Spinicaudatans from the Upper Cretaceous Nenjiang Formation of the Songliao Basin, northeast China: taxonomy and biostratigraphy

The spinicaudatan (conchostracan) Dictyestheria, originally recovered from the Yaojia Formation, is recognized to occur also in the basal part of the overlying Nenjiang Formation. This means that the Dictyestheria elongata ( = D. ovata) Zone extends from the Yaojia Formation into the upper Turonian basal part of the First Member of the Nenjiang Formation and underlies the Halysestheria yui Zone at Yaojia Railway Station, Dehui City, Jilin Province, northeast China. The fossil record shows that the basal part of the Halysestheria yui Zone is dominated by Plectestheria songhuaensis in the study area. The diagnoses of Dictyestheria and Plectestheria are revised on the basis of an examination of specimens under a scanning electron microscope. In both genera the radial lirae on the growth bands near the umbo are long and relatively widely spaced. There is a fine reticulum between them in the umbonal part of the carapace and cross-bars occur on the ventral surface. The ornament of both genera indicates that they can be assigned to the family Halysestheriidae rather than to the Jilinestheriidae, in which they have been placed previously. The discovery of serration along the lower margins of growth lines in Plectestheria indicates its close relationship with Orthestheria (Migransia), supporting an interpretation of parallel evolution of O. (Migransia) to Plectestheria in East Asia and Bauruestheria in South America.