Reptile egg fossil

Reptile egg fossils are the fossilized remains of eggs laid by reptiles. The fossil record of reptile eggs goes back at least as far as the Early Permian. However, since the earliest reptile eggs probably had soft shells with little preservation potential, reptilian eggs may go back significantly farther than their fossil record. Many ancient reptile groups are known from egg fossils including crocodilians, dinosaurs, and turtles. Some ancient reptiles are known to have given live birth and are therefore not anticipated to have left behind egg fossils.

The earliest reptile eggshells probably had leathery membranes instead of hard shells. Eggs like this decay so quickly that fossilization is very unlikely. Therefore the fossil record is too incomplete for scientists to determine what kinds of eggshell most fossil reptile groups had. A 5.9 cm by 3.79 cm fossil from the Lower Permian was described in 1939 by Alfred Romer and Lewellyn Price as the oldest hard-shelled fossil egg. However, in a 1979 paper Karl Hirsch disputed Romer and Price's claim, since the fossil in question didn't show evidence for a calcite shell. Hirsch found enough phosphorus in the object's outer layer to propose that the fossil was actually an egg with a leathery shell. If the contentious fossil really is a reptile egg, it's the oldest known.

The earliest known fossil rigid eggs were laid by South African crocodilians during the Late Triassic or Early Jurassic; however, the rigid eggshell itself was probably much older than these specific fossils.

The oldest known fossil crocodilian eggs are from Early Cretaceous rocks of Galve, Spain. These eggs had microstructures identical to those of modern crocodiles. This suggests that once the crocodilian's rigid eggshell first evolved it changed very little over time. Crocodilian eggshell is regarded as primitive because it only contains one type of calcite crystal and these crystals are arranged in "tight columns".

...
Wikipedia