ALBERT

Description: Much uncertainty remains as to the temporal relationship between the Ediacaran and Cambrian biotas, yet this is critical to our understanding of the rise of metazoans. Here we present new high-resolution carbon isotope chemostratigraphy and biostratigraphy for a terminal Ediacaran to Cambrian succession on the eastern Siberian Platform, Russia, which shows the presence of a succession of diverse fossil assemblages before the start of the basal Cambrian negative carbon isotope excursion (BACE). Soft-bodied Ediacaran biota ( Beltanelliformis ) occur before the start of the late Ediacaran positive carbon isotope plateau (EPIP), a mixed Ediacaran and Cambrian skeletal biota ( Cloudina , Anabarites , Cambrotubulus ) appears within the EPIP, and diverse Cambrian-type small shelly fossils including Protohertzina and other protocondonts, halkieriids, chancelloriids, hyoliths, hyolithelminthes, and the burrowing trace fossil Diplocraterion appear at the beginning of the BACE. These integrated data show that taxa attributed to so-called Ediacaran and earliest Cambrian skeletal biotas in fact overlap without notable biotic turnover, and thus refute the presence of a large isotope excursion coincident with mass extinction of all Ediacaran biota. We propose a new biozone, the Cloudina - Namacalathus - Sinotubulites Assemblage Zone, to precede the known small shelly fossil (SSF) zones. These observations raise doubts as to whether there is any true separation between the Ediacaran and Cambrian skeletal biotas, and suggest that there is a deep root for the Cambrian explosion of metazoans.

Description: The trigger for biomineralization of metazoans in the terminal Ediacaran, ca. 550 Ma, has been suggested to be the rise of oxygenation or an increase in seawater Ca concentration, but geochemical and fossil data have not been fully integrated to demonstrate cause and effect. Here we combine the record of macrofossils with early marine carbonate cement distribution within a relative depth framework for terminal Ediacaran to Cambrian successions on the eastern Siberian Platform, Russia, to interrogate the evolution of seawater chemistry and biotic response. Prior to ca. 545 Ma, the presence of early marine ferroan dolomite cement suggests dominantly ferruginous anoxic "aragonite-dolomite seas", with a very shallow oxic chemocline that supported mainly soft-bodied macrobiota. After ca. 545 Ma, marine cements changed to aragonite and/or high-Mg calcite, and this coincides with the appearance of widespread aragonite and high-Mg calcite skeletal metazoans, suggesting a profound change in seawater chemistry to "aragonite seas" with a deeper chemocline. By early Cambrian Stage 3, the first marine low-Mg calcite cements appear, coincident with the first low-Mg calcite metazoan skeletons, suggesting a further shift to "calcite seas". We suggest that this evolution of seawater chemistry was caused by enhanced continental denudation that increased the input of Ca into oceans so progressively lowering Mg/Ca, which, combined with more widespread oxic conditions, facilitated the rise of skeletal animals and in turn influenced the evolution of skeletal mineralogy.

Description: The Ediacaran skeletal tubular putative metazoan Cloudina occurs globally in carbonate settings, which both provided lithified substrates and minimized the cost of skeletonization. Habitat and substrate preferences and the relationship of Cloudina to other metazoans have not been fully documented, so we know little as to its ecological demands or community dynamics. In situ Cloudina from the Nama Group, Namibia (ca. 550–541 Ma), formed mutually attached reefs composed of successive assemblages in shallow, high-energy environments, and also communities attached to either stromatolites in storm-influenced deep inner-ramp settings or thin microbial mats in lower-energy habitats. Each assemblage shows statistically distinct tube diameter cohorts, but in sum, Cloudina shows an exponential frequency distribution of diameter size. In reefs, we document a periodicity of size variation, where mean, minimum, and maximum tube diameters vary together and show a systematic increase toward the top of each assemblage. We conclude that most Nama Group Cloudina represent one ecologically generalist taxon with highly variable size, that size was environmentally mediated, and that Cloudina could respond rapidly to periodic environmental changes. While Nama Group skeletal metazoans coexisted with soft-bodied biota, there was no apparent ecological interaction, as they were segregated into lithified carbonate and non-lithified clastic microbial mat communities, respectively. We infer that ecological flexibility allowed Cloudina to form varied communities that colonized diverse carbonate substrates under low levels of interspecific substrate competition. This is in notable contrast to the earliest Cambrian skeletal epibenthos that formed biodiverse reef communities with specialist niche occupancy.

Description: The complex nature of growth and decomposition in microbial mats results in a broad range of microbial preservation. Such taphonomic variability complicates both the description of microbial elements preserved within geologic materials and the potential interpretation of microbial biomarkers. This study uses a taphonomic assessment to explore the preservation of different microbial components within silicified microbial mats of the late Mesoproterozoic (~1.0 Ga) Angmaat Formation, Bylot Supergroup, Baffin Island. The Angmaat Formation consists of unmetamorphosed and essentially undeformed strata that represent intertidal to supratidal deposition within an evaporative microbial flat. Early diagenetic silicification preserved microbial communities across a range of environments, from those episodically exposed to persistently submerged. Here, we present the development of a new methodology involving the use of high-resolution image mosaics to investigate the taphonomy of microfossils preserved in these mats. A taphonomic grade is assigned using a modified classification that accounts for both the taphonomic preservation state (good, fair, poor) of individual microfossils, as well as the degree of compaction of the overall mat. We show that although various taphonomic states occur within each of the silicified mats, the overall taphonomic assessment differentiates between well-preserved mats that are interpreted to have been silicified during active growth, to highly degraded and compacted mats that are interpreted to represent preservation during later stages of biological decomposition. These data indicate that even small changes in the timing of silicification may have substantial implications on our identification of microbial biomarkers and, therefore, our interpretation of early Earth ecosystems.