Protists of the Upper Mesoproterozoic Ruyang Group in Shanxi Province, China

Organic-walled microfossils described in this paper have been obtained from the Beidajian Formation of the Mesoproterozoic Ruyang Group (ca. 1300 Ma) in southern Shanxi Province, China. They are characterized by many eukaryotic organisms, such as acanthomorphic acritarchs, sphaeromorphic acritarchs with a distinct circular excystment, acritarchs with multilayered walls and ornamentation, and multicellular filaments and algal thallose. Recently, more eukaryotic microfossils have been found and the triaromatic dinosteroid biomarker was extracted from the whole rock sample containing abundant acritarch specimens of Dictyosphaera delicata and Shuiyousphaeridium. One acritarch form, Spiromorpha segmentata, shows morphological similarities to the extant green alga Spirotaenia. It means that sexual reproduction by conjugation probably arose in the late Mesoproterozoic. genus Spiromorpha is proposed here. In combination with known Mesoproterozoic microfossil assemblages from the Roper Group of Australia and the Bahraich Group of Ganga Basin, India, we infer that marine protests had a rapid development during the Mesoproterozoic Era.