Final answer:
The development of the amniotic egg was the key adaptation that allowed reptiles to reproduce on dry land, by maintaining moisture and allowing gas exchange to protect the embryo. Along with scaly skin and efficient lungs, this adaptation contributed to the success of reptiles in terrestrial habitats.
Step-by-step explanation:
Reptile reproduction can occur on dry land due to a crucial adaptation known as the amniotic egg. This evolutionary leap was a significant development for the ancestors of today's reptiles because it allowed embryos to be protected from drying out. The amniotic egg is surrounded by membranes that facilitate gas exchange while retaining moisture, ensuring the embryo's survival on land. Furthermore, the shells of reptile eggs can be either hard or leathery, offering additional protection.
While lungs, scales, and limbs are also important adaptations that have enabled reptiles to thrive on land, it was the amniotic egg that truly freed them from the need to return to water for reproduction, setting them apart from their amphibian relatives. This adaptation, along with other developments like scaly skin containing keratin to reduce water loss and efficient lung respiration, allowed reptiles to fully exploit terrestrial environments.