Cretaceous Period, 100-66 million years BP. A large fossil Charonosaurus sp. hadrosaur egg on matrix, retaining evidence of the original leathery surface. 4 kg, 18cm (7"). From a Lincolnshire, UK, collection. Dinosaur eggs are known from about 200 sites around the world, the majority in Asia and mostly in terrestrial (non-marine) rocks of the Cretaceous Period. It may be that thick calcite eggshells evolved during the Cretaceous (145 to 65 million years ago"). Most dinosaur eggs have one of two forms of eggshell that are distinct from the shells of related modern animal groups, such as turtles or birds; however, some eggs closely resemble the type of shells seen in present day ostrich eggs.