Trilobite‐based biostratigraphy of the Xiaoshiba Lagerstätte

A large number of exquisitely preserved fossils, including eight trilobite species, from the Xiaoshiba Lagerstätte in the Hongjingshao Formation, Kunming, southern China, shed new light on the evolutionary biology of early animal life. These trilobites also have significance for the biostratigraphical division of the lower Cambrian of southern China. Herein holaspid exoskeletal characteristics of seven trilobite species, Yunnanocephalus yunnanensis Mansuy, Dolerolenus (Malungia) laevigata Lu, Zhangshania typica Li & Zhang, Hongshiyanaspis yiliangensis Zhang & Lin, Chengjiangaspis chengjiangensis Zhang & Lin, Kuanyangia pustulosa Lu and Kuanyangia shaanxiensis Zhang & Lin, are described based on material from the Hongjingshao Formation in the Xiaoshiba area. A new variant of Z. typica is recognized that shows many small, serrated spines on the posterior margin of the pleural spine and pygidium. If refined biostratigraphical zones are to be defined for these rocks, they should be based on species occurrences, rather than genus-level assemblage zones. By refining the stratigraphical occurrence of trilobites within highly fossiliferous individual sections of the Xiaoshiba Lagerstätte, we suggest that a species-based zone based on the occurrence of species of Chengjiangaspis or Hongshiyanaspis will likely be useful, as the ranges of species belonging to these genera are markedly shorter than those of Yunnanocephalus. Knowledge of the occurrence Chengjiangaspis and Hongshiyanaspis in the upper part of the Yu'anshan Member of the Heilinpu Formation is presently too poor to permit proposal of a formal zone herein, but any zone so defined will extend into the lower part of the subsequent Hongjingshao Formation and will represent the latest Chiungchussian Stage in the traditional Chinese scheme (equivalent to the middle part of the Nangaoan Stage). While needing substantial improvement, the trilobite biostratigraphy permits the Chengjiang, Xiaoshiba, and Malong soft-bodied biotas to be arranged in successive stratigraphical order, collectively representing less than 5 m.y. of early Cambrian time.