Although rarely seen in modern avian eggshells, secondary eggshell units are remarkable structures in non-avian dinosaur eggshells and provide paleobiological information. However, the origin of secondary eggshell units lacks an in-depth study, which has resulted in controversies in former studies. An abiogenic origin has also been suggested for the formation of secondary eggshell units. Here, we show that secondary eggshell units of non-avian dinosaur eggshells are biogenic structures using a combination of microscopic techniques such as electron backscatter diffraction (EBSD), and scanning electron microscope (SEM). The features of secondary eggshell units suggest that the formation mechanism of non-maniraptoran dinosaur eggshells is comparable to that of turtle and crocodile eggshells. Matrix fibers appear to be the initiators of secondary eggshell units. A weakening trend of secondary eggshell units toward non-avian maniraptoran dinosaurs supports that maniraptoran dinosaurs evolved a more organized physiological mechanism of eggshell formation.
 

dinosaur eggshell;,secondary eggshell unit;,biogenic origin,formation mechanism