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Sponges have great significance for understanding the origin and evolution of metazoans. Biomarker fossil and molecular clock studies suggest a Precambrian origin of sponges, but no unambiguous fossil of sponge spicules have been recovered from units older than the early Cambrian to date. Here, we report abundant siliceous spicules from Ediacaran-Cambrian transition strata, representing a deep-slope setting on the southeastern margin of the Yangtze Block in South China. The first appearance of monaxon spicules in the Sifangjing section is in the lower Liuchapo Formation (uppermost Ediacaran), from below the Basal Cambrian Carbon isotope Excursion (BACE), which is consistent with the timing of their initial appearance in shallow-platform settings (e.g., the Three Gorges area). These spicules are monaxons that are isolated or aggregated as spiculate bundles, the latter having a structural arrangement consistent with construction by demosponges. Polyaxial spicules first appear in phosphatic nodules of the basal Jiumenchong Formation (early Cambrian Stage 2). Relative to shallow-water settings, the appearance of polyaxial spicules in deep-water slope settings was delayed (∼10 Ma). Oxygenation of anoxic deep watermasses and increasing concentrations of dissolved Si in seawater during Cambrian Stage 2 may have facilitated the expansion of the Hexactinellida into deep-water settings. The triaene spicules from the basal Jiumenchong Formation are the oldest (∼522 to 525 Ma) Tetractinellida spicule fossils identified to date. • Sponge spicules were recovered from the Liuchapo Formation at Sifangjing section. • The first appearance of monaxons is near the Ediacaran-Cambrian boundary. • Polyaxial spicules appear in Cambrian Stage 2, marking evolution of Tetractinellida. • Siliceous sponges had an evolutionary tendency from monaxon to triaxon spicules. [ABSTRACT FROM AUTHOR]

On the basis of expanded documentation of complex fossils, early Mesoproterozoic has already been proven as an important Era for eukaryotic evolution. In order to portray the biological community of this critical era, we studied a new silicified microbiota from the ∼1560 Ma Gaoyuzhuang Formation in the classic Jixian section, Tianjin, China. In total, eighteen species have been recognized, four species have been identified informally and five species have been identified in open nomenclature. Similar to other Proterozoic microbial mat biota, the new microbiota is dominated by cyanobacteria filaments and coccoids as well. Potential eukaryotic microfossils, such as ornamented coccoids, filaments with flanges and terminal cell, were firstly recovered. Amongst them, filaments with flanges and conidia-like terminal cell probably were the earliest unidirectional growing multi-cellular organisms as well as the earliest phylogeny-resolved multicellular eukaryotes. Combined with increasing records of Proterozoic eukaryotes, these microfossils indicated that eukaryotes have evolved into a new level in Early Mesoproterozoic. [ABSTRACT FROM AUTHOR]

• The oldest radiolarian fossils were recovered. • Most of the radiolarians are related with the order Spumellaria. • Early morphology of radiolarians is multilayered spherical without spicule(s). • Molecular appearance of radiolarian was estimated deep in Neoproterozoic. • Advent of radiolarians brought biogenetic influence into silica cycle and sediment. Radiolarian, as a group of siliceous skeleton-bearing heterotrophic protist, has a long history from modern ocean back to the Cambrian. The earliest stage in the evolution of radiolarians, although poorly understood, is important not only for itself but also with respect to the development of the global silica cycle and biogenetic siliceous sediments. It has been hypothesized that radiolarians evolved from an ancestral form constructed by spicules, although this is not without controversy since origin and their early morphology is still in dispute. However, the record of Cambrian radiolarians, especially the early Cambrian, is still sparse. Here we report a research in chert of the Liuchapo Formation from the latest Ediacaran to early Cambrian in South China that yielded the most probable oldest radiolarians from deep-water settings. Accompanied fossils and stratigraphic correlation indicate a Fortunian age. Judging by the prominent concentric multilayered shell and absence of spicules, the fossils were classified into the order Spumellaria according to the advance of radiolarian taxonomy and phylogeny. They can be compared with radiolarians from Series 2 to Furongian of the Cambrian. Molecular study conducted here also supports the existence of spumellarian during the Cambrian. Together with other reports of Cambrian radiolarians in recent years, the occurrence of non-spicule forms predates the ones constructed by spicules, which do not support the previous assumption about primitive radiolarian. The discovery not only added to the sparse fossils record of the Cambrian radiolarian, but also demonstrated their contribution to silica cycle and siliceous sediments. Evolution of early radiolarians still subjects to open discussion and awaits more evidence. [ABSTRACT FROM AUTHOR]

