Sergeev V N, Knoll A H, Grotzinger J P
Geological Institute, Russian Academy of Sciences, Moscow.
J Paleontol. 1995 Jan;69(1 Pt 2):1-37.
Silicified peritidal carbonates of the Mesoproterozoic Kotuikan and Yusmastakh Formations, Anabar Uplift, northeastern Siberia, contain exceptionally well-preserved microfossils. The assemblage is dominated by ellipsoidal akinetes of nostocalean cyanobacteria (Archaeoellipsoides) and problematic spheroidal unicells (Myxococcoides); both are allochthonous and presumably planktonic. The assemblage also includes distinctive mat-forming scytonematacean and entophysalidacean cyanobacteria, diverse short trichomes interpreted as cyanobacterial hormogonia or germinated akinetes, rare longer trichomes, and several types of colonial unicells. Although many taxa in the Kotuikan-Yusmastakh assemblage are long-ranging prokaryotes, the overall character of the assemblage is distinctly Mesoproterozoic, with its major features shared by broadly coeval floras from Canada, China, India, and elsewhere in Siberia. Microfossils also occur in middle to inner shelf shales of the Ust'-Il'ya and lower Kotuikan Formations. Leiosphaerid acritarchs (up to several hundred microns in diameter) characterize this facies. As in other Mesoproterozoic acritarch assemblages, acanthomorphic and other complex forms that typify Neoproterozoic assemblages are absent. The combination in Billyakh assemblages of exceptional preservation and low eukaryotic diversity supports the hypothesis that nucleated organisms diversified markedly near the Mesoproterozoic-Neoproterozoic boundary. The assemblages also demonstrate the antiquity of cyanobacteria capable of cell differentiation and suggest the importance of both changing peritidal substrates and evolving eukaryotes in determining stratigraphic patterns of Proterozoic prokaryotes. The permineralized assemblage contains 33 species belonging to 17 genera. Ten new species or new combinations are proposed: Archaeoellipsoides costatus n. sp., A. elongatus n. comb., A. dolichos n. comb., A. minor n. nom., A. crassus n. comb., A. major n. comb., A. bactroformis n. sp., Veteronostocale medium n. sp., Filiconstrictosus cephalon n. sp., and Partitiofilum yakschinii n. sp.
西伯利亚东北部阿纳巴尔隆起中元古代科图伊坎组和尤斯马斯塔赫组的硅化潮间带碳酸盐岩中保存着异常完好的微化石。该组合以 nostocalean 蓝细菌（古椭球体）的椭圆形厚壁孢子和有问题的球形单细胞生物（粘球藻属）为主；两者均为异地来源，推测为浮游生物。该组合还包括独特的形成席状的伪枝藻科和内共生藻科蓝细菌、被解释为蓝细菌藻殖段或萌发厚壁孢子的各种短丝状体、罕见的较长丝状体以及几种类型的群体单细胞生物。尽管科图伊坎 - 尤斯马斯塔赫组合中的许多分类单元是分布广泛的原核生物，但该组合的总体特征明显是中元古代的，其主要特征与来自加拿大、中国、印度和西伯利亚其他地区的大致同期植物群相同。微化石也出现在乌斯季 - 伊利亚组和下科图伊坎组的中到内架页岩中。光面球藻疑源类（直径可达数百微米）是该相的特征。与其他中元古代疑源类组合一样，缺少新元古代组合中典型的棘球形态和其他复杂形态。比利阿克组合中保存异常完好和真核生物多样性低的特点支持了这样一种假说，即有核生物在中元古代 - 新元古代边界附近显著多样化。这些组合还证明了能够进行细胞分化的蓝细菌的古老性，并表明在确定元古代原核生物地层模式方面，潮间带基质的变化和真核生物的演化都很重要。该矿化组合包含 17 属 33 种。提出了 10 个新种或新组合：肋状古椭球体新种、细长古椭球体新组合、长古椭球体新组合、微小古椭球体新名称、粗壮古椭球体新组合、大型古椭球体新组合、杆菌形古椭球体新种、中型古原 Nostocale 新种、头状硅缢缩体新种和雅克斯奇尼分区藻新种。
