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The lower member of the Poleta Formation is well known for its abundant archaeocyaths, including an exceptionally preserved reef at Stewart’s Mill in Nevada. Large archaeocyath-bearing reefs are relatively uncommon, with most occurring as small nearshore patch reefs. Here, we describe a new 70 m-thick archaeocyath reef from the lower Poleta Formation, herein referred to as the Gold Point Reef, located near the townsite of Gold Point, Nevada. In spite of broad similarities with the Stewart’s Mill exposure, the Gold Point Reef shows an interplay between shoal and reef development over the ~ 70 m exposure that differs from what is observed at Stewart’s Mill. This reef also preserves a small shelly fauna in its uppermost 10 m, dominated by Hyolithellus. Three successive ecological stages of reef development are observed here: the pioneer stage characterized by a single rare frame-building species; the diversification stage involving about nine species of both frame-building and mud-sticking archaeocyaths; and the final foundering stage represented by one to two species of archaeocyaths and facies indicating reef demise, including abundant grainstone/packstone shoals and a silicified upper bed with a settlement of hyolitheminthes. During the apex of reef development here, there are large modular archaeocyaths, which were able to produce thick secondary skeletal tissue for a firm attachment; these archaeocyaths were capable of building large reefs in a variety of settings. Additionally, the carbon isotope profile recorded in the Gold Point Reef shows similarities to nearby exposures of the equivalent lower Poleta Formation, with the exception of a small negative downturn at the top of the reef that is not found in other locations. Combined with the documentation of facies changes over short lateral distances, the isotope data suggest that archaeocyathan reefs may have been time-transgressive across the shelf during deposition of the lower Poleta. Additionally, our work suggests that both here and globally, the specific biotic assemblage reflected the local influences of the environment on reef development, creating highly variable coeval paleocommunities even within a restricted area.

An assemblage of Late Precambrian macrofossils consisting of five species (Aspidella terranovica, Beltanelliformis brunsae, Nenoxites curvus, Palaeopascichnus renarius, and Suvorovella aldanica), was discovered in the type sections of the Yudoma Group of the Siberian Platform. A unique feature of the assemblage is the presence of carbonate discoidal remains of Suvorovella, formed as a result of secondary incrustation of basal structures, similar to Aspidella. The accumulations of redeposited fragments of Suvorovella formed the rock resembling the Phanerozoic shell beds. Except for this taxon, all members of the Yudoma assemblage have a wide geographic distribution. Their stratigraphic distribution tends to occur in the youngest deposits of the Upper Vendian, directly underlying the beds with small shelly fauna (SSF) of the Nemakit-Daldynian Stage, Cloudina tubes, and traces of the Treptichnus pedum Zone.

The Ediacaran–Cambrian (E-C) transition witnessed dramatic tectonic, climatic, oceanic, and biological co-evolutions at the dawn of the ‘Cambrian explosion’. Although glaciations were reported in some mid-high-latitude continents during this interval, large uncertainties remain concerning timing and extent, as well as their effect on the entire Earth's surface system. We here report magnesium (Mg) isotopic compositions, along with chemical index of alteration (CIA), Al/Ti ratios from a time-dated, nonglacial E-C boundary section on the mid-low-latitude Yangtze Block, South China, to constrain chemical weathering intensity and associated climatic, oceanic and biological co-evolution during this critical interval. Our data demonstrate a rapid and significant negative δ26Mg excursion (from +1.28 to −0.34‰) along with similar CIA and Al/Ti decreases across the E-C boundary, reflecting an apparent attenuation in chemical weathering intensity likely in response to severe climate cooling. This finding further affirms the far-field glaciation reported in the mid-high-latitude continents during the E-C transition. Under this condition, the intense climate cooling could have enhanced oceanic circulation and/or upwelling, and simultaneously caused prominent variations of seawater geochemistry (including alkalinity elevation) and nutrient flux as well as ecological stresses on metazoans. All these could reasonably account for extensive deposition of phosphorites and organic-rich black shales, and faunal turnover (extinction of Ediacaran fauna and appearance of skeletonized small shelly fauna) across this interval. These novel datasets thus shed new insights into the climatic-oceanic-biological interactions and time-specific deposition around the E–C transition at the prelude of the Cambrian explosive bioradiation.

