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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]

The Gujingling (古井岭) Section in Xiaodong (小董), Guangxi (广西) Zhuang Autonomous Region, China, consists of cherts and siliceous siltstones, which belongs to the Bancheng (板城) Formation of the Qinzhou (钦州) allochthon. Three successive radiolarian assemblage-zones related to the Guadalupian (Middle Permian) appear in the section in the following ascending order: Pseudoalbaillella sp. aff. Pseudoalbaillella longicornis-Pseudoalbaillella fusiformis, Follicucullus monacanthus, and Follicucullus scholasticus-Follicucullus porrectus. The radiolarian content in this section is generally greater than that of sponge spicules. The radiolarian fauna commonly consists of abundant Albaillellaria and spherical radiolaria with minor Latentifistularia and Entactinaria. The sponge fauna is composed mainly of hexactinellids with minor demosponges. These siliceous fossil faunal features, comparable with those in a deep basin of the western belt of the Phosphoria Basin in the western United States, indicate that the Bancheng Formation in the Gujingling Section was deposited in a basin deeper than 1 000 m. The siliceous siltstones in the section are characterized by inclusion of silt-sized quartz and no inclusion of sand-sized materials, suggesting that the Gujingling Section was located at least a few hundred kilometers from the South China Block in the Guadalupian. [ABSTRACT FROM AUTHOR]

The Ediacaran-Cambrian (E-C) transition was a critical stage for the origin and evolution of early complex life on Earth. Because studies of E-C biotas and environments are relatively fewer for deep-water than for shallow-water facies, we undertook a systematic paleontological analysis of the E-C transition in the deep-water Liuchapo Formation at Longbizui, South China. Two sequential but distinct assemblages were recovered, namely the Lower Liuchapo Assemblage (LLA) and the Upper Liuchapo Assemblage (ULA). The LLA consists mainly of soft-bodied fossils, including Longbizuiella hunanensis gen. et sp. nov., Nenoxites jishouensis sp. nov., Nenoxites irregularis sp. nov., Nenoxites sp., Helanoichnus sp., Shaanxilithes ? sp. and tiny-sized spherical fossils. The ULA is dominated by skeletal organisms comprising the small shelly faunas (SSFs, mainly protoconodonts, hyoliths and Cambroscleritida), sponge spicules, and the first reported Poratusiramus xiangxiensis gen. et sp. nov. Biostratigraphic correlation support a latest Ediacaran age for the LLA, whereas the majority of the ULA belongs to the early Cambrian—thus the change from the LLA to the ULA coincided with the E-C transition. Our results suggest that, during the E-C transition, a biological revolution may have occurred in deep-water settings synchronously with the well-studied faunal turnover in shallow-water environments. At Longbizui, fossil abundance and diversity decreased gradually during the latest Ediacaran and recovered stepwise after the E-C transition. The deep-water fauna, including both benthic and pelagic members, developed a biomineralization ability during the E-C transition. As calcareous and possibly phosphatic skeletal organisms began to flourish in shallow-water environments, siliceous skeleton-forming organisms, represented by the spicule-bearing sponges, colonized the deep seafloor in great abundance. In addition, the siliceous spicules documented in this study record fast evolutionary development during the early history of sponges, one of the earliest-branching metazoan phyla. The evolution of spicules from monaxon to diaxon-triaxon and then to tetraxon and polyactinal forms, as well as their considerable increase in diameter and length, was achieved during the first SSF biozone. • First detailed study of deep-water fossil assemblages of Ediacaran-Cambrian age. • Deep-water fauna may have experienced a turnover during the E -C transition. • Patterns of biodiversity and abundance paralleled those of shallow-marine habitats. • Record of diversification of earliest sponges and other siliceous biomineralizers. • Utility of evolutionary sequence of sponge spicule types as biostratigraphic index. [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]

• 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]