Fig. 7
a A type 5 oncoid, marked by an irregular shape with a clearly external enveloping micritic cortex and the nucleus; b Local magnification of a, showing the typical Girvanella fossils inside the dark micritic cortex. Some spherical cross-sections can be observed; c A type 6 oncoid, marked by a larger individual size, irregular shape and thin cortex. The local magnifications show the two arrangements of Girvanella fossils, which are parallel and intertwined