The Ediacaran-Cambrian (E-C) transition witnessed significant ecosystem perturbations of the oceanic environment (e.g., oxygen concentrations of seawater) and biological evolution. However, in spite of abundant Ediacara-type megafossils, trace fossils, small shelly fossils (SSFs), macroscopic algae and metazoans recovered from shallow-water E-C transition strata throughout South China, the manifestation of geological events in deep-water settings remains elusive. Here, we present new paleontological and geochemical evidence from the ~38-m-thick deep-water Liuchapo Formation (ca. 551–523 Ma) of the Yangtou section, northeastern Guizhou Province, China, which consists mainly of chert through the E-C transition. The Liuchapo Formation can be divided into two members based on biostratigraphy and redox proxies. The lower member (0–32 m) is characterized by low Mo/Al (0.15–1.82 ppm/%) and U/Al (0.47–2.51 ppm/%) values that suggest deposition under suboxic conditions. These deposits are dominated by Horodyskia minor , Paleopascichnus jiumenensis , spherical cyanobacteria (? Myxococcoides) and random acritarchs. The upper member (32–38 m) records a transition to anoxic (non-sulfidic) conditions as suggested by marked increases in Mo/Al (5–47 ppm/%) and U/Al (8–810 ppm/%). The upper member contains SSFs, sponge spicules, filamentous cyanobacteria, multicellular algae and sheet-like fossils. The survival of diverse organisms under strongly reducing conditions reflected in the upper member suggests that, marine redox fluctuations may limit the growth of Late Ediacaran organisms and provide the living space and elevated nutrients for early Cambrian biota. • Organismal shifts and redox condition variations during the Ediacaran-Cambrian transition • Flourishing multicellular organisms are reported from the upper Liuchapo Formation. • Geochemical proxies confirm the studied chert is of seawater origin. • Diverse organisms may have survived under oxygen-depleted condition. • Marine redox fluctuation was important for the biotic turnover. [ABSTRACT FROM AUTHOR]

• Nong Pong Formation chert was deposited in a deep marine environment on continental crust. • Radiolarian indicate a late middle Permian age for the studied section. • Deposition was probably in a rift setting, flanked north and south by platform carbonates. The deepwater chert-shale-limestone-tuff Permian Nong Pong Formation (NPF) of the Saraburi Group is a major unit within the Triassic (Indosinian Orogeny) Khao Khwang Fold and Thrust Belt (KKFTB). Whether the NPF was deposited in a continental margin or ocean basin remains uncertain and impacts structural, sedimentary and stratigraphic interpretations of the KKFTB. This study investigates chert geochemistry from a 60 m section of the formation, near Muak Lek town, to determine its depositional environment. Radiolarian fauna from the chert beds includes Follicucullus porrectus Rudenko, (probable late Guadalupian, late middle Permian), indicating the NPF is time-equivalent to adjacent carbonate platform deposition (Khao Khwang and Khao Khad formations). High ratios of SiO 2 /(SiO 2 + Al 2 O 3 + Fe 2 O 3) and Fe-Al-Mn plots indicate a biogenic origin (radiolarians, sponge spicules) for the cherts, far from a hydrothermal source. Trace element ratios for U/Th, Sc/Th and Sr/Ba and Ce anomalies in the NASC-normalized REE distribution are ambiguous, and range between shallow to deep ocean basin, and continental margin settings. Major elements analysis of cherts show: (1) high ratios of SiO 2 /(SiO 2 + Al 2 O 3 + Fe 2 O 3), range: 0.60–0.82 (0.76 average), (2) Al/(Al + Fe + Mn), range: 0.67–0.82 and (3) average MnO/TiO 2 = 0.14. The MnO/TiO 2 and Al 2 O 3 /(Al 2 O 3 + Fe 2 O 3) ratio, 100 × Fe 2 O 3 /SiO 2 vs. 100 × Al 2 O 3 /SiO 2 and Fe 2 O 3 /(100-SiO 2) vs. Al 2 O 3 /(100-SiO 2) diagrams indicate deposition on a continental margin. The La n /Ce n versus Al 2 O 3 /(Al 2 O 3 + Fe 2 O 3) discrimination diagram indicates a pelagic, continental margin setting. Combined geochemical, structural and sedimentary facies analyses suggests the NPF was deposited in a deep marine, normal fault-bounded basin on a rifted, ribbon continent margin where carbonate platforms grew on structural highs. [ABSTRACT FROM AUTHOR]

