Triassic cephalopods from the Zhuganpo and Xiaowa Formations in Guan ling,Guizhou,with a discussion on the age of the Guanling biota

The Middle and Upper Triassic Zhuganpo Formation and Xiaowa Formation in the Guanling area,Guizhou,yield abundant cephalopods,including11families,19genera and37species.Among them,the Enoploceras,Alloceratites and Sibyllites and Trachyceras aon were first reported in China.Five ammonoid zones may be recognized,they are in ascending order:(1)the Xenop rotrachyceras primum Zone,(2)the Protrachyceras deprati Zone,(3)the Protrachyceras costulatum Zone,(4)the Tra chyceras multituberculatum Zone,and(5)the Sirenites cf.senticosus Zone.The characteristics of these ammonoid zones are discussed and correlated with international contemporaneous zones.The first zone in the Zhuganpo Formation may correspond with the Early Ladinian R.reitzi Zone in western Europe,the sec-ond and third ones are equivalent to the Late Ladinian E.curionii Zone and P.archelaus Zone respective-ly,the fourth one in the Lower Xiaowa Formation may be correlated with the early Early Carnian T.aon Zone in Europe,and the fifth one in the Middle Xiaowa Formation is equivalent to the late Early Carni-an Sirenites nanseni Zone in northern America.The horizon bearing Guanling biota may correspond with the Trachyceras multituberculatum Zone,which should be of early Early Carnian age.