stable" conditions; during extinction events; and during radiations. The Westermann Morphospace method, presented in Chapter 7, displays fundamental morphotypes and hypothesized life modes of measured ammonoid fossils in a ternary diagram. By linking measured shells to hypothesized life modes, this method can address hypothetical ecospace occupation in collections with tight stratigraphic, lithologic, and abundance control, even when taxonomy is in dispute. Chapter 8 uses Westermann Morphospace to compare and contrast the ammonoid faunas that flourished directly after the Permian/Triassic (251 Mya) and the Triassic/Jurassic (201.3 Mya) mass extinctions. Abundant, cosmopolitan, and large ammonoids which flourished globally after the Triassic/Jurassic mass extinction produced simple coiled, "serpenticonic" shells, to which current hydrodynamic assessments ascribe a lack of dynamic directed swimming capabilities. In contrast, the small abundant ammonoids which flourished following the Permian/Triassic mass extinction produced significantly more species with different shell morphotypes, implying different ecological roles. It is further shown that the Early Triassic ammonoids maintained dynamic shell shapes despite repeated extinctions, which were triggered by high temperature and sudden low-oxygen events. The Early Triassic ammonoids also had significantly more species of streamlined "oxyconic" shells in the tropics than in temperate settings within two million years of the extinction. These ecology dynamics are much more complex than those demonstrated for the Early Jurassic, which can only be interpreted to have complex ecology if more studies can show support for more dynamic motion within the limited shell shapes.;Overall, though the Permian/Triassic event had a higher death toll and longer-lasting (5 Ma) duration before recovery, the Triassic/Jurassic extinction had extensive global ecological consequences in both benthic and pelagic habitats. Though it may have lasted only 2 Ma, the switch from carbonate to siliceous sponge dominated habitats on the sea floors, and the switch to ubiquitous drifting ammonoids in the seas, presents a very different picture from pre-extinction conditions.