EVOLUTION OF THE SHELLED EGG
The evolution of the shelled egg (also known as the 'cleidoic' or 'closed' egg) was a significant evolutionary step, as it allowed eggs to be laid on land without drying out. Animals able to lay shelled eggs are called amniotes. As the eggs no longer need to be laid in water, it was also possible to eliminate the vulnerable 'tadpole' stage of life. This occurred in the Late Carboniferous, possibly in forms such as Hylonomus, a small, egg-laying reptile. With the shelled egg, land animals evolved into a wide range of primitive reptiles in the Carboniferous, such as Petrolacosaurus and the large predator Ophiacodon.