First record of organic-walled microfossils from the Tonian Shiwangzhuang Formation of the Tumen Group in western Shandong, North China

Organic-walled microfossils offer an important window onto the biospheric evolution in pre-Cryogenian and provide biostratigraphic implications for many Proterozoic fossiliferous sequences which are poorly age constrained for the lack of reliable radiometric date. Recently, macroscopic carbonaceous compression fossils have been reported for the first time from the Tonian Shiwangzhuang Formation of the Tumen Group in western Shandong, North China. However, organic-walled microfossils have never been discovered in the Shiwangzhuang Formation before. To improve our knowledge about Proterozoic biodiversity in North China, we conducted a micropaleontological survey on the argillaceous limestone samples of the Shiwangzhuang Formation, which also contain macroscopic carbonaceous compression fossils, from the Baishicun section in Anqiu, Western Shandong, North China. Our investigation shows that the Shiwangzhuang microfossil assemblage is dominated by smooth-walled sphaeromorphic acritarchs and cyanobacterium-like filaments and relatively low abundance of other acritarchs, including 15 taxa, such as Polysphaeroides filliformis, Ostiana microcystis, Simia annulare, ?Jacutianema sp., Pellicularia tenera, Polythrichoides lineatus, and Navifusa actinomorpha. The Shiwangzhuang organic-walled microfossil assemblage, although consisting of long-ranging taxa and without age diagnostic taxa, is not contradictary with a Tonian age suggested by macroscopic carbonaceous compression fossils, including the Chuaria-Tawuia and Sinosabellidites-Protoarenicola-Pararenicola assemblages, revealed from the same horizon of the Shiwangzhuang Formation and organic-walled microfossil assemblage, including the late Mesoproterozoic to Tonian index fossils Trachyhystrichosphaera aimika, from the underlying Tongjiazhuang Formation. However, it is also worth noting that a Cryogenian or Ediacaran age cannot be completely excluded based just on the microfossils because of their limited biostratigraphic utility.

Methods
Specimens were measured by ImageJ 1.45.