云南丽江奥陶纪鹦鹉螺

ORDOVICIAN NAUTILOIDS FROM LIJIANG, YUNNAN, SOUTHWEST CHINA

首次系统描述云南丽江鸣音下胖罗、当子落两地奥陶纪鹦鹉螺 10属 19种 ,包括Meitanocerassubglobo sumdiscoides,Changyangocerasgaojiabianense,Michelinoceraselongatum ,M .paraelongatum ,M .parasquamatulum ,Michelinoceravariabilum ,M .xuanxianense ?,M .aff.M .xuanxianense ,M .zangbeiense,Eosomichelinocerasguizhouense,Archigeisonoceraselegatum ,A .sp .1,A .sp .2 ,Stereoplasmocerinasp .1,S .?sp .2 ,Camerocerascf.C .compressum ,Diderocerassp .,Discocerascf.rarospira ,Trocholitesaff.T .lativentrosus。依据鹦鹉螺组合特征 。

Few Ordovician nautiloids from the western part of Yunnan have been described systematically before. During the field excursions in 1973 and 1981, the junior author collected many fairly preserved nautiloid specimens from two localities in Lijiang County. The nautiloid bearing beds consist mainly of limestones with some siltstone and shale intercalations. The present paper describes all of nautiloid fossils available so far from Lijiang County, totally, 19 species within 10 genera. Most of species are easily recognized, several specimens are too fragmentary to assign to any known species, only generic names can be specified. Two nautiloid assemblages could be determined. The first is lower stratigraphically with the following components, Archigeisonoceras elegatum, A . sp. 1, A. sp. 2, Changyangoceras gaojiabianense, Meitanoceras subglobosum discoides, Michelinoceras variabilum . Among them, Meitanoceras is significant in stratigraphical correlation, it was found only in the middle Darriwilian before, the specific horizon is the upper part of the Guniutan Formation of the Yangtze Block, so the age of the lower nautiloid bearing bed of the present locality is determined as the middle Darriwilian. The second assemblage is from the upper horizon, including Cameroceras cf. C. compressum, Dideroceras sp., Discoceras cf. rarospira, Eosomichelinoceras guizhouense, Michelinoceras elongatum, M. paraelongatum, M. parasquamatulum, M. xuanxianense?, M. aff. M. xuanxianense, M. zangbeiense, Stereo plasmocerina sp. 1, S. ? sp. 2, Trocholites aff. T. lativentrosus . The genus Michelinoceras is dominant quantitatively in this assemblage. Two species, Michelinoceras elongatum, M. paraelongatum , were known from the middle Caradoc of the Yangtze Block, Tibet, the western margin of the North China Block. Eosomichelinoceras was also thought to be a typical nautiloid genus of the middle Carodoc. Obviously, the age of the upper horizon is the middle Caradoc. Some taxonomic revisions about M. paraelongatum have been done in the part of systematic paleontology of the present paper.