中元古代罕见的实体宏观藻类化石——来自天津蓟县中元古界高于庄组第三段非叠层石碳酸盐沉积序列中的新材料

天津蓟县剖面的中元古界高于庄组可以划分为4个段,其中第三段组成一个特别的非叠层石碳酸盐岩序列。在该非叠层石碳酸盐岩序列上部的隐晶质泥晶灰岩中,发育着还存在成因争议的臼齿状构造,与其伴生的是多种类型的实体宏观藻类化石。根据宏观形态特征,可以将这些宏观藻类化石暂时归为球状丘尔藻、椭球状寿县藻、带状塔乌藻和豆荚状荚藻、卵圆状卵圆藻、舌状舌藻等多种类型,以前二种居多,组成一个球状丘尔藻—椭球状寿县藻组合。这些宏观藻类化石,曾经被认为是臼齿状构造的一种而称为"球状臼齿状构造",或被简单地当作燧石结核、钙质结核等等。但是,由于其具有规则的宏观形态,其中的球形或次球形化石在常规岩石薄片下还显示出可能的原始多细胞组织分化和可能的细胞显微结构,与充填臼齿状构造的方解石微亮晶形成明显的区别,所以不应该将其定义为臼齿状构造,更不能解释为钙质结核和燧石结核,其以罕见的实体化石不同于多年来所描述的、以碳质压型化石的形式产出的宏观藻类化石。其规则多样的宏观形态所表明的多种化石类型,以及那些球状或次球状实体宏观藻类化石显示出可能的多细胞组织及相似的细胞显微结构,表明具明显的原始多细胞真核生物特征,这为探索前寒武纪真核生物的起源与演化提供了宝贵的实际材料。

Unusual Three-Dimensional Fossils of Megascopic Algae of the Mesoproterozoic: A Contribution from the Non-stromatolitic Carbonate Succession Making up the Third Member of Gaoyuzhuang Formation at the Jixian Section

Lots of unusual three-dimensional fossils of megascopic algae are fortunately found in the aphanitic micrites(similar to leiolite limestones)of a non-stromatolitic carbonate succession making up the third member of the Mesoproterozoic Gaoyuzhuang Formation at the Jixian section in North China.Petrologically,the Mesoproterozoic Gaoyuzhuang Formation at the Jixian section in North China can be subdivided into four members,and the third member is marked by a particular non-stromatolititic carbonate succession in the Precambrian.Lots of molar-tooth structures,which are frequently paragenetic with the possibly three-dimensional fossils of megascopic algae in beds of aphanitic micrites of the upper part of this non-stromatolitic carbonate succession.According to the macro-morphological features,these three-dimensional fossils of megascopic algae can be grouped into many categories,the spherical category of the Genus Chuaria,the ellipsoidal category of the Genus Shouhsienia,the podiform-like category of the Genus Phascolites and the tube-and ribbon-like category of the Genus Tawuia etc.All of them make up an assemblage of the Chuaria-Shouhsienia.Several features of these three-dimensional fossils of megascopic algae,i.e.the regularly megascopic morphological feature,the obvious differentiation of the possible multicellular tissue and the clear cellular microstructure under the petrographic microscope,show that these three-dimensional fossils of megascopic algae are strongly different from molar-tooth structures filled with the particular calcite microspars and also distinct from the common fossils of megascopic algae that have been described and always preserved as carbonaceous compressions.Further,three-dimensional fossils of megascopic algae can not be simply grouped into the lime concretion,the chert concretion or the tuff concretion.Although the taxonomic affinity of these three-dimensional fossils of megascopic algae remains uncertain,special microstructure and interesting phenomena under the petrographic microscope reflect that these three-dimensional fossils of megascopic algae from the Mesoproterozoic Gaoyuzhuang Formation might be the representatives of the eukaryotes in the Precambrian.Many categories reflected by variously and regularly megascopic morphological features,and the possibly obvious multicellular tissue and the clear cellular microstructure under the petrographic microscope for those spherical or sub-spherical three-dimensional fossils of megascopic algae,provide preciously actual materials for the further understanding on the origin and evolution of the Precambrian eukaryotes.