Abstract:

In the long Precambrian, there are several problems like the enigmas such as the growth mechanism of stromatolites, the origin of molartooth structures, the forming mechanism of dolomites and the biological ascription of algae megafossils. Megascopic algae are also nominated as the cellprothalli plant. Both the megascopic algae and the cell-prothalli plant are the temporary-pseudo denomination. Thus, studies on the algae megafossils are very important for the research on the origin and the systematic evolution of the metaphyte as well for stratigraphic correlation of the Precambrian strata. Algae megafossils refer to those fossils of the lowgrade plant that can be seen by the naked eyes. Because of the absence of modern analogy,  the confirming of the biological classified location and attribution of megascopic algae of the Precambrian is very difficult and is highly contentious, which results in a Precambrian enigma. Algae megafossils in beds of the leiolite limestones of the third member of Mesoproterozoic Gaoyuzhuang Formation at the Jixian section are particularly characterized by the three-dimensional fossils and are frequently parageneic with the molartooth structures whose origin is also highly contentious. These threedimensional fossils can be grouped into several genuses, i.e. the spheroidal or sub-spheoidal category of the Genus Chuaria, the ellipsoidal category of the Genus Shouhsienia, the beanpodshaped category of the Genus Phascolites and the ribbon-or vein-shaped category of the Genus Tawuia, and all of them make up an assemblage of the Chuaria Shouhsienia. These algae megafossils can still interpret the sprereoidal molartooth structure. Several features of algae megafossils, such as the older forming duration that is more than 1,500 Ma, the paragenesis with the molar-tooth structure, the regular shape of threedimensional fossils and the possibly celluar microstructure, show a strong difference from both microspars filling the molartooth cracks and carbonaceous compression. Thus, these megafossils of algae are not the molartooth structures. Further, both the silication in the central part and the calcification in the marginal part make these possible threedimensional algae megafossils become strong different from those common concretions, such as lime, chert, tuff concretions or gas-bubble structure. The forming age of algae megafossils from the Gaoyuzhuang Formation at the Jixian section is older than that of three-dimensional algae megafossils from the Little Dal Group in Canada reported by H. J. Hofmann in 1984, and these algae megafossils are possibly the oldest three-dimensional algae megafossils. Ultimately, the threedimensional algae megafossils in the molar-tooth limestone provide insights to ponder some enigmatic problems of the Precambrian.

Key words: three-dimensional fossil, cellular microstructure, megafossils of algae, Precambrian enigma