ALBERT

Description: The Zavkhan terrane in western Mongolia preserves thick, fossiliferous, carbonate-rich strata that span the Ediacaran-Cambrian transition. Measured stratigraphic sections and geological mapping of these strata, which include the Zuun-Arts, Bayangol, Salaagol, and Khairkhan Formations, reveal large lateral facies changes over short distances that necessitate revisions to previous lithostratigraphic correlations and biostratigraphic range charts. Here, we integrate new geological mapping and measured stratigraphic sections across the Zavkhan terrane with high-resolution carbon isotope ( 13 C) chemostratigraphy, sequence stratigraphy, and biostratigraphy. Using these data, we revise correlations within the Zavkhan terrane and place small shelly fossil and ichnofossil horizons in a refined temporal and spatial framework. Finally, we integrate these data from Mongolia with absolute ages and chemostratigraphy from sections in Morocco and Oman. Our revised age model suggests that deposition of the Bayangol Formation and the lower part of the Salaagol Formation was limited to the Nemakit-Daldynian and Tommotian Stages. With the new correlations and age model, we place the small shelly fossil first appearance datum in the basal Bayangol Formation instead of the basal Zuun-Arts Formation, which moves this horizon hundreds of meters higher in the stratigraphy, above the large negative excursion in the Zuun-Arts Formation. The first appearance datum of Treptichnus pedum is ~275 m above the large negative excursion in the Zuun-Arts Formation and ~250 m above the first appearance datum of small shelly fossils, highlighting the rarity and facies dependence of its preservation. We shift the first appearance datums of tommotiids, orthothecimorphs, hyolithelminths, cap-shaped fossils, protoconodonts, and Salanacus to just below the positive peak 3p. This interpretation differs from previous chronostratigraphic placements of first appearance data of genera between positive excursions 1p and 2p. Using this level as the first appearance datum in Mongolia for these genera, we replot global fossil first appearances. With this new compilation, there are still three distinct pulses of fossil first appearances, as was suggested in previous compilations; however, we suggest that this pattern is controlled largely by regional sedimentation and taphonomy rather than the rate of taxonomic origination.

Description: Evaluation of hypotheses that relate environmental to evolutionary change across the Ediacaran-Cambrian transition has been hampered by a dearth of sections that preserve both the last appearance of Ediacaran body fossils and the first appearance of Treptichnus pedum within carbonate-rich strata suitable for chemostratigraphic studies. Here, we report two new exceptionally preserved latest Ediacaran fossil assemblages from the Deep Spring Formation at Mount Dunfee, Nevada (USA). Further, we report these occurrences in a high-resolution carbon isotope chemostratigraphic framework, permitting correlation on a regional and global scale. The lower of the two horizons, at the base of the Deep Spring Formation, hosts a body fossil assemblage that includes Gaojiashania, other vermiform body fossils, and possible Wutubus annularis interbedded with Cloudina shell beds. The upper of the two fossil horizons, in the Esmeralda Member of the Deep Spring Formation, contains Conotubus and occurs within the basal Cambrian negative carbon isotope excursion, establishing it as the youngest Ediacaran fossil assemblage discovered to date. This is the first report of Gaojiashania , Conotubus, and Wutubus in Laurentia, extending the known stratigraphic ranges and biogeographic distributions of these taxa to a global scale. These data refine the relative ages of defining characteristics of the Ediacaran-Cambrian boundary and confirm that a large perturbation to the carbon cycle and surface ocean conditions coincided with the extinction of Ediacaran organisms.

Description: The fossilization of organic remains and shell material by calcium phosphate minerals provides an illuminating, but time-bounded, window into Ediacaran–Cambrian animal evolution. For reasons that remain unknown, phosphatic fossil preservation declined significantly through Cambrian Series 2. Here, we investigate the phosphorus (P) sources for phosphatic Cambrian carbonates, presenting sedimentological, petrographic, and geochemical data from the Cambrian Series 2–3 Thorntonia Limestone, Australia, some of the youngest Cambrian strata to display exceptional phosphatic preservation of small shelly fossils. We find that within Thorntonia sediments, phosphate was remobilized by organic decay and bacterial iron reduction, with subsequent reprecipitation largely as apatite within the interiors of small shelly fossils. We discuss the merits of bioclastic-derived, organic matter–bound, or iron-bound P as potential sources to these strata. Petrographic observations suggest that the dissolution of phosphatic skeletal material did not provide the P for fossil preservation. In contrast, high organic carbon contents imply significant organic fluxes of P to Thorntonia sediments. Sedimentology and iron-speciation data indicate that phosphorus enrichment occurred during times of expanded anoxic, ferruginous conditions in subsurface water masses, suggesting that phosphorus adsorption to iron minerals precipitating from the water column provided a second significant P source to Thorntonia sediments. Simple stoichiometric models suggest that, by themselves, neither organic carbon burial nor an iron shuttle can account for the observed phosphorus enrichment. Thus, we infer that both processes were necessary for the observed phosphorus enrichment and subsequent fossil preservation in the Thorntonia Limestone.