A study of the morphological and microstructural characteristics of helcionelloids from the Nanjiang area(north Sichuan,China)at the northern Yangtze Platform was undertaken,in order to understand their phylogenetic p...A study of the morphological and microstructural characteristics of helcionelloids from the Nanjiang area(north Sichuan,China)at the northern Yangtze Platform was undertaken,in order to understand their phylogenetic position.Helcionelloids fossils from the Kuanchuanpu Formation of the Changtanhe–Maolinzi section include 7 genera and 11 species.There are distinct differences in the degree of shell bending,the apical morphology and apex position,the outline of the aperture and the height of the shell,all of which constitute important identification features.Although helcionelloid apices exhibit varying degrees of rotation towards the aperture,they do not show any signs of evident distortion.A pair of basally-symmetrical muscle scars are discovered on the shells of helcionelloids(such as Bemella simplex),showing a distinct difference when compared with those of the stem-group gastropod(e.g.,Pelagiella).The prismatic shell layer of helcionelloids has been observed,which are common components in Cambrian molluscs,but differ significantly from the dense layer microstructure found in contemporary molluscs.The microstructures,muscle scars and morphological characteristics demonstrate that helcionelloids represent a stem-group of molluscs.展开更多
The Yidun Island Arc in the Three Rivers (Jinsha River, Lancang River, Nujiang River) region of southwestern China is one of the most important Kuroko-type volcanogenic massive sulfide deposits (VMS) in China. Intra-a...The Yidun Island Arc in the Three Rivers (Jinsha River, Lancang River, Nujiang River) region of southwestern China is one of the most important Kuroko-type volcanogenic massive sulfide deposits (VMS) in China. Intra-arc rifting of Yidun Island occurred during the Late Carnian-Norian when VMS deposits such as the Gacun Pb-Zn-Cu deposit were formed. A bivalve fauna was found in fine-grained tuffaceous slate and in mineralized tuffaceous siltstone containing very high contents of Pb (45.01-103.37 ppm) and Zn (135.78-300.03 ppm) of the upper Tumugou Formation in the Changtai-Gacun volcanic-sedimentary rift basin. Stratigraphically, the bivalve-bearing beds are equivalents of the Gacun Pb-Zn-Cu deposits. The diversity of this bivalve fauna is very low. It consists mainly of the thin-shelled, epibyssate suspension-feeding bivalves Pergamidia eumenea and Parapergamidia changtaiensis, the burrowing large, elongated, suspension-feeding Trigonodus keuperinus and Unionites? sp., and occasional specimens of the endobyssate suspension-feeding Trigonodus? sp. and the deep burrowing suspension-feeding Pleuromya markiamensis. Individuals of the first four taxa are so abundant that the specimens are sometimes concentrated in shell beds, probably indicating a gregarious habit. This bivalve fauna is associated with internal moulds of cylindrical, slightly conical tubes most likely produced by a worm-shaped organism. Composition, morphology, diversity, and high abundance of this fauna, chemical features of the surrounding sediment, and the tectonic setting all suggest that this bivalve fauna lived in a deep-water environment in or around a hydrothermal vent system.展开更多
基金funded by the National Natural Science Foundation of China(Grant Nos.41872007,41972026)the Sichuan Science and Technology Program(2018JY0491)。
文摘A study of the morphological and microstructural characteristics of helcionelloids from the Nanjiang area(north Sichuan,China)at the northern Yangtze Platform was undertaken,in order to understand their phylogenetic position.Helcionelloids fossils from the Kuanchuanpu Formation of the Changtanhe–Maolinzi section include 7 genera and 11 species.There are distinct differences in the degree of shell bending,the apical morphology and apex position,the outline of the aperture and the height of the shell,all of which constitute important identification features.Although helcionelloid apices exhibit varying degrees of rotation towards the aperture,they do not show any signs of evident distortion.A pair of basally-symmetrical muscle scars are discovered on the shells of helcionelloids(such as Bemella simplex),showing a distinct difference when compared with those of the stem-group gastropod(e.g.,Pelagiella).The prismatic shell layer of helcionelloids has been observed,which are common components in Cambrian molluscs,but differ significantly from the dense layer microstructure found in contemporary molluscs.The microstructures,muscle scars and morphological characteristics demonstrate that helcionelloids represent a stem-group of molluscs.
基金supported by National Science Foundation of China (Grant Nos. 40372014, 40172011, 40472013, 40743016 and 41173058)the fund of Cross Century Talent of Ministry of Education of China, the Distinguished Young Scholar grant of Sichuan Province and China Scholarship Council
文摘The Yidun Island Arc in the Three Rivers (Jinsha River, Lancang River, Nujiang River) region of southwestern China is one of the most important Kuroko-type volcanogenic massive sulfide deposits (VMS) in China. Intra-arc rifting of Yidun Island occurred during the Late Carnian-Norian when VMS deposits such as the Gacun Pb-Zn-Cu deposit were formed. A bivalve fauna was found in fine-grained tuffaceous slate and in mineralized tuffaceous siltstone containing very high contents of Pb (45.01-103.37 ppm) and Zn (135.78-300.03 ppm) of the upper Tumugou Formation in the Changtai-Gacun volcanic-sedimentary rift basin. Stratigraphically, the bivalve-bearing beds are equivalents of the Gacun Pb-Zn-Cu deposits. The diversity of this bivalve fauna is very low. It consists mainly of the thin-shelled, epibyssate suspension-feeding bivalves Pergamidia eumenea and Parapergamidia changtaiensis, the burrowing large, elongated, suspension-feeding Trigonodus keuperinus and Unionites? sp., and occasional specimens of the endobyssate suspension-feeding Trigonodus? sp. and the deep burrowing suspension-feeding Pleuromya markiamensis. Individuals of the first four taxa are so abundant that the specimens are sometimes concentrated in shell beds, probably indicating a gregarious habit. This bivalve fauna is associated with internal moulds of cylindrical, slightly conical tubes most likely produced by a worm-shaped organism. Composition, morphology, diversity, and high abundance of this fauna, chemical features of the surrounding sediment, and the tectonic setting all suggest that this bivalve fauna lived in a deep-water environment in or around a hydrothermal vent system.
