Functional Morphology Of An Unknown Structure In The Posterior Section Of Xidazooon In The Chengjiang Biota

Vetulicolia is an extinct phylum of mysterious and peculiar Cambrian creatures with controversial phylogenetic positons.With the discovery of Gills of pharynx in Vetulicolia,which is the most important feature of Deuterostomia,more and more scholars tend to believe that Vetulicolia is likely to represent the most primitive stem group of Deuterostomia.Vetulicolia has attracted much attention because it is closest to the origin of the Deuterostomia that people have explored so far.The research of Vetulicolia has a long history,and it has gradually become clear to people about its overall external shape and internal pharyngeal cavity,gill system,digestive tract and other structures and functions.But more details about its morphological structure still need to be revealed.Vetulicolian fossils are distributed world wide.Among them,fossils from the Chengjiang biota in eastern Yunnan of China are abundant in species and well-preserved.This thesis studies the fossils of Vetulicolia from the Chengjiang biota and found a special structure in the tail of one of the species—Xidazoon stephanus Shu,Conway Morris et Zhang,1999.The structure constantly appears between or within the third and/or fourth segment of the tail.The whole structure is nearly circular,with a diameter of about one-fifth of the maximum width of the tail,and there are horizontal folds parallel to the front and rear axis and organic masses.Based on the statistics and analysis results of Xidazoon fossil specimens,we infer that the structure is an organ in Xidazoon.It may be a simple gonad that has not evolved into a complete organ of Xidazoon.The primitive nature of this structure is also consistent with phylogenetic positions of the stem group of Deuterostomia.The in-depth study of this structure will enrich people’s understanding of Vetulicolia,and be beneficial to further exploration to the origin and early evolution of the organs and physiological functions of early Deuterostomia.