Why did reptiles become the first typically terrestrial vertebrates?

Reptiles are different from amphibians because, if necessary, they can live their lives away from water. They have waterproof skins, so if the weather gets very hot, the water inside their bodies doesn’t evaporate, reducing the chances of dehydration. They were also the first creatures to lay eggs on land. They were surrounded by rigid, waterproof shells that contained a portable ocean in the form of a liquid membrane to protect the embryo and provide it with all the food it needed until it was old enough to hatch and survive on its own in the air.