Skeletonized Microfossils And Depositional Environments At The Precambrian-Cambrian Boundary

The Precambrian-Cambrian boundary is a critical paleobiological concern and receives increasingly important scientific attentions. Ediacaran successions are well-exposed in the Ningqiang area of southern Shaanxi Province, South China where abundant fossil materials have been well-known and thus would be an ideal study area providing important insights into our understanding of Precambrian-Cambrian biotic evolutions.Filed sampling, laboratory analysis, SEM photography, thin-sectioning, and slabbing are conducted in this study. More than 4,000 Early Cambrian specimens of Small Shelly Fossils and the co-occurring animal embryos and more than 200 specimens of latest Ediacaran skeletonized fossils Cloudina and Sinotubulites are recovered from phosphorites and cherts of the Early Cambrian Kuanchuanpu Formation and dolostones of the latest Ediacaran Beiwan Member of the Dengying Formation, respectively. In addition, more than 20 polished slabs and more than 200 thin-sections are studied to constrain the depositional environments of these fossils.Morphology, taxonomy, and paleoecology of a variety of taxa, including Conulariids, Anabarites, Protoconodonts, Cloudinids, are discussed in this paper:1. Morphology of a newly found bilaterial Conulariids is described with a new morphological term. This new taxon is ascribed to Hexangulaconulariids. This taxon is presumed to be a nekton or plankton and the hollow lumen may have been capable of generating buoyancy. Within the lower part of the lumen, however, possesses the respiratory and feeding organs. We postulated that this taxon may have close phylogenetic relationships with the pentagonal fossil Punctatus.2. Anabarites is interpreted as a erect benthos and the curved tube is due to the destructive influence of storm-generated events. Morphology of anabarites is compared with an Ediacaran tri-symmetrical skeletal fossil and they may have close taphonomic links.3. A review and discussion on Protoconodonts are summarized on the basis of previous study. In addition, a well-preserved Protoconodont showing unusual morphological feature is described and interpreted as possible predatorial texture remains on Protoconodont.4. Cloudina has been suggested to be the earliest biomineralized fossil and a potential index fossil in the Ediacaran. Observed from body fossils and thin-sections, we suggest that all specimens share the same morphology and thus only one taxon is existed. The seemingly morphological differences are resulted from taphonomic bias. Cloudina is an erect epi-bethos and funnel-shaped walls of this tubular fossil don't eccentrically intersected.5. This study is focused on the discussion of animal embryos. The lack of cleavage-stage embryo forms has led to inaccurate assumptions on this fossil. Possible cleavage-stage forms are discussed and compared with Ediacaran embryos from the Doushantuo Formation and a "one egg, multiple tubes"-like developmental sequence-an initially developed egg and diversified tubular forms of post-egg stage-is reconstructed. Statistical analysis is also included in this study.6. A sedimentary interpretation-the Kuanchuanpu assemblage was deposited in storm-generated sediments-is drawn on the basis of not only hummocky cross stratification seen on the outcrops but also microfacies evidence observed in thin-sections.