The Cambrian diversifi cation of animals was long thought to have begun with an explosive phase at the start of the Tommotian Age. Recent stratigraphic discoveries, however, suggest that many taxa appeared in the older Nemakit-Daldynian Age, and that the diversifi cation was more gradual. We map lowest Cambrian (Nemakit-Daldynian through Tommotian) records of δ 13 C CaCO 3 variability from Siberia, Mongolia, and China onto a Moroccan U/Pb-δ 13 C CaCO 3 age model constrained by fi ve U/Pb ages from inter bedded volcanic ashes. The δ 13 C CaCO 3 correlations ignore fossil tie points, so we assume synchro neity in δ 13 C trends rather than synchroneity in fi rst appearances of animal taxa. We pre sent new δ 13 C org , 87 Sr/ 86 Sr, uranium, and vanadium data from the same carbonate samples that defi ne the Moroccan δ 13 C CaCO 3 curve. The result is a new absolute time line for fi rst appearances of skeletal animals and for changes in the carbon, strontium, and redox chemistry of the ocean during the Nemakit-Daldynian and Tommotian ages at the beginning of the Cambrian. The time line suggests that the diversifi cation of skeletal animals began early in the Nemakit-Daldynian, with much of the diversity appearing by the middle of the age. Fossil fi rst appearances occurred in three pulses, with a small pulse in the earliest Nemakit-Daldynian (ca. 540-538 Ma), a larger pulse in the mid-to late Nemakit-Daldynian (ca. 534-530 Ma), and a moderate pulse in the Tommotian (ca. 524-522 Ma). These pulses are associated with rapid reorganizations of the carbon cycle, and are superimposed on long-term increases in sea level and the hydrothermal fl ux of Sr.

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Chancelloriids are a group of enigmatic sessile animals that are covered with sclerites shaped as rosettes of spines, producing an appearance like that of a barrel cactus. They are known only from Cambrian rocks. Isolated sclerites of chancelloriids are widespread in small shelly faunas, but they have proven difficult to treat taxonomically due to the variation within and between individual animals. We report on large samples of chancelloriid sclerites from the Meishucunian (pre-trilobitic Lower Cambrian) of eastern Yunnan Province, China, including material from the Dahai Member of the Zhujiaqing Formation (the Zhujiaqing section, Dahai, Huize County) and the Shiyantou Formation (the Xiaotan section, Yongshan County). The material from the Dahai Member appears to come from a single species, Chancelloriella irregularis. Statistical analysis of morphotype co-occurrences in different samples suggests that several species are represented in the material from the Shiyantou Formation, which we herein tentatively place in four species, Allonnia erromenosa, Allonnia tetrathallis, Archiasterella charma sp. nov. and Archiasterella cf. pentactina. We suggest therefore that careful study of large collections of isolated chancelloriid sclerites permits the identification of different species, opening the possibility of their use in evolutionary or stratigraphical studies. In particular, in South China, it seems that Chancelloriella irregularis is characteristic of rocks of the Paragloborilus subglobosus -Purella squamulosa Assemblage Zone and the Watsonella crosbyi Assemblage Zone, whilst Archiasterella and Allonnia first appear in the overlying Sinosachites flabelliformis -Tannuolina zhangwentangi Assemblage Zone. C. irregularis is thus amongst the oldest well-characterized chancelloriids, and the irregularity and poor organization of its sclerites relative to those of younger forms support the hypothesis that chancelloriid sclerites are compound structures arising from the fusion of originally separate elements, perhaps homologous with sclerites of halkieriids and other coeloscleritophorans. The similarity between the arrangement of rays in a chancelloriid sclerite and the arrangement of bubbles in small bubble clusters suggests that in many cases, aspects of the form of chancelloriid sclerites are the result of simple physical processes.

