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The post-protaspid ontogenetic series of the eodiscinid trilobite Tsunyidiscus yanjiazhaiensis S.Zhang in Yin and Li,1978 is described,on the basis of a large amount of articulated specimens together with numerous disarticulated sclerites from the Shuijingtuo Formation,Cambrian Series 2,Shaanxi Province,South China.Additionally,thousands of phosphatized exoskeletons assigned to T.yanjiazhaiensis,were also obtained by diluteacetic acid (5％) dissolution of the thin-bedded limestones from the same horizon.These three-dimensionally preserved eodiscoid exoskeletons,exhibiting unusually detailed features and marked morphological disparity,enable an investigation on their body plan and trunk segmentation schedule,and set up an essential criteria for evaluating the previous taxonomic assignment of Tsunyidiscus from southern China.On this basis,the postembryonic development of these eodiscinids has been thoroughly examined and discussed.