Reptiles have always been a large group of fauna and were the true terrestrial quadrupeds. They laid eggs and hatched them on land. Reptiles first appeared in the late Palaeozoic and Mesozoic and flourished throughout the land, air, seas, rivers, lakes and swamps, but they declined sharply at the end of the Mesozoic. Only a few have survived to the present. Reptiles are mysterious due to their strange forms. Other than turtles, complete fossils of other reptiles are difficult to obtain, so complete sets have high ornamental value. Fossils of turtles, lizards, alligators, pterosaurs and dinosaurs are important fossil types.