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Sponges are among the most common and diverse Cambrian fossils. However, the origin of the crown groups of the extant sponge classes constitutes the principal problem in the understanding of the evolution of these lower metazoans. New intact spiculate sponges from the lower Cambrian Stage 3–Stage 4 Sinsk Lagerstätte of the Siberian Platform enable a better understanding of the early evolution of crown‐group demosponges. The skeletons of Neomenispongia plexa and N. diazoma gen. et sp. nov. consist mostly of simple oxeas, which are organized in relatively regular tufts that are additionally strengthened by sigmoidal spicules. The C‐shaped elements of N. diazoma are megascleres in their size range but have sigmoidal shapes similar to sigma microscleres of extant demosponges; the sigmoidal spicules of N. plexa fully accord with microscleres and are the smallest spicules in known Cambrian demosponges. Together with an unnamed early Cambrian demosponge from the Sirius Passet biota (Greenland) and middle Cambrian Ulospongiella from the Burgess Shale (Canada), the new species represent the earliest heteroscleromorph demosponges and indicate an evolutionary origin of microscleres from megascleres. The thin, homogenous skeleton of Keithospongos loricatus gen. et sp. nov. is built of small, spirally arranged oxeas corresponding to the skeletal structure of the primitive Hazeliidae, which have been interpreted as the ancestral skeletal organization of demosponges. These new sponges therefore provide a link from extant spiculate demosponge groups to their more familiar Cambrian ancestors. [ABSTRACT FROM AUTHOR]

Archaeocyaths were one of the major animal groups contributing to the Cambrian Explosion. Despite an overall poriferan appearance of their hypercalcified skeletons, they exhibit a number of peculiar features such as the presence of septa and an absence of spicules. This sets archaeocyaths apart from general trends in early poriferan evolution. Fossils recently discovered from the early Cambrian Guanshan Lagerstätte of South China exhibit features typical for the suborder Archaeocyathina whilst possessing a silicified skeleton associated with carbonaceous material. This rigid fibrous skeletal framework, on the other hand, fits the diagnosis of the supposed ancestral aspiculate demosponges, the family Vauxiidae. By a detailed description of this specimen and a direct morphological comparison between the Archaeocyathina and the Vauxiidae, we suggest that the supposed Cambrian representatives of horny demosponges, that is, vauxiids, probably, were a progeny lineage of archaeocyathines. They survived the extinction of hypercalcified archaeocyaths due to their ecological adaption to siliciclastic environments. The existence of hypercalcified dictyoceratid ('keratosan') sponge Vaceletia, well known as a 'living archaeocyath', supports such a possibility. [ABSTRACT FROM AUTHOR]

Diverse skeletal assemblage has been discovered in the Ediacaran/Cambrian strata of the Codos locality in NE Spain (Aragón, Cadenas Ibéricas). This assemblage includes at least seven genera, only three of which can be ascribed to known taxa. All the fossils are preserved in phosphate but their original microstructures are traceable by both elementary composition and microstructural features that are indicative for primary aragonite and high-magnesium calcite biomineralogies. Already these early skeletal fossils show sophisticated microstructures represented by heterogeneous multilayered composites to satisfy the requirements for better protection against both chemical dissolution and mechanical predator deterioration. The most common and best-preserved fossil, which is tubicolous Codositubulus grioensis gen. et sp. nov., is described here. The composition of the fossil assemblage and its stratigraphic position are indicative for the lowermost Terreneuvian (lower Cambrian) or even pre-Terreneuvian age of this fauna. [ABSTRACT FROM AUTHOR]

Understanding the mechanisms that sustain high biodiversity and regulate community structure remains a major challenge as past ecological interactions cannot be reconstructed with precision. Here we reveal the dynamics of the earliest diverse skeletal metazoan communities known, the early Cambrian (Tommotian–Atdabanian) archaeocyathan sponge reefs, over a period of 20 million years from their first appearance ~ 535 million years ago (Ma) until the first mass extinction event ~ 512 Ma. Archaeocyaths were restricted to the Siberian Platform until ~ 520 Ma, and so these data provide unique insight into a single, evolving isolated species pool. Systematic and statistical analyses, including α -diversity, evenness, Jaccard similarity, species abundance distributions, and Spearman statistics coefficient were undertaken for 58 sampling units representing 6121 individuals of 84 archaeocyath species from the Siberian Platform. These statistical results indicate an overall similarity to analyses of species co-occurrence in modern tropical coral reefs and rainforest communities. We show a prevalence of niche-assembly mechanisms throughout the entire Tommotian–Atdabanian interval. By the late Atdabanian, however, these earliest metazoan reefs showed the addition of community assembly processes, which can be attributed to neutral behaviour. [ABSTRACT FROM AUTHOR]

