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.