REPTILES! San Diego Natural History Museum

What is a Reptile? . . . and what isn't?

Reptiles are wonderfully diverse in size, shape, color, and lifestyle. Reptiles have a long history; they have lived on Earth for over 300 million years. Reptiles are mysterious and often misunderstood; they are feared and revered.

The reptiles, or Class Reptilia, include turtles, crocodilians, tuataras, lizards and snakes. Birds, dinosaurs, and numerous extinct groups may also be classified as reptiles.

  • Reptiles are vertebrates -- they have an internal skeleton with a backbone.
  • Reptiles are tetrapods -- they have four legs or their ancestors had four legs. (Even snakes, whose legs have been lost, are tetrapods.)
  • Reptiles are amniotes -- they have eggs with a waterproof shell that contains amniotic fluid.
  • Reptiles evolved as land-dwelling creatures -- they have skin that protects them from drying out, and toes with claws. Some reptiles later adapted to life in the water; others took to the air.
  • Most reptiles are ectotherms or cold-blooded -- they use their environment to warm and cool their bodies.

Fish have scales and lay eggs. But, unlike reptiles, all fish live in the water and breathe through gills. The scales of fish and reptiles look very different and are of different composition. Fish eggs are without a shell and are deposited in water. Fish are not reptiles.

Amphibians have four legs, and often look like reptiles. But amphibians, such as salamanders and frogs, have moist skin that does not protect them from drying out, and toes without claws. The eggs of amphibians are gelatinous and have no shell. Amphibians usually go to water or very moist places to breed and lay their eggs. Amphibians are not reptiles.

A few mammals lay eggs and most have claws. Most mammals bear their young alive, but the egg-laying duck-billed platypus and spiny anteaters are exceptions. Mammals are different from reptiles in some obvious ways: they have hair or fur and produce milk from mammary glands to feed their young. Mammals are not reptiles.

Birds have scales, claws, and they lay eggs. Bird feathers are modified scales. Bird legs are covered with scales, and their toes have claws. Birds lay shelled eggs, and they are descendants of small, meat-eating dinosaurs. Scientists who classify animals on the basis of ancestry say: Birds are reptiles!

Reptiles! Real and Robotic