Microproblematica Fossils From The Gaojiashan Biota

It has drew worldwide attention once the Cambrian Explosion was reported,and the reasons also attract the attention of scholars.The biotas on the eve of the Cambrian Explosion play an important role on the study of the Cambrian Explosion.The period from about 3.5 billion years to 540 million years ago is the stage of early life evolution.During this time,the emergence and development of prokaryotes,the origin and evolution of eukaryotes,the origin and evolution of metazoans and metazoa are very important biological evolution events(Yuan et al.,2002),which provide material basis and environmental support for the Cambrian Explosion.The Gaojiashan biota,which is under the Precambrian-Cambrian boundary,is a perfectly important biota with skeletal tubular fossils and many fossils such as vase-shaped fossils and trace fossils.In recent years,with the accumulation of fossils and the deepening of research,using the new experimental methods and new techniques X-ray experiments combining with traditional experimental methods of scanning electron microscopy to obtain precious internal structure images of fossils without destroying the shape and structure,more and more new groups have been discovered and revealed.They not only make the previously unresolved issues more clear,but also play an indispensable role in the study of the Cambrian Explosion.The fossils illustrated in this paper are mainly collected from the interclastic dolostone from the upper part of the Gaojiashan section of the Dengying Formation in the Lijiagou section of the Ningqiang area in southwestern Shaanxi.The main progress and conclusions include: discovering a diverse assemblage of calcified cyanobacteria which are identified as Epiphyton,Girvanella,Obruchevella,Subtifloria,Cambricodium,and others.Among them,Obruchevella and Girvanella were discovered in the Neoproterozoic,but the number is rare and the reports are only sporadic.Epiphyton,Girvanella,and Subtifloria are the most common and important components of reefs in the Cambrian.Cambricodium is currently only found in the Early Cambrian strata.These fossils is the first to be discovered and reported in the Gaojiashan Member of the Dengying Formation in this area,and it is also the first time in the end of the Precambrian around the world.Calcified green algae fossils are also firstly discovered at the end of the Precambrian,including Cyclocrinites and Microcodium.The prosperity of calcified cyanobacteria at the end of the Neoproterozoic is the key to solving the "Precambrian enigma".It was previously believed that there was a so-called ‘fossil record deletion' of calcified cyanobacteria in the Neoproterozoic,and the reason may be the lower precipitation rate of calcium carbonate relative to the Palaeozoic at the end of the Neoproterozoic or the inability of calcification of cyanobacteria and algae during this period.A large amount of phosphatized calcified cyanobacteria and calcified green algae indicates that preservation conditions or insufficient discovery may be the main reason.It is believed that Cambricodium is also a kind of calcified cyanobacteria,belonging to the same family as Subtifloria and can be attributed to Girvanella-Group.At the same time,through the type of filament preservation,the process mode of phosphate replacing calcified sheath is proposed,and Micro-CT technology restored the Cyclocrinites morphological features.In addition to the phosphatized body fossils,microbial pseudofossils have been discovered,that is,the preservation of biological non-mineralized tissues relies mainly on the complete replacement and incomplete replacement of microorganisms.The Doushantuo-Pertatataka giant acanthomorph acritarchs and possible embryo fossils are firstly discovered,including the three genus Meghystrichosphaeridium,Tynnia,Asterocapsoides and embryonic fossils of the genus Megasphaera and Megaclonophycus.The discovery of Doushantuo-Pertatataka acritarchs expands the occurrence range and stratigraphic distribution fossils in the Doushantuo period,and at the same time proves that acritarchs have survived the extinction boundary inferred by the preresearchers.In addition to the reported vase-shaped fossils,several new types of protozoa fossils have been discovered,including long-necked vases and spindles.The discovery of new protozoan groups has greatly increased the biodiversity of the Gaojiashan biota,providing important clues for exploring the evolution of the Late Neoproterozoic ecosystem.The discovery of the above fossil types has enabled the Gaojiashan biota to play a role in a connecting link between the preceding and the following evolution of lives,strengthening continuity of evolution of the Precambrian and Cambrian life.