Upper Ordovician (Katian) to Lower Silurian (Rhuddanian) strata in the Middle–Upper Yangtze Block, South China contain extensive chert beds, although the origin of microcrystalline silica in these cherts is largely unknown. In this study, the mineralogy and structure of chert as well as major, trace, and rare earth elements are used to determine the origin of microcrystalline silica in 29 chert samples from Longmaxi Formation (Rhuddanian) in Baizitian outcrop, west of Kangdian Uplift (KU). Results show that 21 of the 29 chert samples have a biogenic origin, 2 indicate a hydrothermal origin, and 6 show a mixed origin. Cherts from Baizitian outcrop are compared to contemporaneous cherts deposited east of KU. Cherts both east and west of KU are mainly biogenic in origin, which is attributed to high productivity in the paleo-seaway. Hydrothermal microcrystalline silica is relatively uncommon in the Longmaxi Formation across the Middle-Upper Yangtze Block, and only occurs in Baizitian outcrop. Terrigenous influx contributed to chert accumulation both east and west of KU; however, cherts east of KU received much more clastic material than those in Baizitian outcrop. A comparison of chert composition and geochemistry to geological reconstructions of the Middle-Upper Yangtze Block reveal that accumulation of hydrothermal chert is closely linked to tectonic evolution. During the Early Silurian, the Middle-Upper Yangtze Block was in a compressional tectonic regime with limited heat supply, and hence, low hydrothermal circulation; this is manifested in the paucity of hydrothermal microcrystalline silica in cherts east of KU. In contrast, hydrothermal microcrystalline silica in cherts west of KU is interpreted to have formed in an extensional tectonic regime with sufficient heat supply. This research demonstrates that chert source is, in part, tectonic controlled, and chert geochemistry can be used in reconstructing paleoenvironments. • Cherts in Longmaxi Formation (Fm) are mainly biogenic in origin. • Hydrothermal cherts in Longmaxi Fm only occur west of Kangdian Uplift (KU). • Hydrothermal chert west of KU was derived from active hydrothermal circulation. • East of KU there is no evidence of hydrothermal circulation. • Cherts east of KU received more clastic material than those west of KU. [ABSTRACT FROM AUTHOR]

The Early Phanerozoic witnessed one of the most important events in Earth history - the Cambrian Explosion. There is also a consensus that at roughly the same time oceanic dissolved silicon (DSi) concentrations decreased from close to saturation to somewhere below saturation. Yet the timing and magnitude of the putative DSi decrease, and any relationship with biological innovations, remain poorly known. Here, we study the widespread chert formed during the Ediacaran-Cambrian transition (ca. 551-522 Ma) on the Yangtze Block, South China. Major and trace element geochemistry suggest the silicon for the chert is predominantly sourced from seawater, and most likely precipitated via adsorption on organic matter and uptake by silica-secreting organisms. New and compiled silicon isotope data from seven sections spanning the E-C boundary show an increase in divergence of δ 30 Si values at ca. 533 Ma. Using mass-balance arguments, we interpret this as evidence that the radiation of siliceous sponges and radiolarians contributed to a substantial (ca. 80%) decrease in oceanic DSi concentrations, which is corroborated by fossil records and simple box-models. Reduced oceanic DSi may reduce the effort needed to maintain intracellular Si concentrations below the level at which harmful Si precipitation in organismal cytoplasm occurs. This energy can be reallocated to other metabolic pathways of new functional types, potentially the establishment of suspension-feeding communities that triggered Phanerozoic-type mixground ecology. Therefore, the early appearance of silicifiers (∼540-530 Ma) may have made conditions more favorable for the main stage (∼520 Ma) of Cambrian Explosion in functional and ecological diversification. • Silica for the Ediacaran-Cambrian cherts derives mainly from seawater. • Silicon isotopes suggest the radiation of biosilicifiers drove a decline in ocean dissolved silicon in the early Cambrian. • A box-model suggests the degree of the dissolved silicon drawdown is high (ca. 80%). • The removal of silicon toxicity may have promoted the main stage of Cambrian Explosion. [ABSTRACT FROM AUTHOR]

