A hypothetical cladogram of rigid eggshells evolution in sauropsids.

<p>The red branches represent the rigid calcified eggshells, the blue represent the “tuatara-styled” eggshells, and the black represents the derived squamate eggshell. The archosaur eggshells are characterized by the nucleation sites at their inner margin. The tuatara and soft gekkotan eggshells are morphologically similar to each other and characterized by the “stem-like” and “cap-like” structure in the inner and outer regions, respectively. The derived squamate eggshells have a shallow calcareous layer on the surface without calcite “stem-like” structure into the shell membrane. The eggshell microstructure of the Dibamidae is unknown.