Yanjiestheria, Yanshania and the development of the Eosestheria conchostracan fauna of the Jehol Biota in China

Re-examination of specimens of the type species of Yanjiestheria (Y. bellula), from the Albian Dalazi Formation in eastern Jilin, and Yanshania (Y. dabeigouensis), from the Berriasian Dadianzi Formation in northern Hebei, under a scanning electron microscope has confirmed that Yanshania is a junior synonym of Yanjiestheria. This means that the earliest occurrence of Yanjiestheria precedes that of Eosestheria in northern Hebei, indicating that it cannot have descended from the latter, as has been suggested previously; rather the two genera must have evolved in parallel. An analysis of the changes in diversity of components of the Eosestheria fauna of the Jehol Biota indicates that the increase in the number of genera and species, which began during the deposition of the Jianshangou Beds of the Yixian Formation, may have been connected with the dispersal of conchostracan taxa to north-east China, Inner Mongolia, west and south Mongolia and elsewhere from what appears to have been their main centre of origin in northern Hebei. The Eosestheria fauna continued to flourish until deposition of the Jiufotang Formation came to an end.