Studies Of Fossil Stems From Late Triassic And Middle Jurassic Of Baojishan Basin,Gansu Province

The Baosjishan Basin contains Mesozoic formations(such as the Upper Triassic Nanying’er Formation and the Middle Jurassic Xinhe Formation),is situated in Pingchuan,Baiyin City,Gansu Province,China,and adjacent to the Ordos Basin.Abundant plant fossils were reported in the Nanying’er Formation,about 38 genera and80 species,but the researches on fossil woods has been relatively less.However,only a few plant wood fossils have been reported from the Middle Jurassic Xinhe Formation in the Baojishan Basin,Xenoxylon,Euestheria and Fossils of vertebrate were reported.In this thesis,we study the fossil of Sphenopsida,based on the comparison with fossil records in morphological characters,10 species of 2 genera were recognized,which are Equisetites baojishanense sp.nov.,Equisetites arctatus sp.nov.,Equisetites lateralis,Equisetites acanthodon,Equisetites cf.shaerhuensis,Equisetites sp.1,Equisetites sp.2,Neocalamites hoerensis,Neocalamites rugosus and Neocalamites sp.Equisetites baojishanense sp.nov.is characterized by tubers,no underground stem or fibrous roots.Tubers are arranged in series along the long axis,and the number of tubers is 1-3.The tubers are oblong to fusiform,relatively flat and long,with 2 ribbed prominences on the surface,and the joints between the tubers contract or break.Most single preserved tubers have sharp ends,while a few are more blunt.Equisetites arctatus sp.nov.composed of scabbard tooth,covered a layer of brown to black carbon film.Leaf at the bottomconnection into leaf sheath,strong connection degree,with six to eight pieces of long and thin blade tooth,tend to be from both sides to middle together;The bottom leaf sheath is wider.The fossil records of tuber of Equisetum from all over the world have been counted in this thesis.Through the analysis and discussion on the basis of records,we found that the morphology of the tuber is related to temperature,and would be close to round for improve the efficiency of obtain the nutriment to against the bad living environment or seasonal variation.The tuber of Equisetum reported in this thesis is oldest,so the morphology of this specimen maybe represent the original morphology of tuber of equisetum.In addition,as the oldest tuber of Equisetum in the fossil record,the morphological characteristics of the tuber reported in this paper may represent the relatively primitive morphology and early evolution of the tuber.We study the fossil wood collected from the late triassic Nanying’er Formation,It was found that 5-8 layers of parenchyma in the shape of irregular polygons existed in the edge area of the medullary part of the specimen,and 2-3 layers of tracheid were distributed outside the xylem in a concentric ring,which may be vascular tissue.These characteristics indicate that the specimen might be a fossil stem of Kodachida.In this paper,the mineral composition characteristics of this specimen were observed under a polarizing microscope.It was found that the mineralization process of this specimen can be divided into three stages,which are calcite with large euhedral crystal,small euhedral crystal and xenotopic crystal.After the analysis of its mineral composition and preservation status,this specimen indicates that the burial area at that time belongs to the humid condition with rich water source.The p H of the water body is high and the depositional environment has reducing action.The microorganism in the water body is dominated by anaerobic bacteria.Furthermore,we study the fossil wood collected from the Middle Jurassic Xinhe Formation,because of it’s poorly state of preservation,it is hard to determine to a genus,so it was just be classified in the family Taxodiaceae.In addition to the above studies,this paper studied the climate and temperature of the Late Triassic in Baojishan Basin through plant fossil records,and concluded that the area was temperate with warm and humid climate at that time.Ginkgoaleans and tubers of Euqisetum indicate that there were seasonal changes in climate at that time.The diversity of phytocoenosium reflecting different landform was high,mainly in upland,slope,lowland,wetland,understory and delta regions.lakes and rivers exist in the area.