The Study Of Microfossils In The Lower Cambrian From The Southern Shaanxi Province And Northern Sichuan Province

Earliest skeletal fossils (Small shelly fossils) is an important part in microfossils. Thesmall shelly fossils thrived in the early Cambrian, marking the beginning of “Cambrianexplosion”. The small shelly fauna, lived the period between Ediacaran fauna and Chengjiangfauna, probably originated from Ediacaran and giving rise to Chengjiang fauna. Additionally,these small shelly faunas are representative of the first skeletal organisms which indicates asudden change on biomineralization. As a result, the study on earliest skeletal fossils is of animportant significance on investigating Cambrian explosion and life’s evolution. The LowerCambrian system in Southern Shanxi and Northern Sichuan, which contain rich earliestskeletal fossils, micropaleobotanical fossils and embryo fossils which witness multicellularityand biodiversity on the eve of Cambrian life’s explosion are widely distributed at the northedge of Yangtzi platform.In this study, we sampled abundant earliest skeleton fossils and embryo fossils andanalyzed these specimens by scan electronic microscope (SEM), slicing and X-ray phasecontrast micro-imaging and so on. After that, a number of earliest skeleton fossils and embryofossils were obtained. At last, we make the following progress:1. Embryo fossils of cleavage stage in Kuanchuanpu faunaSeveral exceptionally-preserved three-dimensional embryo fossils, respectivelycorresponding to cleavage, blastula and gastrula stage, were obtained through acetic acidtreatments and slicing, and a complete developmental sequence being from olivooides toblastula and gastrula then toward the juvenile stage of Punctatus, is proposed. The new fossilsfill the gap on embryo fossil of cleavage stage in Kuanchuanpu fauna and make a perfect onintegrated embryo development. In comparison with fossil embryos of Wen’an Biota inGuizhou Province and their modern equivalents, the cleavage course of embryo is revealed. Inthis paper, the developmental data facilitate a thorough integration of palaeontology intoevolutionary developmental biology on understanding the biological evolution.2. Probable gastrulation patterns in Kuanchuanpu faunaThe rich embryo fossils in gastrulation stage in Kuanchuanpu fauna provide probabilityfor researching gastrulation patterns of early life. Based on abundant embryo fossils ingastrulation stage and their microstructure observed from rock sections and Micro-CTtechnology, we found two types of gastrulation in Kuanchuanpu fauna, namely,invagination of a coeloblastula to form a coelogastrula and delamination of a coeloblastula toform a coelogastrula, suggesting that characters of blastula are influenced by gastrulation. It also supports the viewpoint of Wolpert et al.(1992) that the gastrulation pattern depend onuniformity degree of yolk distribution in embryo, but has no significant relationship withbiological development and evolution.3. The discovery of a new species in Punctatus and foundation of integrated developmentalsequenceHere, we obtained several intact specimens of Punctatus. They were identified as a newspecies, named Punctatus costalis sp. nov. In addition, the specimens assigned to Punctatusemeiensis were found in Nanjiang region, which expands geographical distribution ofPunctatus. The fossils being from cleavage, blastula, gastrula stage to larva, were obtained,which improves developmental sequence from Olivooides to Punctatus. This investigationprovides significant proof for genesis, systematics, evolution and ontogeny of metazoan.4. The discovery of a new species in HexangulaconulariaAbundant protoconulariids fossils in both number and category were obtained. Theyinclude Carinachites and Hexangulaconularia. It is worth indicating that we established anew species assigned to Hexangulaconularia ningqiangensis sp.nov., which superinduce aspecies into Hexangulaconularia. Contemporarily, it is significant to stratigraphy.5. The discovery of Bifurcate fossils and their biological propertyWe found plenty of bifurcate including dichotomous and trichotomous column-shapedtube fossils characterized by fragmentation and budding. Finally, their biological property wasdiscussed based on detailed observation and description of the fossils.