Discovery of conchostracan genus Clithrograpta and its significance in the Dadianzi Formation,Luanping basin,northern Hebei

In an intensive study of fossil conchostracans from the Lower Cretaceous Dadianzi Formation in the Luanping basin,northern Hebei,the authors found the genus Clithrograpta W.Wang,1987in the upper part of this formation.It is characterized by occurrence in many beds,large numbers of individuals and a high diversity of species.This discovery provides key and strong paleontological evidence for the stratigraphic division and correlation in the Lower Cretaceous Dadianzi Formation in the Luanping basin,northern Hebei and the correct establishment of the continental Jurassic-Cretaceous stratigraphic framework in northern Hebei-western Liaoning.According to this discovery and the conchostracan assemblage se-quence in the Dabeigou Formation and Xiguayuan Formation,the upper part(3rd and4th Members)of the Dadianzi Formation in the area may be correlated with the lower part(original Daxinfangzi Formation)of the"Yixian Formation"in the Jianchang basin,western Liaoning,suggesting that the lower part(1st and2nd Members)of the Dadianzi Formation of the area is absent in western Liaoning;the Xiguayuan Formation is largely equivalent to the Yixian Formation of western Liaoning.