Simple SummaryMost previous studies on eggshell mechanics only focused on single species or taxa and often lacked a side-by-side comparison between eggshell microstructures, mineral content, and mechanical properties. However, those properties are essential to understanding amniotes’ ecology and evolution. Studies on non-avian reptilian eggshells are even more lacking than that of birds. In contrast to birds’ parental incubation, most reptilian eggs are hatched by environmental heat without parental care and are subjected to significant external disturbances. It is, however, still unclear whether reptilian eggshells have evolved different properties from birds due to their different incubation strategies. To fill this knowledge gap, we extend our previous work on bird eggs to cover various reptilian groups, including turtles, tortoises, crocodiles, and geckos, with 214 freshly laid eggs belonging to 16 species across three orders of Class Reptilia. This new result is compared to our previous work on birds and may shed new light on the correlation between eggshell properties and incubation strategies across a broader range of amniotic taxa (birds, crocodilians, turtles, tortoises, and geckos).AbstractWe analyze 214 freshly laid eggs belonging to 16 species across three orders of Class Reptilia. Using mechanical compression tests, we measure each egg’s absolute stiffness (K, unit: N m−1) and relative stiffness (Cnumber). The effective Young’s modulus,E, was obtained by combining experimental and numerical methods. The mineral (CaCO3) content was measured by acid–base titration, the microstructures by scanning electron microscopy (SEM), and the crystallography by electron backscatter diffraction (EBSD). We find that theCnumber of reptilian eggs is, on average, higher than that of bird eggs, indicating that reptilian eggs are stiffer with respect to the egg mass than birds. However, Young’s moduli of the reptilian eggshells (32.85 ± 3.48 GPa) are similar to those of avian eggshells (32.07 ± 5.95 GPa), even though those eggshells have different crystal forms, microstructures, and crystallography. Titration measurement shows that the reptilian eggshells are highly mineralized (>89% for nine Testudines species and 96% forCaiman crocodilus). Comparing the species with aragonite and calcite crystals, we find that calcite shells, including those of the Kwangsi gecko (inner part) and spectacled caiman (outer part), generally have larger grains than the aragonite ones. However, the grain size is not correlated to the effective Young’s modulus. Also, as measured by theCnumber, the aragonite shells are, on average, stiffer than the calcite ones (except for the Kwangsi gecko), primarily due to their thicker shells.