Fig. 4

Microscopic characteristics of types 1 and 2 oncoids. a A type 1 oncoid from the Sandaogou section, characterized by concentric fine-laminar structure and clear nucleus (made up with trilobite debris and dark micrite); b Local magnification of a, showing the interlaced and intertwined calcified filamentous fossils of Girvanella; c A type 2 oncoid from the Diaoquan section, characterized by a concentric rough-laminar structure with lower frequency alternation than that in type 1; d Local magnification of c, showing the micrite conglomeration and rough edges of the oncoid. Girvanella fossils can be observed inside the dark micrite laminae with a disordered winding occurrence