Reptiles have several notable adaptations which help them survive, including their ability to regulate their own body temperatures, their coverings of protective scales, their internal fertilization processes, amniotic eggs and in most cases claws for digging and defense. Some reptiles, like glass lizards and snakes, do not have limbs and thus are not effective diggers or burrowers.
Reptiles are limited only by their inability to withstand year-long cold. Even in climates with cold seasons lasting for months at a time reptiles can survive by burrowing or congregating to conserve warmth and then sleeping through the winter or entering a state of suspended animation to prevent substantial loss of nutrients.
Amniotic eggs are equipped with amniotic fluid, nutrient-rich goop which allows developing reptile hatchlings a good chance to grow strong before hatching. New-hatched crocodiles, snakes and lizards all have a substantial edge based on the energy they are able to extract and use from these amniotic eggs which reptiles have adapted to lay.
Scales are a strong defense mechanism which help reptile skin shrug off damage from predators and from the environment. Reptile are much less easily snagged or torn by environmental hazards like sharp rocks than are soft-skinned animals like mammals, making wounds and infection less likely in everyday life.