Enveloping Eggs

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.

Top-right: Reptile relations — lizards, snakes, crocodiles, pterosaurs and birds — later take advantage of the egg innovation. Communication systems emerge that become critical to chicks' survival. Unhatched chicks hear one another chirping and clicking, which somehow enables the baby birds to hatch at the same time. While still in the incubating egg, they recognize parental alarm calls in such a way that they hush-up until they receive a parental all-clear. (Photograph by Patrick O'Connor, coutesy New England Science Center)

Bottom: This herbivorous reptile, just under two meters in length, represents the transition from amphibian to reptile. (Illustration by Zdenek Burian, © Jiri Hochman and Martin Hochman)

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