Final answer:
Reptiles are distinguished from amphibians by their amniotic egg and terrestrial skeletal modifications. The amniotic egg allows development on land with its protective membranes, and specific adaptations in reptiles cater to a fully terrestrial lifestyle, unlike amphibians which need moist environments for reproduction.
Step-by-step explanation:
The key evolutionary novelties that distinguish reptiles from amphibians are the amniotic egg and skeletal modifications for terrestrial locomotion. The amniotic egg is significant as it allowed vertebrates to lay eggs on land without the risk of the eggs drying out, thanks to its extraembryonic membranes and occasionally a shell. These membranes, which include the yolk sac, amnion, chorion, and allantois, provide nutrients, gas exchange, and waste containment for the developing embryo.
Terrestrial adaptations such as thicker skin and rib ventilation of the lungs are other distinct features that amniotes, which include reptiles, birds, and mammals, have developed in comparison to amphibians. Amphibians, whose eggs lack a waterproof covering, are generally restricted to moist environments and underwent a two-stage development process that is different from the terrestrial life cycle of reptiles.