The two phases of the Cambrian Explosion

Abstract The dynamics of how metazoan phyla appeared and evolved – known as the Cambrian Explosion – remains elusive. We present a quantitative analysis of the temporal distribution (based on occurrence data of fossil species sampled in each time interval) of lophotrochozoan skeletal species (n = 430) from the terminal Ediacaran to Cambrian Stage 5 (~545 – ~505 Million years ago (Ma)) of the Siberian Platform, Russia. We use morphological traits to distinguish between stem and crown groups. Possible skeletal stem group lophophorates, brachiopods, and molluscs (n = 354) appear in the terminal Ediacaran (~542 Ma) and diversify during the early Cambrian Terreneuvian and again in Stage 2, but were devastated during the early Cambrian Stage 4 Sinsk extinction event (~513 Ma) never to recover previous diversity. Inferred crown group brachiopod and mollusc species (n = 76) do not appear until the Fortunian, ~537 Ma, radiate in the early Cambrian Stage 3 (~522 Ma), and with minimal loss of diversity at the Sinsk Event, continued to diversify into the Ordovician. The Sinsk Event also removed other probable stem groups, such as archaeocyath sponges. Notably, this diversification starts before, and extends across the Ediacaran/Cambrian boundary and the Basal Cambrian Carbon Isotope Excursion (BACE) interval (~541 to ~540 Ma), ascribed to a possible global perturbation of the carbon cycle. We therefore propose two phases of the Cambrian Explosion separated by the Sinsk extinction event, the first dominated by stem groups of phyla from the late Ediacaran, ~542 Ma, to early Cambrian stage 4, ~513 Ma, and the second marked by radiating bilaterian crown group species of phyla from ~513 Ma and extending to the Ordovician Radiation

The Rise of Animals in a Changing Environment: Global Ecological Innovation in the Late Ediacaran

The evolutionary trajectory of early complex life on Earth is interpreted largely from the fossils of the Precambrian soft-bodied Ediacara Biota, which appeared and evolved during a time of dynamic biogeochemical and environmental fluctuation in the global ocean. The Ediacara Biota is historically divided into three successive Assemblages—the Avalon, the White Sea, and the Nama—which are marked by the appearance of novel biological traits and ecological strategies. In particular, the younger White Sea and Nama Assemblages record a “second wave” of ecological innovations, which included not only the development of uniquely Ediacaran body plans and ecologies, such as matground adaptations, but also the dual emergence of bilaterian-grade animals and Phanerozoic-style ecological innovations, including spatial heterogeneity, complex reproductive strategies, ecospace utilization, motility, and substrate competition. The late Ediacaran was an evolutionarily dynamic time characterized by strong environmental control over the distribution of taxa in time and space

Ediacaran-Cambrian transition of the Southwestern USA—Field Trip of the North American Paleontological Convention, June 19–22, 2019

Participants should plan to arrive in Riverside by June 18. We will depart from the University of California, Riverside campus the morning of June 19. We will drive 5 hours to the White-Inyo Mountains. We will spend the remainder of June 19 and most of June 20 visiting the upper Ediacaran through lower Cambrian succession of this area, including a rich assemblage of trace fossils and some of the earliest Laurentian biomineralized fossils. We will drive to Beatty, NV the evening of June 20. On the morning of June 21, we will visit upper Ediacaran strata and an Ediacaran-Cambrian boundary section of the Reed Dolomite and Deep Spring Formation at Mount Dunfee and see Ediacaran microbialites, tubular body fossils and some of the oldest complex trace fossils. We will spend midday at the nearby ghost town of Gold Point, NV, and have an opportunity to see the spectacular early Cambrian archaeocyathan reefs of the Poleta Formation at Stewart’s Mill in the afternoon. We will depart Beatty the morning of June 22 and return to Riverside by way of Death Valley, where we will make brief stops to discuss the stratigraphy of the Death Valley region, Neogene lake deposits near Shoshone and Cryogenian diamictites and the Marinoan cap carbonate exposed in the Saddle Peak Hills and at Sperry Wash. We will be staying in motels in Big Pine, CA (June 19) and Beatty, NV (June 20–21); participant lodging for these days will be covered by field trip fees. Lunches on all four days (June 19–22) and breakfast on the third day of the trip (June 21) will be provided; participants will be responsible for all other meals. We will return to Riverside in the late afternoon on June 22, in time for participants to check in to conference lodging. See Figure 1 for a map of field trip destinations

Publisher Correction: The role of calcium in regulating marine phosphorus burial and atmospheric oxygenation

An amendment to this paper has been published and can be accessed via a link at the top of the paper

The evolution of the marine carbonate factory

Calcium carbonate formation is the primary pathway by which carbon is returned from the ocean-atmosphere system to the solid Earth (1,2). The removal of dissolved inorganic carbon from seawater by precipitation of carbonate minerals-the marine carbonate factory-plays a critical role in shaping marine biogeochemical cycling(1,2). A paucity of empirical constraints has led to widely divergent views on how the marine carbonate factory has changed over time(3-5). Here we use geochemical insights from stable strontium isotopes to provide a new perspective on the evolution of the marine carbonate factory and carbonate mineral saturation states. Although the production of carbonates in the surface ocean and in shallow seafloor settings have been widely considered the predominant carbonate sinks for most of the history of the Earth(6), we propose that alternative processes-such as porewater production of authigenic carbonates-may have represented a major carbonate sink throughout the Precambrian. Our results also suggest that the rise of the skeletal carbonate factory decreased seawater carbonate saturation states

Evaluation of shallow-water carbonates as a seawater zinc isotope archive

•Primary marine carbonate phases are characterized by highly variable δ66Zn values.•Skeletal and microbial aragonite δ66Zn values are comparable to seawater δ66Zn.•Organically complexed zinc does not appear to contribute to carbonate-hosted zinc. Carbonate zinc (Zn) isotopes have increasingly been used to track changes in seawater Zn isotopic composition. A large body of recent work, in particular studies conducted under the GEOTRACES program, have added significantly to our knowledge of how Zn behaves in the water column. However, our understanding of how water-column Zn isotopic signals become incorporated into carbonate sediments is at a much more nascent stage. To help solve this issue, we have measured the Zn isotopic composition of modern surficial and core-top carbonate sediments and seawater from the Bahamas, Panama, and the Persian Gulf. Whereas modern Bahamian seawater has a δ66Zn of 0.14 ± 0.12‰ (2σ), Bahamian carbonates show a large range of δ66Zn—from −0.56 to 1.11‰. The δ66Zn of carbonate mud is ∼0.3‰ higher than that of the Bahamian seawater, potentially due to Zn isotopic fractionation during precipitation. Carbonate ooids and peloids and certain skeletal and microbial aragonites, including gastropods, green algae, stromatolites and precipitates associated with cyanobacterial (Dichothrix) filaments, have δ66Zn indistinguishable, within analytical error, from that of modern seawater. This suggests that these carbonate phases may be promising archives for seawater δ66Zn