Answer
The major development that allowed reptiles to move inland was the evolution of amniotic eggs during the Carboniferous period, around 310 million years ago. Amniotic eggs have a protective, waterproof membrane that allows them to be laid on land, enabling reptiles to reproduce away from water. This adaptation freed reptiles from the dependence on aquatic environments for reproduction, allowing them to colonize diverse terrestrial habitats and become successful land-dwelling animals.
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