Animals with radial symmetry are abundant in the Cambrian Fortunian Stage of South China, but with relatively low diversity: representatives include Olivooides, Quadrapyrgites, carinachitiids, hexangulaconulariids and Pseudooides. Here, we report a new radial animal, Qinscyphus necopinus gen. et sp. nov., from the Fortunian small shelly fauna of southern Shaanxi Province, South China. Qinscyphus necopinus has a cup-shaped profile, with slightly raised annuli and five groups of triangular thickenings in pentaradial symmetry. This organism has a comparable morphology to, and thus a close affinity with, Olivooides and Quadrapyrgites, and is interpreted as a coronate scyphozoan. This discovery adds a new crown-group cnidarian to the Cambrian Explosion.

Widespread Ni-Mo sulfide mineralization in South China during the early Cambrian may have affected the seawater chemistry and biodiversity of this region, but its underlying causes are not well-understood. To better understand the formation of sulfide ores in early Cambrian marine systems, this study evaluated the paragenesis and sulfur isotopic composition (δ34S) of sediment-hosted Ni-Mo sulfide ores in the lower Cambrian Niutitang Formation of the Nanhua Basin of South China. Petrographic analysis revealed three types of sulfide aggregates: (1) ubiquitous framboidal and euhedral pyrite (Py-1), (2) a MoSC phase (i.e., Mo mineralization) that was cogenetic with larger pyrite cubes or nodules (Py-2), and (3) intergrown millerite (NiS), sphalerite (ZnS) and veined pyrite (Py-3) (i.e., Ni-Zn mineralization) characterized by laminated and veined textures. Alternating precipitation of Mo and Ni-Zn sulfides was caused by intermittent emission of Mo-Fe or Ni-Zn-rich hydrothermal fluids into euxinic mid-depth waters of the Nanhua Basin, representing a sedimentary exhalative (SEDEX) process. The different sedimentary ore textures are each associated with a characteristic δ34S distribution. At all study sites, the syngenetic Py-1 phase yields δ34S of −15.7 to + 1.9‰, consistent with a seawater sulfate source that was variably fractionated through microbial sulfate reduction (MSR). In the Nayong and Zunyi areas, δ34S values of −11.8 to +4.4‰ are associated with the MoSC phase, and −20.6 to −6.8‰ with the Py-2 phase. In the Zhangjiajie area, Py-2 yields similar values (−25.1 to −10.8‰) but MoSC higher and less variable values (+8.6 to +18.7‰). These phases are thought to contain a mixture of sulfur sourced from seawater sulfate (via MSR) and hydrothermal fluids in varying proportions. A mode of Ni-Zn sulfides δ34S values between −9.1 and +8.0‰ is likely to represent the isotopic composition of hydrothermal sulfur inputs to the Ni-Zn mineralization. As a consequence, both hydrothermal emissions and biogenic production of H2S had a significant influence on seawater chemistry, with consequences for contemporaneous evolution of early marine animal life (e.g., the small shelly fauna and Chengjiang Biota) in the early Cambrian Nanhua Basin.

The early Cambrian (541–514 Ma) was a crucial interval for the evolution of life on Earth, popularly known as the “Cambrian Explosion”. Here, we report the timing of changes in hydrothermal and depositional inputs, as well as paleo-redox state, which may have influenced biogeochemical changes. According to high-resolution petrology, fossil distributions, isotopic records, and inorganic geochemistry, the lower Cambrian of the South China can be subdivided into four intervals: the lowermost Cambrian Zhujiaqing Formation (Cam-I); the Cambrian Stage 2 Shiyantou Formation (Cam-II); the lower part of the Cambrian Stage 3 Yu'anshan Formation (Cam-III); and the mid–upper Cambrian Stage 3 and the middle–upper part of the Yu'anshan Formation, continuing into the Canglangpu Formation (Cam-IV). Hydrothermal events are detected during the early Cam-I, Cam-II, and Cam-III intervals. During the early Cam-I and Cam-II intervals, these events coincided with extensive bottom water euxinia, which in turn may have restricted the spread or proliferation of Ediacaran fauna and small shelly fauna. Through the whole Cam-III interval, further hydrothermal events occurred concurrently with euxinic and ferruginous conditions, probably within a single spatially stratified water column, again plausibly restricting the spread of aerobic organisms. In conjunction with the cessation of hydrothermal events and the gradual lowering of sea level during the late Cam-III, oxic water environments gradually spread into relatively deep-water regions, concurrent with the emergence of the Chengjiang and Qingjiang faunas. These data suggest that periodic hydrothermal events may have had a significant impact on the spread, radiation and extinction of macroscopic fauna during the early Cambrian in South China.