Amphibians transform into reptiles with a grand innovation: internal fertilization, which results in the "closed" egg. A new class of vertebrates spins off, generating the Great Age of the Reptiles. The reptiles' new reproductive strategy allows them to move inland to drier territories where they rapidly expand the vertebrate presence on land.
Egg architecture evolves. A biomineralized shell prevents fluids from evaporating and protects the growing embryo. Separate compartments for pantry and for waste are easily accessed by the embryo.