Abstract:

The Mesoproterozoic Gaoyuzhuang Formation at the Jixian section in Tianjin makes up a stromatolitic and  non-stromatolitic carbonate succession, in which the third member is a particular sedimentary succession of the non-stromatolitic carbonate. Various ribbon-shaped molar-tooth structures are developed in the leionite limestones (aphanitic limestones) of the upper part of the succession of the non-stromatolitic carbonate, and lots of spherical or sub-spherical bodies with the sizes from several millimeters to more than one centimeter are frequently paragenetic with the molar-tooth structures. These spherical or sub-spherical bodies can be interpreted as the spherical molar-tooth structure, the lime concretion, the chert concretion, or the tuff concretion. Preliminary observation under the common microscope shows that there is a differentiation of primitive multicellular tissues within these spherical or sub-spherical bodies, i.e. the possible meristem in the central part and the possible mature tissue like the parenchyma. And some possible mature cells indicate the obvious microstructure. These features of the microstructures of the spherical or sub-spherical bodies show the strong difference from the molar-tooth structure filling with the special substance, i.e. the calcite microspar. Thus, these spherical or sub-spherical bodies should be grouped into the fossils of megascopic algae such as the Chuaia or the Parachuaria but not the spherical molar-tooth structure. Furthermore, most fossils of megascopic algae that had been found and described are mainly preserved as the fashion of carbonaceous compressions in fine deposits such as mudstones or shales, but fossils of megascopic algae in the leiolite limestones of the third member of the Gaoyuzhuang Formation at the Jixian section are characterized by three-dimensional fossils. Other features such as the older geological time that is more than 1,400 Ma, the obvious but primitive differentiation of the multicellular tissue and the  possibly  cellular microstructure, endow these fossils of megascopic algae with the biologically eukaryote attributes, which provides an unusual material and is meaningful for the further research on the evolution and origin of the eucargote.

Key words:  three-dimensional fossil, fossil of megascopic algae, multicellular tissue, molar-tooth structure, Mesoproterozoic, Jixian section in Tianjin