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Cambrothyra ampulliformis Qian and Zhang, 1983, is a jar- or vase-shaped fossil known from the Lower Cambrian of Shaanxi and Hubei provinces, China. It has been interpreted as a protistan test or cyst or a metazoan sclerite. A large collection of specimens from the Xihaoping Member of the Dengying Formation in southern Shaanxi Province permits its detailed redescription. These fossils are highly variable in shape but this variation is continuous and does not support the current recognition of multiple species for this material. They were originally hollow with a restricted basal foramen and a calcareous wall probably composed of fibrous aragonite. All of these features support the identification of Cambrothyra as sclerites of a coeloscleritophoran, a problematic group of Cambrian scleritome-bearing metazoans. Furthermore, the walls of Cambrothyra sclerites contain numerous pores, a feature shared with other coeloscleritophorans. Cambrothyra resembles chancelloriids in particular due to the shared presence of a verruculose texture around the foramen and the absence of mirror-image pairs of asymmetric sclerites. However, unlike chancelloriids, which have rosette-like compound sclerites, the scleritome of Cambrothyra was dominated by isolated sclerites, with only a few pairs and clusters of sclerites and twin sclerites. Consequently, we hypothesize that Cambrothyra forms a clade with other chancelloriids, but represents a basal lineage that plesiomorphically retained isolated sclerites. The morphology of Cambrothyra sclerites, which shares features with both chancelloriids and halkieriids, thus supports the hypothesis that all coeloscleritophorans form a natural group.

Hyoliths are a group of Palaeozoic fossils with calcareous shells whose affinities remain controversial. As their shells were originally aragonitic, their fossils are usually coarsely recrystallized, and few data on their microstructure are available. We report hyoliths from the middle Cambrian (Drumian, Floran) Gowers Formation of the eastern Georgina Basin, Queensland. These are preserved as phosphatic internal moulds, often with the inner layers of the shell also partly replaced by phosphate. Microstructural details preserved by this early diagenetic phosphatization show that these hyolith conchs were originally composed of fibrous crystallites, c. 0.5 μm wide, parallel to one another and to the inner surface of the shell. In several species, the fibres are arranged in a plywood‐like structure composed of multiple lamellae with a different fibre orientation in each lamella: often they are transversely oriented (relative to the long axis of the conch) in the inner part of the wall and longitudinally oriented in the outer part. Opercula also show a microstructure of parallel fibres. The lamello‐fibrillar microstructure we report from hyoliths is reminiscent of microstructures of many Cambrian molluscs; that this microstructure is found in both conchs and opercula suggests that these structures are serial homologues of one another, and in this respect they resemble brachiopod valves. As with many other biological plywoods, the hyolith shell probably records self‐organization in a liquid‐crystal‐like organic matrix. This provided a straightforward way to construct a material that could resist stresses from different directions, offering an effective defence against predators.

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Chancelloriids are a poorly understood group of phylogenetically problematic Cambrian metazoans; complete specimens show they were sessile, radially symmetrical, club‐shaped organisms covered with sclerites in the form of rosettes of spines. While isolated sclerites are common components of Cambrian shelly assemblages, they have been relatively little studied. We describe chancelloriid sclerites from a series of nine sections spanning the upper Dyeran to lower Delamaran stages (latest Stage 4 to perhaps basal Wuliuan) from the Pioche–Caliente region of east‐central Nevada, USA. Acid maceration of samples from the Combined Metals, Comet Shale and Susan Duster Limestone members of the Pioche Formation yielded more than 2000 sclerites. Based on careful examination of these sclerites and statistical analyses of co‐occurring sclerite types, we distinguish six species, each with a restricted stratigraphic range. Chancelloria impar Moore sp. nov. is the dominant species in most upper Dyeran samples. Archiasterella cometensis Moore sp. nov. and A. auriculata Moore sp. nov. are rare in the upper Dyeran but abundant in the lowest Delamaran; A. uncinata Moore sp. nov. and C. lilioides Moore sp. nov. replace these in younger samples. A. auriculata is noteworthy for sharing features with species of both Archiasterella and Chancelloria. These results provide further support for the taxonomic tractability and biostratigraphical utility of chancelloriid sclerites; large collections from single horizons allow intraspecific variability to be assessed and species to be distinguished. Our results document a taxonomic turnover in chancelloriids at the Dyeran–Delamaran boundary, showing that not only trilobites were affected at this time.