东秦岭北坡中元古代晚期微体生物群——一个早期生命的新窗口

LATE MESOPROTEROZOIC MICROFOSSIL ASSEMBLAGE ON NORTHERN SLOPE OF EASTERN QINLING MOUNTAINS, CHINA: A NEW WINDOW ON EARLY EUKARYOTES

山西南部永济地区位于东秦岭北坡 ,在该地区出露良好未变质的中—新元古代地层。在水幽剖面的中元古界汝阳群北大尖组中保存类型多样的微体化石 ,包括具刺疑源类 (Shuiyousphaeridium (Du)Yan ,emend .Yin ,1 997;TappaniaYin ,1 997) ,球形、舟形疑源类和多种带状、管状藻类化石。其大的膜壳 ,突起附属物 ,脱囊开口 ,以及同平面不规则分枝丝体等都显示了真核原生物的形态特征。线形和螺旋形微细管体和网状结构物首次见于部分带状丝体和膜状碎片 ,推测这些管状物是底栖藻类为适应干旱缺水环境而发育的输导或加固支撑的结构物。当前 ,以具刺疑源类Tappania为特征的相似微体化石组合相继在印度、澳大利亚南部中元古代地层中发现 ,揭示了约 1

Qinling Mountains extend in middle part of China. The eastern Qinling Paleoplate was ultimately formed by collision and spell of the North China Plate and the Yangtze Plate through a long course of development from Mesoproterozoic to early Calidonian orogeny (Jia et al ., 1988). Meso Neoproterozoic unmetamorphosed and slightly metamorphosed rocks underlain by igneous Xionger Xihe groups are in ascending order subdivided into the Ruyang Group, the Luoyukou Group and Sinian System (Guan et al ., 1988). Abundant sphaeromorphic acritarchs, acanthomorphic acritarchs Shuiyousphaeridium, Tappania, filamentous microfossils and some doubtful animal traces have been found from the Ruyang Group on eastern slope of Qinling Mountains (Guan et al ., 1980; Yan and Zhu, 1992; Yin, 1997; Hu et al ., 1991,1996; Yang and Zhou, 1995 ). Summarily, the microbiota of the Ruyang Group is characterized by the following morphological forms: 1) Large (normally 110 250 μm) acanthomorphic acritarchs, Shuiyousphaeridium (Yan, 1992)emend. Yin, 1997, bearing small polygonal ‘plates' vesicle and dissymmetrical processes (Plate I, figs.1,3,4,6,7,10; Plate II, fig.3). 2) Sphaeromorphic acritarchs with a large opening (normally over 1/2 diameter of vesicle; Plate I, fig.8; Plate II, fig.2). 3) Valeria lophostriata , bearing fine stripes on the surface of vesicle(Plate I, figs.2.5). 4) Remarkably morphological acanthomorphic acritarchs Tappania Yin, 1997, possessing one neck like extension and irregularly distributed processes (Plate II,figs.4,6,7,9,10; Plate III,figs.1,4,6 9). 5) Belt like filaments being with longitudinal stripes on the surface, in which some show irregular branches (Plate IV,figs.5,6) and some have small helical tubes (0.5 5 m) and network cells like ‘pseudoparenchyma'(Plate III, figs.2,3,5; Plate IV, figs.7,8). 6) Navifusa segmentatus Prasad and Asher, 2001(Plate I, figs.9,12), showing ellipse outline and having linea transversa on the surface. 7) A filamentous form is same as Archaeolata prima Hermann, 1989 to have longitudinal threads on the surface and irregular branches arised from a single plane (Plate IV, fig.2). In addition, some moulage specimens (?) have been discovered in the upper part shale of Beidajian Formation (Plate II, fig.8). Based on the known microfossil assemblage from Mesoproterozoic shales of the Ruyang Group in China, the Roper Group in Australia and the Bahraich Group in India, the Mesoproterozoic marine ecosystem could be reconstructed as follows: 1) Eukaryotic protists had already been enrolled in the marine environment. 2) Some morphologic structures, such as cytoskeletal architecture possibly characterized by Tappania (Javaux et al ., 2001), excystment opening shown in many sphaeromorphic acritarchs and small helical tubes in some filamentous specimens, indicate that the organisms had distinct advances in dynamic function , reproductive cells and possible adaptability for drought conditions during Mesoproterozoic epoch. 3) The fact that Tappania have been found in Meso Proterozoic rocks of China, India and Australia implies the remarkable acanthomorphic acritarchs are potentially index fossils for intercontinental correlation of Meso Proterozoic strata.