Study On The Phosphatized Lagerst(?)tte Fossils Of Xixiang Biota

The Cambrian Lagerst?tte biota has great significance in study of origin and evolution of early life.The Xixiang biota is a Lagerst?tte biota of small skeleton fossils,which occurs in phosphorus bearing stratigraphy of Cambrian bottom in Xixiang,Sourthern Shaanxi.The fossil association features of this biota could compare with that of small skeleton fossils of Early Cambrian Meishucun stage.This biota has great amount of well-preserved fossils which cover varieties of species,and many fossils found in this biota enriches the Cambrian explosion,is a rare scientific window for origin and evolution study of early life.Based on the field recording,sample cllection and a mass of sample preparation,this article summarizes the genera,species and features of six kinds of fossils: Hyolithes,Hyolithelminthes,Anabarites,Cycloneuralia,Quadrapyrgites and Acanthocassis.The main achievements of this article are as follows:1.Study of Hyolithes and its internal featuresPrimary statistics shows that the amount of Hyolithes in Xixiang Biota is enomous,accounted for 60% ~ 70% of the total fossils in this biota.Mainly is Circotheca Sysoiev,1958,with a small amount of Pseudovalitheca Qian,1989,all belonging to Orthothecimorpha,Circothecida.The Hyolithes of this biota are in the first stage of Hyolithes evolution in China.According to the study of rock slice,the internal structure of Hyolithes is divided into sevral categories,the single-layer shells and concentric multilayer shells are protogenous(about 70%)and the excentric multilayer shells are secondary structure that produces by potamic transport(about 30%).The multilayer shell structure should reinforce the shell,is the survival strategies in response to the appearance of carnivorous animals.2.Study of Anabarites and its evolution of symmetryAnabarites trisulcatus Missarzhevsky,1969,Aculeochrea tripartitus Missarzhevsky,1969 and Anabarites isiticus Missarzhevsky,1974 are three main species of Anabarites in Xixiang Biota.Among them,the A.isiticus has two kinds of growth lines that differ from each other by their width.These growth lines reflect the different growth periods of Anabarites.Meanwhile,some special fossils of A.trisulcatus showing the transform stage between tetraradial symmetry and Hexagonal symmetry,forming an evolutionary sequence for the symmetry.It is showing that in the progress of evolution,the new secondary grooves are not present at the same time,but presents one by one.Because of the conservatism of symmetry evolution,this law should apply to the evolution of higher multiples radial symmetry.3.New meterial of Quadrapyrgites embryoPrevious research on the morphological characteristics,internal structure and growth pattern of Quadrapyrgites are detailed,and the individual development sequence has been established primaryly,while some significance develop stage,especially stages before the larval stage are still inadequate.Recently,a new embryo fossil of Quadrapyrgites are found in Xixiang Biota,it has smooth epidermis and are already showing the rudiment of Quadrapyrgites,thus is considered as the late preincubation stage.The new meterial supplemented and improved the individual development sequence.In addition,the comparison among this fossil and two species of punctatus is showing in this article.