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

Light microscopy reveals multiple layers of eggshell, compacted and disturbed by sediments. The two-dimensionally preserved egg reveals between four and six layers in cross-section (a). Parts of the cuticle (cu, dark brown layer) and shell membrane (sm) are also preserved (b). A close-up of the shell membrane (pink box) shows some fibrils (pink arrows), that are proteinaceous in living birds, but in this case might be at least partially mineralized. The eggshell layers are highly diagenetically altered (c, d). A four-layered area with a mirror-image pattern most likely represents an abnormal double-layered egg (d, e) that displays the tripartite ornithoid microstructure typically found in avian eggshells: a mammillary layer (ml) with organic cores (white arrows), a prismatic layer (pl), and an external layer (el). However, this mirror-image pattern (e) could also be the product of sediment displacement and lithostatic compaction, as exemplified in a second ground section (f, g). In this section (g), sediment displacement and shear have also partially created a mirror-image pattern, mimicking an abnormal multi-layered eggshell (f). Scale bar is 500 μm in a and g; 50 μm in b, 100 μm in c, d and f; and 20 μm in e