https://www.nature.com/articles/s41467-019-09259-x

SEM images showing the cuticle, eggshell, and shell membrane of the egg of Avimaia. Eggshell, cuticle, and shell membrane in cross-section (a). One fibril from the shell membrane is clearly visible (pink arrow). Close-up of the eggshell (b) and parts of the overlying cuticle and shell membrane in cross-section (b). Close-up of the cuticle of IVPP V25371, showing nanostructures of calcium phosphate (c), an adaptation found in modern birds, such as the Japanese quail, Coturnix japonica (d), to protect the embryo in infection-prone environments. It also represents the hypothesized ancestral morphological condition for avian cuticles. Abbreviations: same as Fig. 2. Scale bar is 50 μm in a, 20 μm in b, 3 μm in c, and 2 μm in d