AMMONOIDS ASSEMBLAGES OF THE MIDDLE AND LATE JURASSIC FROM NIELAMU COUNTY, TIBET

The ammonoids described in the paper were collected from the middle and upper Jurassic in Nielamu County of southern Tibet in 1980 and 1981, Stratigraphically, these fossils can be divided into four assemblages and an ammonoids-bearing stratum as follows: 1. Macrocephalous-Indocephalous assemblage located at the basement of Menbu Formation, mainly consists of Macrocephalitidae, 2.Aulacosphinctoibes-Ptychophylloceras lalonglaensis assemblage derives from the middle and upper parts of Menbu Formation with few Katroliceras and Metagravesia genera etc, 3.Virgatosphinctes-Haplophylloceras pinque assemblage from the upper section of Xiumo Formation is characterized by plenty of Virgatosphinctes (sPP.). 4.Berriasella-Blanfordiceras assemblages at the lower part of Gucuo Formation are principally composed of Berriasellidae. Mayaitids-bearing stratum lies at the lower part of Menbu Formation, Based on the correlation analysis among the ammonid faunas and the other faunas from some parts of the world, it is considered the faunas in Tibet present a completed series from Callovian to Tithonian, It is interested that the Tithonian may be divided into three parts with three ammonoids-bearing strata of different features.