Final answer:
Reptiles exhibit key adaptations for terrestrial life, most notably the development of scaly skin with keratin, which helps prevent water loss and is crucial for their fully terrestrial life cycle.
Step-by-step explanation:
One adaptation that first occurred in reptiles includes the development of scaly skin containing the protein keratin and waxy lipids, which helped to significantly reduce water loss from the skin, a critical factor for completely terrestrial living. Reptilian scales serve as a barrier against dehydration while providing protection from environmental hazards. This adaptation, along with the production of amniotic eggs with protective shells, enabled reptiles to occupy a variety of terrestrial habitats and reduced their dependence on water bodies for reproduction.
Further adaptations in reptiles to terrestrial life include their completely terrestrial life cycle, which is supported by lungs for air breathing since their skin is not suited for respiration like that of amphibians. Reptiles also excrete waste as uric acid paste to conserve water, and they have behavioral and physiological adaptations for thermoregulation, as most are ectothermic. These adaptations allowed reptiles to exploit diverse ecological niches and become one of the dominant groups of land vertebrates.