Development And Morphometric Analysis Of Mollusk Pelagiella From The Cambrian Stage 3 Of Southern China

Molluska,being the second largest phylum in the animal kingdom,encompass a great diversity of animal taxa.Nowadays researchers are keen on its origin,development and evolution,because they have left numerous fossils in the sediments throughout the whole Phanerozoic.There are two major views about the origin of gastropods in palaeontology:one is that they evolved directly from monoplacophorans,the other is that both gastropods and monoplacophorans came from a common stem ancestor.Pelagiella have made their first appearance as an important representative group of molluska since the early Cambrian,but their subsequent development in body configurations and continuous evolution are not clearly recognized by researchers due to the lack of enough stratigraphical data and poor preservation of available fossils.Nevertheless,this research area remains a hot topic and is full of different academic views and controversiesRecently we have collected a large number of phosphatized specimens from the nodular limestone of the Cambrian Stage 3 Yu'anshan Formation in Leibo,Sichuan province.Most specimens of Pelagiella sp.in our collection are well preserved as internal models,making it possible to apply quantitative detection methods to them.Measurement and statistics show that the outer edge length of shell and the length of aperture grow at the isometric;the length and width of the aperture grow atanisometric.According to the morphological characteristics,shells with different sizes and various shapes are representatives of different growth stages,belonging to the same Pelagiella sp.The simulation method of logarithmic spiral and the principal component analysis method(PCA)have been applied for the research of shell outer edge,so as to obtain meaningful data related to the development and evolution of this taxon.In the application of simulation method of logarithmic spiral,the polar coordinate formula ? =Aea ? is used to simulate the outer edge curve of shells,where the value of a can indicate the degree of curling.Of the shell in the same growth stage,the smaller the value a is,the greater is the tightness of coiling shells.The quantitative study on the outer edge of Pelagiella sp.shows that with growth the outer edge of some shells followed the same value a,while this value may decrease among some shells belonging to different growth stages,or the value of the outer edge of different-sized shells in the same growth stage may be large or small.The results of the PCA show that the outer edges of the larger shells with an umbilicus being nearly rounded display a greater degree of curvature at the beginning of the protoconch,and the outer edge of these shells displays a greater degree of corling.These Pelagiella sp.may belong to intraspecial,and the morphology of shell is more similar to that of the palaeogastrids.These shells can also clearly show distinctive individual morphology in various development stages from a protoconch to an adult shell,such as the changes in shape in the umbilicus and apertures of shells.The statistical results on umbilicus show that the width of umbilicus varies significantly in different individuals.The appearance of specific aperture lips is considered to be intraspecific feature.In comparison with other known typical molluska species,Pelagiella was found to have some similar rmicrostructures of monoplacophorans and gastropods.The new information above reveals the basic shell structure and developmental characteristics of molluska Pelagiella individuals,which are worldwide distributed.It would be convenient for a comprehensive comparison of this taxa around the world and may provide new reference data for studies on the origin,development and evolution of the early Cambrian gastropods.