BIOSTRATIGRAPHY OF ORGANIC-WALLED MICROFOSSILS FROM THE TONIAN LIULAOBEI FORMATION IN THE HUAINAN REGION OF NORTH CHINA

Biostratigraphic subdivision and correlation of Tonian strata is hampered by the limited number of widely distributed and age diagnostic fossils. The acanthomorphic acritarch Trachyhystrichosphaera aimika, among a few other microfossil taxa, has emerged as a potential Tonian index fossil, but its temporal and spatial ranges have not been thoroughly documented. To improve our knowledge about Tonian microfossils and to specifically test the biostratigraphic significance of Trachyhystrichosphaera aimika, we carried out a micropaleontological investigation of the Tonian Liulaobei Formation in northern Anhui of North China. Our investigation using a low-manipulation maceration technique revealed a diverse assemblage of organic-walled microfossils dominated by sphaeromorphs and filaments, with relatively few acanthomorph taxa. Combined light and electron microscopic analysis recovered several complex organic-walled taxa, including Eotylotopalla? Grandis (a species characterized by a large vesicle decorated with hemispherical processes), Trachyhystrichosphaera aimika (an acanthomorphic species widely present in pre-Cryogenian Neoproterozoic rocks), T. botula (a species characterized by a sausage-like vesicle ornamented with irregularly distributed processes), and Pololeptus rugosus (a species characterized by an ellipsoidal vesicle with transverse annulations). The new data add to the growing diversity of Tonian fossils and strengthen the basis for improved biostratigraphic correlation of Tonian strata. Our data confirms Trachyhystrichosphaera and T. aimika as promising index fossils diagnostic of the Tonian period.