GIRVANELLAFOSSILS AND THEIR MICROSTRUCTURE FROM CAMBRIAN MICROBIALITES OF WESTERN HENAN

Girvanella is a kind of cyanobacterial fossil with interconnected cell row and enveloping polysaccharide mucilaginous sheaths and is the most common calcified microbes in Cambrian microbialites of western Henan． Girvanella filaments display different distributions in different microbialites． The filaments in stromatolite of intertidal flat environment mainly occur in dark laminations and display as a regular horizontal distribution．The oncoids in low-energy coastal oolitic inter-shoal or back-shoal environments are dominated by irregular shape and cortex stru-ctures．Girvanella filaments are mainly irregular densely winding in nuclei and dark cortex of the oncoids．The oncoids in high-energy oolitic shoal environments have a round shape and regular cortex structures in which Girvanellafilaments are distributed in the dark cortex of the oncoids along the tangent direction． The thrombolites were formed in oolitic inter-shoal of restricted platform environments in which Girvanella filaments are mainly densely winding，being intertwined with each other with irregular distribution in the clots．The giant ooids were preserved in oolitic shoal of open platform environments in which Girvanella filaments appear as densely winding in inner cortex and as the tangent direction in the outer cortex．Each individual tube of Girvanellafilament display similar features although Girvanellafilaments have the different distributions in different micro-bialites．Each individual tube consists of a dark grey tube wall(sheath)and bright tube core(cell row)under polarizing microscope．Viewed under electronic microscopy，the tube wall of Girvanella consists of radial-arrayed granular or short columnar calcites which have a diameter of 1-2μm and are considered to be biominerals induced by photosynthesis of Girvanella．The tube core is made up of cylindrical calcites with the diameter of 10-20μm and the variation length of 50-200μm，which are the products of recrystallization during diagenesis． According to the microstructure of Girvanella in combination to the features of modern tubular cyanobacteria，Girvanellais made up of short rod cells instead of coccoid cells．Its microstructure includes strands of short rod cells attached end-to-end，cell walls and extracellular sheaths(EPS)．

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