The rise of siliceous organisms is generally considered to have driven the abrupt decrease of silica in the ocean during the Ediacaran-Cambrian (E-C) interval, even if the exact timing and biological triggers of this event are still debated. Although Ediacaran-Cambrian chert depositions are widely distributed, multiple evidences converge to support their hydrothermal origin. In this study, we provide petrographic study supplemented by elemental concentration, SEM and Raman spectral data on the siliceous rocks from the Yanjiahe Formation (the Three Gorges area, South China), in order to ascertain the origin of included silica and discuss the involved processes of silica deposition. Contrary to most previous studies, our analyses suggest that studied cherts mainly resulted from biogenic silica, and subsidiary from abiotic precipitation influenced by surrounding hydrothermal activity and from detrital input. The deposition model of silica includes direct deposition of siliceous bioclasts, secondary replacement of carbonate by dissolved biosilica, and microbial induced silicification from seawater. The sink of silica recorded in the Yanjiahe Formation was characterized of a transition from typical Precambrian to Phanerozoic process. In addition, micropalaeontological and taphonomical analyses of the Yanjiahe Formation revealed two pulses of microfossil abundance and diversity in the Fortunian and Stage 2, respectively. They can be correlated with a regression and an enrichment in dissolved phosphate that probably derived from enhanced detrital flux. • Silica source in chert of the Yanjiahe Formation is mainly biogenic. • Silicification process was discussed in the Yanjiahe Formation. • Two pulses of blooming of microfossils are recognized in the studied chert. • Varied sources of silica from different settings in the Yangtze Block are discussed. [ABSTRACT FROM AUTHOR]

• We document sponge spicules from Ediacaran-Cambrian boundary interval. • Earliest spiculites are reported here in the lower Yanjiahe Formation. • Geochemical signal confirms the studied chert is mainly of biogenic origin fixed by sponges. • Ecosystem engineering by sponges was probably an important trigger for the first establishment of modern-type ecosystems. • Taphonomy may have affected morphology of spicules and their scarcity in Precambrian. The lack of unequivocal sponge fossils before the Cambrian despite their probable deep origin during the Cryogenian period has been a conundrum to geologists. Their impact on the dramatic evolution of ecosystems and the seawater silica cycle during the Ediacaran-Cambrian (E-C) transition is also speculative. In this study, abundant sponge spicules and spicule-like structures that probably represent original sponge fossils were recovered from four sections of the E-C boundary interval in the Yangtze Gorges, South China. The paleontological and geochemical data presented herein provides evidence for a continuous distribution of relatively abundant sponge spicules, some even forming spiculites, as well as the earliest biogenic deposition of silica by metazoans in the Ediacaran-Cambrian boundary interval. These results further confirm that a biological takeover of oceanic dissolved Si co-occurred with the evolution of silica biomineralization in the E-C interval, resulting in the widespread deposition of biogenic chert. Hydrothermal input in spicule-bearing cherts is also observed. Such hydrothermal activity might have favored the blooming of sponges through significant nutrient supply. These results provide the first conjunction of geochemical and paleontological proxies in support of previous models considering that sponges, by ventilation, filtration and oxygenation of seawater, were important ecosystem engineers of the E-C bioradiation event and of the related establishment of modern-type ecosystems. [ABSTRACT FROM AUTHOR]