A Preliminary Study On The Morphology And Locomotion Of Posterior Section Of Cambrian Vetulicolians

The morphology and phylogenetic position of vetulicolians,which appeared in many Cambrian L(?)gerstatten around the world,have always been controversial.They were initially considered to be a strange group of large bivalved arthropod.Later,based on discovery of a series of important structures such as pharyngeal gill slits,Shu et al.demonstrated that vetulicolians were a primitive branch of metazoans,and established the phylum Vetulicolia.The body of the Vetulicolians is divided into two parts,the anterior section and the posterior section.Vetulicolians is bipartite,cuticularized body,anterior segmented region and voluminous mouth,ventral margin flattened,and widens posteriorly.There are a lateral groove on either side of the anterior section.Each lateral groove contain five pharyngeal gill pouches with slits.The posterior section(tail)is paddle-like or latifoliate.It is in general divided into seven segments,along which there is a alimentary canal running through and an anus is located near the end.The posterior section of the Vetulicolians has obvious segments,and there are arthropod-like articulation,articular membranes and other joint structures between each segment.Based on these features,it is inferred that the posterior section is the most important organ of movement for Vetulicolians.However,there has been controversy about the flattening direction and the movement mode of the Vetulicolians posterior section.Some scholars believe that the posterior section of Vetulicolians is dorsal-ventral flat,swinging up and down;other scholars believe that the it is laterally compressed,driving the body forward by swinging laterally.In this paper,by studying the burial status of the Vetulicolians posterior section,analyzing the articulation/twist characteristics of the articulation structure between each segment of the posterior body,judging the symmetry of the paddle and the position of the anus,in order to restore the flatness dtrection and locomotion of the posterior section.In addition,it provides further evidence for the morphology and locomotion of the hind body of the archaea by comparing the living fish and conducting simulation research.The results of the study indicate that the posterior section of the Vetulicolians is laterally compressed,and the locomotion is mainly lateral swing.The morphology and movement mode of the posterior section further supports the taxonomic status of Vetulicolians as the primitive group of metazoans.