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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.

The basal Cambrian marks the beginning of an important chapter in the history of life. However. most paleontological work on the basal Cambrian has been focused on skeletal animal fossils, and our knowledge about the primary producers--cyanobacteria and eukaryotic phytoplankton (e.g., acritarchs)--is limited. In this research, we have investigated basal Cambrian acritarchs, coccoidal microfossils, and cyanobacteria preserved in phosphorites and cherts of the Yanjiahe Formation in the Yangtze Gorges area (South China) and the Yurtus Formation in the Aksu area (Tarim Block, northwestern China). Our study confirms the occurrence in these two formations of small acanthomorphic acritarchs characteristic of the basal Cambrian Asteridium-Comasphaeridium-Heliosphaeridium (ACH) assemblage. These acritarchs include abundant Heliosphaeridium ampliatum (Wang, 1985) Yao et al., 2005, common Yurtusia uniformis n. gen. and n. sp., and rare Comasphaeridium annulare (Wang, 1985) Yao et al., 2005. In addition, these basal Cambrian successions also contain the clustered coccoidal microfossil Archaeophycus yunnanensis (Song in Luo et al., 1982) n. comb., several filamentous cyanobacteria [Cyanonema majus n. sp., Oscillatoriopsis longa Timofeev and Hermann, 1979, and Siphonophycus robustum (Schopf, 1968) Knoll et al., 1991], and the tabulate tubular microfossil Megathrix longus L. Yin, 1987a, n. emend. Some of these taxa (e.g., H. ampliatum, C. annulare, and M. longus) have a wide geographic distribution but occur exclusively in basal Cambrian successions, supporting their biostratigraphic importance. Comparison between the stratigraphic occurrences of microfossils reported here and skeletal animal fossils published by others suggests that animals and phytoplankton radiated in tandem during the Cambrian explosion.

Although knowledge of exceptional Cambrian fossil Lager-statten has increased dramatically, relatively little information has been uncovered about the early ontogeny of ancient Metazoa. Here we describe new phosphatized eggs and embryos, partly indicating germ band formation, and the earliest known larvae of early bilaterians. Developmental sequences are reconstructed for Pseudooides- and tetramerous-type embryos. Together with these developmental sequences, patterns of germ band formation, irregular 'radial' cleavage types, and unciliated larva with serial divisions and a posterior segment indicate affinities with protostomes, most probably within an ancestral arthropodan lineage. Keywords: Yangtze platform, Cambrian, embyros, larvae, ontogeny.

The Tannuolinidae includes only two known genera--Micrina Laurie, 1986 and Tannuolina Fonin and Smirnova, 1967, both from the Lower Cambrian. They are phosphatic bimembrate small shelly fossils, consisting of two types of sclerites--the mitral and sellate sclerites. These sclerites show basal-internal accretional growth, spaced growth lamellae, carinae (in mitral sclerites of Tannuolina) or teeth (in mitral sclerites of Micrina), and unevenly distributed pores and canals. The scleritome of tannuolinids is poorly known because they are often preserved as disarticulated sclerites. Here we describe Micrina xiaotanensis new species and Tannuolina zhangwentangi Qian and Bengtson, 1989 from the Lower Cambrian Shiyantou and Yu'anshan formations (upper Meishucunian to Qiongzhusian stages) at Xiaotan, eastern Yunnan. South China. The new material not only extends the geographic range of Micrina, previously known only in Australia, but also includes two composite specimens of T. zhangwentangi: one with a pair of dextral and sinistral mitrals juxtaposed along the apertural margin of their decrescent sides, and the other with a smaller sellate ontogenetically merged with the sella of a larger sellate. Perhaps as a result of being contacting or imbricating surfaces, the decrescent, sella, and duplicature sides are also characterized by negative allometry (relative to other sides) and a sparse distribution of pores. The new fossils do not support reconstructions that place a sellate and a mitral sclerite, respectively, at the anterior and posterior end of the Tannuolina animal, in a way similar to Halkieria evangelista Conway Morris and Peel, 1995. Instead, they are consistent with the traditional view that the scleritome of Tannuolina consisted of four anterior-posterior files of sclerites: two opposing mitral rows flanked by two imbricated sellate series. The bimembrate Micrina mayor may not have had a similar scleritome. If Tannuolina and Micrina forma rnonophyletic group outside the total group of brachiopods, then Micrina is likely to have hada multisclerite scleritome similar to that of Tannuolina. This implies that microstructural similarities between the tannuolinids and some linguliformean brachiopods are probably symplesiomorphic of convergent. Alternatively, Tannuolina and Micrina may represent stem groups leading to the linguliformeans while Halkieria represents a stem group leading to the calcareous brachiopods--a radical hypothesis that would imply that the two brachiopod groups independently evolved the bivalved body plan.