The Ediacaran-Cambrian transition, which incorporates the radiation of animals, lacks a robust global temporal and spatial framework, resulting in major uncertainty in the evolutionary dynamics of this critical radiation and its relationship to changes in palaeoenvironmental geochemistry. We first present a new δ13C carb composite reference curve for the Ediacaran Nama Group of southern Namibia, and we then outline four new possible global age models (A to D) for the interval 551–517 million years ago (Ma). These models comprise composite carbonate‑carbon isotope (δ13C carb) curves, which are anchored to radiometric ages and consistent with strontium isotope chemostratigraphy, and are used to calibrate metazoan distribution in space and time. These models differ most prominently in the temporal position of the basal Cambrian negative δ13C carb excursion (BACE). Regions that host the most complete records show that the BACE nadir always predates the Ediacaran-Cambrian boundary as defined by the first appearance datum (FAD) of the ichnospecies Treptichnus pedum. Whilst treptichnid traces are present in the late Ediacaran fossil record, the FAD of the ichnospecies T. pedum appears to post-date the last appearance datums (LADs) of in situ representatives of the skeletal organisms Cloudina and Namacalathus in all environments with high-resolution δ13C carb data. Two age models (A and B) place the BACE within the Ediacaran, and yield an age of ~538.8 Ma for the Ediacaran-Cambrian boundary; however models C and D appear to be the most parsimonious and may support a recalibration of the boundary age by up to 3 Myr younger. All age models reveal a previously underappreciated degree of variability in the terminal Ediacaran, incorporating notable positive and negative excursions that precede the BACE. Nothwithstanding remaining uncertainties in chemostratigraphic correlation, all models support a pre-BACE first appearance of Cambrian-type shelly fossils in Siberia and possibly South China, and show that the Ediacaran-Cambrian transition was a protracted interval represented by a series of successive radiations. [ABSTRACT FROM AUTHOR]

Abstract: Assembly of the necessary biochemical machinery for biomineralisation long-predated the appearance and rapid diversification of metazoan skeletons in the late Ediacaran to Middle Cambrian (~550–520million years ago (Ma)), and the independent acquisition of skeletons of differing mineralogies suggests a trigger that conferred selective advantage to possession of a skeleton even though this involved physiological cost. The cost–benefit ratio of biomineralisation has changed over geological time, varying not only with the availability of precursor ions in seawater, but also with evolutionary innovations, as the energy required to produce a skeleton will change as a function of community ecology, particularly with increases in predation pressure. Here, we demonstrate that during the Cambrian Radiation the choice of biomineral was controlled by an interaction between changing seawater chemistry and evolving ecology. The record also reveals the successive skeletonisation of groups with increasing levels of activity from the Ediacaran to Middle Cambrian. The oldest (~550–540Ma) biomineralised organisms were sessile, and preferentially formed low-cost, simple, skeletons of either high-Mg calcite coincident with high mMg:Ca and/or low pCO2 (aragonite seas), or phosphate during with a well-documented phosphogenic event. More elaborate, but tough and protective, aragonitic skeletons appeared from ~540Ma, dominantly in motile benthos (mostly stem- and crown-group Lophotrochozoa). The first low-Mg calcite skeletons of novel organic-rich composite materials (e.g. trilobites) did not appear until the late early Cambrian (~526Ma), coincident with the first onset of low mMg:Ca and/or high pCO2 (calcite seas). Active, bentho-pelagic predatory groups (vertebrates, chaetognaths, some arthropods) appearing mainly in the late early Cambrian preferentially possessed phosphatic skeletons, which were more stable at the low pH ranges of extracellular fluids associated with intense activity and high-energy ecologies. These trends suggest that the increasing physiological cost of biomineralisation in successively more demanding metabolisms was offset by the increased chance of survival conferred by a protective skeleton, so indicating a driver of escalating community ecology, in particular an increase in predation pressure. [Copyright &y& Elsevier]