Many reptiles share a variety of similar traits, including scaling skin, ectothermic metabolisms and egg-laying reproductive methods. However, when all living reptile species are considered, few traits are common to all. This is because the group scientists call “reptiles” includes not only turtles, lizards, snakes and alligators, but also birds and tuataras as well. Scientists use shared ancestry to define a group of animals not a collection of shared traits.
Scientists do not consider animals to form a natural group because they share a collection of traits. Accordingly, as birds descended from primitive reptiles, they are modern reptiles. While most reptiles have scale-covered skin on at least some portion of their body, birds have scales only on their feet. Additionally, while snakes, lizards, turtles, alligators and tuataras are all “cold-blooded” or ectothermic animals, birds are endothermic or “warm-blooded” animals. The majority of reptiles deposit eggs that hatch into young. However, some snakes and lizards give birth to live young.
Many of these similar traits, specifically scales, are an adaptation to living on dry land. Reptiles evolved from amphibian-like animals that lived in the water. As reptiles began colonizing the land, they had to evolve a method for preventing dehydration. Scales evolved to suit this need, as they are impervious to water.
Turtles are easily identified by their unique shells, which are actually built from their vertebrae and ribs. They are toothless with keratin-based beaks. Snakes and lizards constitute the majority of conventional reptile species and are typified by their flexible jaws with an ability to manipulate food items. Crocodilians include crocodiles, alligators, caimans and the gharial. These are large reptiles with elongated snouts and strong jaws. Their eyes and nostrils are located on top of their heads to aid them in their partially aquatic lifestyles.