Full
ZHOU Wenjun,born in 2002,is a master’s degree candidate. He is engaged in researches on basin structure and palaeogeography. E-mail: zwenjun94@163.com.
Outline
The formation of the “Conchostracan shale” layer at the top of the lower section of the Middle Jurassic Shaximiao Formation in the Sichuan Basin marks a major shift in the paleolake depositional environment and palaeogeography. This transition corresponds to the faunal succession of the Shulong-Emeilong fauna to the Mamenchisaurus fauna. Reconstructing the sedimentary and palaeogeographic transformation during this period is crucial for understanding the evolution of dinosaur faunas in the Sichuan Basin. Through the buried characteristics of Conchostracan fossil assemblages and whole-rock geochemical analyses,samples from the lower “Conchostracan Shale” layer,as well as the mudstone and siltstone samples from both the upper and lower sections of the Shaximiao Formation,were examined. The results indicate that Conchostracans thrived in warm,humid shallow lacustrine environments. A transition in sedimentary environments—from deltaic systems to shallow lacustrine,fluvial,and deltaic systems—occurred between the lower and upper sections of the Shaximiao Formation. Analysis of paleoclimate indices(CIA,CIW),redox indicators(δCe,V/Cr,U/Th,V/(V+Ni),V/Sc),and paleosalinity proxies(B/Ga,Sr/Ba)reveals a dynamic evolution of the paleoenvironment within the Shaximiao Formation. Palaeoclimatic conditions shifted from semi-humid and semi-arid to warm and humid,and later reverted back to semi-humid and semi-arid. Concurrently,the paleo-water environment experienced a transition from oxidizing and weakly reducing conditions to more reducing conditions,followed by a return to oxidizing and weakly reducing conditions. Paleosalinity remained predominantly freshwater throughout,with only localized,minor salinization observed in the upper sections. These findings suggest that during the late Middle Jurassic,the Sichuan Basin experienced significant precipitation,leading to the establishment of a warm and humid palaeoclimate and a reducing lake environment conducive to the extensive proliferation of Conchostracans. Subsequently,a shift toward drier climatic conditions likely drove the adaptive evolution of herbivorous dinosaur lineages,transitioning from the Shulong-Emeilong fauna to the Mamenchisaurus fauna.
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| 科 Family | 属数 Number of genus | 种数 Number of species | 占总种数比例 Percentage of total species (%) | 属 Genus | 种数 Number of species | 占总种数比例 Percentage of total species (%) |
|---|---|---|---|---|---|---|
| 鹅膏菌科Amanitaceae | 2 | 11 | 5.26 | 鹅膏菌属 Amanita | 10 | 4.78 |
| 小菇科 Mycenaceae | 2 | 12 | 5.74 | 丝盖伞属 Inocybe | 5 | 2.39 |
| 多孔菌科 Polyporaceae | 8 | 14 | 6.70 | 蜡蘑属 Laccaria | 5 | 2.39 |
| 红菇科 Russulaceae | 3 | 23 | 11.00 | 小皮伞属 Marasmius | 6 | 2.87 |
| 小菇属 Mycena | 11 | 5.26 | ||||
| 光柄菇属 Pluteus | 5 | 2.39 | ||||
| 红菇属 Russula | 17 | 8.13 | ||||
| 栓菌属 Trametes | 5 | 2.39 |
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