The Succession Of Marine Reptiles From The Middle To Upper Triassic Of Guizhou And Yunnan Provinces, Southwest China

The geological mutation between the Paleozoic and the Mesozoic, resulted in that the Paleozoic type marine ecological system ended, and that the new modern system, in which the active bethic and endogenetic creatures and the zoophagous animals flourished, was formed and improved among the Mesozoic-Cenozoic. The growth of marine reptiles among the Mesozoic and Cenozoic is the outstanding representative of the modern marine ecological system. Marine reptile is not a monophynetic taxon, but, includes various reptiles mainly lived in ocean. In evolutionary progress, marine reptiles not only change their body shape for water adaption, but develop different assemblage in dirrerent era. Some researchers got opinion that there excist four extinction events of marine reptiles in Mesozoic. The four extinction events happened in Middle-Late Triassic (Ladinian-Carnian), Latest Jurassic (Tithonian-Berriasion), early Late Cretaceous (Cenomanian-Turonian), Latest Cretaceous (Maastrichtian-Danian), respectively. Especially, the Middle-Late Triassic (Ladinian-Carnian) marine reptiles extinction event is biggest. More and more fossils of marine reptiles were reported, especially, very abundent fossils of marine reptiles appeared in South China, in the teen years. They are changing our ideas about the succession of marine reptiles in Mesozoic.Triassic is important period of marine reptiles evolution, because the animals origin from the Early Triassic Spathian, and get an unprecedented prosperity between the Middle Triassic and the Late Triassic. They are distributed in Tethys and East Pacific from the Middle to Late Triassic. Compared to other area of the world, not only oldest fossil record exists in the Triassic from South China, but also abundents materials appear in almost each stage from the Middle to Upper Triassic marine strata in southwest China. The continuity of Triassic marine reptiles record in South China is unique of the world. The Triassic marine reptiles materials of South China is very significant for studying the succession of marine reptiles in this period.The exinsting data indicates that the assemblage of marine reptiles is different from the Middle to Late Triassic. It is interesting that Eosauropterygia is in fashion in the Middle Triassic, but disappeared in the Late Triassic. Besides, aquatic protosaurs and mixosaurs origined in the Middle Triassic are not any more appeared in the Late Triassic. However, ichthyosaur Toretocnemidae and Merriamosauria, thalattosaur and placodont Cyamodontoidea expand in the Late Triassic. What reason results that the obvious succession from the Middle to Late Triassic. This scientific problem is the main aim in this thesis.The Middle Triassic Anisian marine reptiles fauna between Yunnan and Guizhou provinces is represented by Panxian Fauna from shoutwestern Guizhou and Luoping Fauna from eastern Yunnan. The above two faunas are yielded in same strata in the second member of the Middle Triassic Guanling Formation. The conodonts assemblage indicates that the age of the two faunas is the Middle Triassic Pelsonian. The two faunas are close each other in geographic distribution. The characteristics of marine reptils and fishes assemblage are very similar. All above evidences indicate that the two faunas should be a fauna - Panxian-Luoping Fauna. Zercon U-Pb LA-ICP-MS analysis from the vocanic tuff in the strata yielded marine reptiles gave 242.0±1.6Ma about Panxian-Luoping Fauna. This result confirms the age of Panxian-Luoping Fauna, furtherly.Xingyi Fauna was yielded in the laminated limestone from the lower Zhuganpo Formation. Former Xingyi Fauna was considered existing in Dingxiao area, Xingyi County, southwestern Guizhou Province. Recently, geological survey and fossils excavation showed that the marine reptiles fossils belonged to Xingyi Fauna were not only appeared but also more abundant in Fuyuan and Luoping counties of Yunnan Province and Wusha Village, Xingyi county of Guizhou Province. Some fossils clue was appeared in the area of Anlong County of Guizhou Province and Longlin County of Guangxi Province. It indicated that the range of Xingyi Fauna was much bigger than Panxian-Luoping Fauna. The age of Xingyi Fauna was controversial, because the assemblage of marine reptiles conflicted to