Big Picture

Reptiles are four-legged vertebrates that lay amniotic eggs. They are ectothermic and have many adaptations for living on land such as tough skin and efficient lungs. Reptiles reproduce sexually and have internal fertilization, and their amniotic eggs have hard shells so that they can be laid on land. Reptiles are placed in four orders, and all are descendants of Sauropsida.

Key Terms

Reptile: Tetrapod vertebrate that produces amniotic eggs.

Tetrapod: Vertebrate with 4 limbs.

Keratin: A tough protein in the skin, feathers, nails, and other parts of tetrapod vertebrates.

Diaphragm: Muscle under lungs that control breathing.

Ectothermy: Body temperature is controlled from the outside through behavioral changes such as basking in the sun.

Cloaca: Body cavity that stores waste/gametes and has an opening that allows them to leave the body.

Amniote: Animal that produces eggs with internal membranes that allow gases but not water to pass through so the embryo can breathe without drying out (reptile, bird, or mammal).

Sauropsid: Early amniote that evolved into reptiles

Synapsid: Early amniote that evolved into mammals.

Structure & Function

Reptiles have more adaptations for living on dry land than amphibians do.

  • Skin has very tough keratin; protects reptiles and prevents water loss
  • Breathe through lungs only
  • Breathing is controlled by diaphragm
  • Ectothermic
  • Can absorb heat energy from the environment, so they don’t need as much food
  • Have hearts with 3 or 4 chambers and good sense of eyesight and smell


  • Most reptiles reproduce sexually, fertilization occurs internally
  • Males pass sperm from their cloacoa into females
  • Female release fertilized eggs through the clocoa and usually don’t take care of the eggs or the hatchlings
  • Eggs have hard shells and contain food to protect and nourish embryos
  • Baby reptiles look like small versions of adults - no larval stage


4 sprawling legs used to gallop; teeth are replaced throughout life; strong jaws; more advanced brain and greater intelligence than other reptiles; 4-chambered heart
crocodiles, alligators, caimans, gharials
Least specialized of all reptiles; brain very similar to amphibian brain; 3-chambered heart that’s more primitive than the hearts of other rep-tiles.
Squamata (lizards, snakes)
Lizards: 4 legs for running/climbing; can swim; change color when threatened; 3-chambered heart. Snakes: no legs; flexible jaw for swallowing prey whole; some inject poison into their prey through fangs; 3-chambered heart.
lizards, snakes
4 legs for walking; hard shell covering most of their body; 3-chambered heart
turtles, tortoises, terrapins


Reptiles Cont.

Evolution & Ecology


2 types of early amniotes: synapsids and sauropsids

  • Sauropsids evolved into reptiles(315 mya)
  • After Mesozoic Era, dinosaurs went extinct, but surviving reptiles evolved into modern reptiles


  • Reptiles live all around the world
  • Most reptiles are carnivores, but some turtles are herbivores