The Early Triassic Jurong Fish Fauna:Age,Composition,Depositional Environment And Taphonomy

After the most devastated biocrisis in the end of the Permian,marine ecosystems experienced a very long recovery process with a ladder model,and are believed not to be fully recovered until middle-late Anisian.The presence of some predatory faunas such as marine fishes,reptiles that form the top/high trophic functioning group of food web is an important mark indicating ecosystem's restoration following major extinctions.For instance,the Early Triassic fish faunas have often been regarded as index faunas marking marine ecosystem's restoration.In order to get the mechanism of death of fishes,it is necessary to give a detailed research on the taphonomy and depositional environment of fish fossils.Here,we report the known earliest fish faunas following the end-Permian nass extinction from the Lower Triassic of Jurong area of Jiangsu Province,South China.They are controversial in age.In the Qingshan section,seven genera of conodonts have been found,including Neospathodus dieneri in Bed 3 and 6,Novispathodus pingdingshanensis in Bed 16 and Scythogondolella milleri in Bed 9,which respectively represent strata age of Dienerian,Spathian and middle-late Smithian.The carbon isotope of Qingshan section can be compared to that of the early Triassic strata in West Pingdingshan section.Fish fossils layer is bedded in Bed 8,so their age might be middle-late Smithian by comprehensive analysis of conodont biostratigraphy and comparison of carbon isotope.Fishes are preserved in carbonate nodules with microstructures of scales,fins,interior skeletons and cartilages.There might be compact relationships between the exceptionally preservation and the process of nodules'formation.The Qingshan section is mainly composed of mid-thin bedded limestone,thin-bedded marlstone and mudstone.Vast horizontal bedding in marstone and mudstone indicates a low energy environment.Microanalysis reveals that abundant small pyrite framboids,3-6?m in diameter,are embeded in Bed 7-Bed 14 while almost none in Bed 6 and Bed 15-16,which obviously indicate an euxinic or anoxic redox condition from Bed 7 to Bed 14 while dysoxic or oxic condition in Bed 6 and Bed 15-16.Abundant microspheroids and small pyrite framboids are also present in host rocks near fish body,indicating that Sulfate-Reducing Bacteria?SRB?microbes might have participated in fish's burial process.In this paper,a model has been raised to simulate the process of nodules'formation.It is composed of five ordered part in the process:1)fish died and fall down to the bottom of water.2)The dead body attracted lots of bacteria,including SRB,then^-HCO₃ and H₂S were produced under the activities of bacterial.The reduction of sulpahte in sea water would also caused^-HCO_3.3)^-HCO₃ and H₂S respectively combine with Ca²⁺and Fe²⁺in sea water to compound CaCO₃ and FeS₂,thus to resulte in the carbonate nodules with fish body and lots of pyrite framboids inside.In addition,we authenticate more than 40 fish fossils from Jurong and give a detail report on six genera,which are Plesioperleidus jiangsuensis,Lepidotes jurongensis,Jurongia fusiformis,Suius brevis,Stensionotus dongchangensis and Qingshania cercida belonging to Perleididae and Paraseminotidae.