Amy E. Kelly

Fossil steroids record the appearance of Demospongiae during the Cryogenian period

The Neoproterozoic era (1,000–542 Myr ago) was an era of climatic extremes and biological evolutionary developments culminating in the emergence of animals (Metazoa) and new ecosystems. Here we show that abundant sedimentary 24-isopropylcholestanes, the hydrocarbon remains of C30 sterols produced by marine demosponges, record the presence of Metazoa in the geological record before the end of the Marinoan glaciation (∼635 Myr ago). These sterane biomarkers are abundant in all formations of the Huqf Supergroup, South Oman Salt Basin, and, based on a new high-precision geochronology, constitute a continuous 100-Myr-long chemical fossil record of demosponges through the terminal Neoproterozoic and into the Early Cambrian epoch. The demosponge steranes occur in strata that underlie the Marinoan cap carbonate (>635 Myr ago). They currently represent the oldest evidence for animals in the fossil record, and are evidence for animals pre-dating the termination of the Marinoan glaciation. This suggests that shallow shelf waters in some late Cryogenian ocean basins (>635 Myr ago) contained dissolved oxygen in concentrations sufficient to support basal metazoan life at least 100 Myr before the rapid diversification of bilaterians during the Cambrian explosion. Biomarker analysis has yet to reveal any convincing evidence for ancient sponges pre-dating the first globally extensive Neoproterozoic glacial episode (the Sturtian, ∼713 Myr ago in Oman).

Carbon and Sulfur Stable Isotopic Systems and Their Application in Paleoenvironmental Analysis

Stable isotope geochemistry has important applications for understanding past environments and should be incorporated, where appropriate, into paleontological research. This chapter aims to provide a basic introduction to the field of stable isotope geochemistry, highlight fundamental isotopic systems used in paleoenvironmental analysis, outline methods used to acquire isotopic data, explain common primary and secondary processes that can affect the interpretation of that data, and discuss the secular trends of carbon and sulfur isotopes through Earth history. In order to properly interpret stable isotope data, it is critical to understand the origin of the sample used in the geochemical analysis and use consistent sampling methodologies and preparation techniques. Careful attention should be made to assess the sample for diagenetic alteration and contamination prior to preparation for geochemical analysis, including the use of petrographic analysis, CL and SEM petrography, and spot sampling obvious diagenetic zones to compare against samples considered less diagenetically altered. Secular variation of carbon and sulfur isotopes has varied dramatically over geological history. Most isotopic excursions in the geological record appear to manifest from sudden imbalances carbon and sulfur cycles. Long-term variation has important implications on productivity, redox state of the atmosphere and oceans, and potentially the extinction and radiation of organisms. However, geochemists still struggle to understand the mechanisms, duration, and triggers of isotopic excursions. In many case studies, such as the Shuram anomaly, there are multiple scenarios that can produce isotopic excursions. The challenge for future research is to better address stable isotopic variation by integrating datasets across disciplines to improve our understanding of the complex interactions between environmental and biological systems.