AMMONOID FAUNAS AROUND THE EARLY AND MIDDLE PERMIAN BOUNDARY IN CHINA AND THEIR BIOSTRATIGRAPHIC IMPLICATIONS

The Early-Middle Permian(Cisuralian-Guadalupian) boundary is marked globally by tremendous changes in terms of the development of ammonoid faunas.At that time the new Order Ceratitida appeared and their first known representatives are found in the lowest part of the Middle Permian Roadian Stage.The earliest Ceratitida therefore serve as reliable markers for the boundary between the Lower and Middle Permian in the Tethyan region and North America in terms of ammonoids.In other biogeographic regions,how-ever,where Ceratitida have not been found,representatives of the families Pseudogastrioceratidae and Spirolegoceratidae play an important role in biostratigraphical subdivision.The GSSP(Global Stratotype Section and Point) between the Lower and Middle Permian is defined in the Glass Mountains of Texas based on the first appearance of the conodonts species Jinogondolella nanking-ensis.However,the first Middle Permian representatives ammonoids appear slightly below this level in the Texas section.As the Roadian Stage was originally defined by Furnish based on ammonoid data.Many ammonoid experts do not agree with a definition based on conodonts in view of a fact that a typical set of Middle Permian ammonoids existed in the Early Permian based on the conodont definiton.Ammonoid experts therefore strongly suggest that the ceratitid Paraceltites elegans and other ceratitid ammonoids should all be attributed to Middle Permian.In this study,characteristic ammonoid faunas around the boundary between the Lower and Mi-ddle Permian in China are re-identified and reviewed.Based on a number of ammonoid occurrences materials near the Kungurian-Roadian boundary previously reported,I consider that ammonoid faunas near the Lower-Middle Permian boundary mainly occur in South and northwest China.Compared with the coeval ammonoid faunas in Russia,it can be concluded that endemic elements including Pseudohaloritinae and Shou-changoceratinae dominate the ammonoid faunas near the boundary.Based on a new study of a number of the ammonoid faunas,the lower part of the Shuangbaotang Formation is assigned to the Kungurian,the middle to upper part of the formation to the Roadian;the Fanjiatun Formation to the Roadian or Wordian;the Jiala Formation from Xigaze,Tibet and the Urulung Formation at Duilongdeqing,Tibet,to Roadian.The Shuangbaotang Section in Gansu Province and the Dongwuli Section in Zhejiang Province are considered the most promising localities to solve the problem of the Kunguian-Roadian boundary in terms of ammonoids.