Remarkable preservation of microfossils and biofilms in mesoproterozoic silicified bitumen concretions from Northern China

Prokaryotes, often generally referred to as “bacteria,” are the original and thus oldest life on Earth. They have shaped the chemical environment of the Earth, but they are difficult to find as ancient fossils due to their subtle structure. Here we report well-preserved fossilized microbial communities in silicified bitumen concretions from unit 3 of the Xiamaling Formation (1.39 Ga) in northern China. The numerous silicified bitumen concretions are in a variety of forms including ellipsoidal, spindle, and pancake ones, with diameters of 1~16 cm and thicknesses of 0.5~3 cm. The principal planes of the concretions are at low angle or directly parallel to the depositional plane level, showing obvious depositional characteristics. The concretions are silicified with abundant bitumen inside. Many different kinds of microbial fossils are found in the bitumen, including spherical forms, rods, and filaments, and some of the microbes are aggregated together in the forms of multicellular structures. These concretions preserve a delicate Mesoproterozoic biotic community.