that of conodont. The GSSP of the Upper Triassic Carnian stage gave the evidence for studying the age of Xingyi Fauna. The FAD of ammonoid Daxatina Canadensis is the sign of the boundary between the Ladinian and Carnian. The FAD of conodont Paragondolella polygnathiformis is reffered sign. The age of the Xingyi Fauna should be latest Middle Triassic Ladinian, due that the FAD of conodont Paragondolella polygnathiformis appeared at about three miters above the strata of the fauna. The Zercon SHRIMP U-Pb age is 240±2Ma from the vocanic tuff in the top of Xingyi Fauna. The Zercon SHRIMP U-Pb data indicated the age of Xingyi Fauna, furtherly.For analyzing the succession progress of marine reptiles between the Middle and Late Triassic, some new materials of sauropterygians and an enigmatic marine reptile were detailly described in this thesis. Based on the description, the phylogenetic relationships were erected and revised in marine reptiles. Atopodentatus unicus, from the second member of the Middle Triassic Guanling Formation, was reported. This animal was defined by very small head, intensive and fence-like teeth, vertically bending down rostrum, very high neural spine, and hoof-shaped distal phalange. Cladistic analysis indicated that Atopodentatus unicus had close relationship to Sauropterygia. Specialized characteristic indicated that the reptile may have been adapted to a way of bottom-filter feeding in water. It is obvious that such delicate teeth are not strong enough to catch prey, but were probably used as a barrier to filter microorganisms or benthic invertebrates such as sea worms. Two new genera and four species, including Paraplacodus luopingensis, Lariosaurus sanxiaensis, Fuyuansaurus huabiensis, and Qujingsaurus wangi, were erected. The phylogenetic relationships of sauropterygian ingroup were revised, because the above new taxa were added in the sauropterygian matrix. Diandongosaurus, Fuyuansaurus, Keichousaurus, Dianpachysaurus, Wumengosaurus, and Qianxisaurus, form a monophyletic group -Pachypleurosauria, together with pachypleurosaurs from San Monte Giorgio. Pachypleurosauria is sister-group with Nothosauridae.The succession progess of the Triassic marine reptiles of South China can be devided to three stages, including the Early Triassic Spathian-the Middle Triassic Anisian, the Middle Triassic Anisian-Ladinian, the latest Middle Triassic Ladinian-the Late Triassic Carnian. Hupehsuchia dies out before the Middle Triassic, however, other marine reptiles develop the most adaption diversity in the first stage. Ther diversity reduces, however, homogeneous taxa bloom in the second stage. The third stage is the most significant among the succession progress of marine reptiles in Mesozoic. Eosauropterygian and Placodontoidea with lower swimming ability die out before the Late Triassic, however, ichthyosaurs, which can swim to deeper water, bloom in this stage. The succession progress is similar between South China and other area. Except that thalattosaur appears earlier in North America and Europe, Placodontia, Pachypleurosauria, Nothosauridae, Pistosaurroidea, and scarce aquatic reptiles appears earlier in South China. Ichthyosaurs of the most successfully aquatic adapting almost origin at the time of the world. The above indicates that South China should be a main centre of the Triassic marine reptile origin.Macroelement, microelement, and sedimentary environment of the strata from the Middle to Upper Triassic indicate that the palaeogeographic pattern changes from the Middle Triassic to the Late Triassic Carnian, and that the water becomes deeper abruptly in the early Carnian. The environmentary change should result in that most small fishes, as the main food of marine reptiles, die out. These should be the main reason the assemblage of marine reptiles change between the Middle and Late Triassic. Food reduction should result that the marine reptiles near shore decrease. However, the taxa with better swimming ability not only can survive, but also occupy bigger ecological realm. The δ13C isotope strongly negative curve should result from that the lightweight carbon from the vocanic eruption is poured into ocean. The reptiles in land and some marine reptile near shore exist between the Middle and Late Triassic, so the vocanic event should be not the main reason of the succession of marine reptiles between the Middle and Late Triassic.