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19.4: Reptiles

Difficulty Level: At Grade Created by: CK-12
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Lesson Objectives

  • Give an overview of form and function in reptiles.
  • Describe the amniotic egg and reptile reproduction.
  • Identify the four living orders of reptiles
  • Summarize how reptiles evolved.
  • Describe where reptiles live and what they eat.


large, sheet-like muscle below the lungs that allows breathing to occur when it contracts and relaxes
ectothermic, tetrapod vertebrate that lays amniotic eggs; includes crocodiles, lizards, snakes, and turtles
type of early amniote that evolved during the Carboniferous Period and eventually gave rise to dinosaurs, reptiles, and birds
type of early amniote that evolved during the Carboniferous Period and eventually gave rise to mammals


Reptiles are a class of tetrapod vertebrates that produce amniotic eggs. They include crocodiles, alligators, lizards, snakes, and turtles. The reptile class is one of the largest classes of vertebrates. It consists of all amniotes except birds and mammals.

Structure and Function in Reptiles

Reptiles have several adaptations for living on dry land that amphibians lack. For example, as shown in Figure below, the skin of most reptiles is covered with scales. The scales are made of very tough keratin, and they protect reptiles from injury, and also prevent them from losing water.

Crocodile Scales. These crocodiles are covered with tough, waterproof scales.

Reptile Respiration

The scales of reptiles prevent them from absorbing oxygen through their skin, as amphibians can. Instead, reptiles breathe air only through their lungs. However, their lungs are more efficient than the lungs of amphibians, with more surface area for gas exchange. This is another important reptile adaptation for life on land.

Reptiles have various ways of moving air into and out of their lungs. Lizards and snakes use muscles of the chest wall for this purpose. These are the same muscles used for running, so lizards have to hold their breath when they run. Crocodiles and alligators have a large sheet of muscle below the lungs, called a diaphragm, that controls their breathing. This is a structure that is also found in mammals.

Ectothermy in Reptiles

Like amphibians, reptiles are ectotherms with a slow metabolic rate. Their metabolism doesn’t generate enough energy to keep their body temperature stable. Instead, reptiles regulate their body temperature through their behavior. For example, the crocodile in Figure below is soaking up heat from the environment by basking in the sun. Because of their ectothermy, reptiles can get by with as little as one tenth the food needed by endotherms such as mammals. Some species of reptiles can go several weeks between meals.

Heat Transfer to an Ectothermic Reptile. This crocodile is being warmed by the environment in three ways. Heat is radiating directly from the sun to the animal’s back. Heat is also being conducted to the animal from the rocks it rests on. In addition, convection currents are carrying warm air from surrounding rocks to the animal’s body.

Other Reptile Structures

Like amphibians, most reptiles have a heart with three chambers, although crocodiles and alligators have a four-chambered heart like birds and mammals. The reptile brain is also similar in size to the amphibian brain, taking into account overall body size. However, the parts of the reptile brain that control the senses and learned behavior are larger than in amphibians.

Most reptiles have good eyesight and a keen sense of smell. Snakes smell scents in the air using their forked tongue (see Figure below). This helps them locate prey. Some snakes have heat-sensing organs on their head that help them find endothermic prey, such as small mammals and birds.

Snake “Smelling” the Air. A snake flicks its tongue in and out to capture scent molecules in the air.

Reptile Reproduction

Most reptiles reproduce sexually and have internal fertilization. Males have one or two penises that pass sperm from their cloaca to the cloaca of a female. Fertilization occurs within the cloaca, and fertilized eggs leave the female’s body through the opening in the cloaca. In a minority of species, the eggs are retained inside the female’s body until they hatch. Then the offspring leave the mother’s body through the cloaca opening.

Amniotic Eggs

Unlike amphibians, reptiles produce amniotic eggs (see Figure below). The shell, membranes, and other structures of an amniotic egg protect and nourish the embryo. They keep the embryo moist and safe while it grows and develops. They also provide it with a rich, fatty food source (the yolk).

Amniotic Egg. The amniotic egg is an important adaptation in fully terrestrial vertebrates. It first evolved in reptiles. The shells of reptile eggs are either hard or leathery.

Reptile Young

Unlike amphibians, reptiles do not have a larval stage. Instead, newly hatched reptiles look like smaller versions of the adults. They are able to move about on their own, but they are vulnerable to predators. Even so, most reptile parents provide no care to their hatchlings. In fact, most reptiles don’t even take care of their eggs. For example, female sea turtles lay their eggs on a sandy beach and then return to the ocean. The only exceptions are female crocodiles and alligators. They may defend their nest from predators and help the hatchings reach the water. If the young remain in the area, the mother may continue to protect them for up to a year.

Classification of Reptiles

There are more than 8,200 living species of reptiles, with the majority being snakes or lizards. They are commonly placed in four different orders. The four orders are described in Table below.

Order Characteristics Example
Crocodilia: crocodiles, alligators, caimans, gharials They have four sprawling legs that can be used to gallop; they replace their teeth throughout life; they have strong jaws and a powerful bite; they have a more advanced brain and greater intelligence than other reptiles; they have a four-chambered heart.


Sphenodontia: tuataras They are the least specialized of all living reptiles; their brain is very similar to the amphibian brain; they have a three-chambered heart, but it is more primitive than the heart of other reptiles.


Squamata: lizards, snakes

Lizards: most have four legs for running or climbing, and they can also swim; many change color when threatened; they have a three-chambered heart.

Snakes: they do not have legs, although they evolved from a tetrapod ancestor; they have a very flexible jaw for swallowing large prey whole; some inject poison into their prey through fangs; they have a three-chambered heart.


Testudines: turtles, tortoises, terrapins They have four legs for walking; they have a hard shell covering most of their body; they have a three-chambered heart.


Evolution of Reptiles

The earliest amniotes evolved about 350 million years ago. They resembled small lizards, but they were not yet reptiles. Their amniotic eggs allowed them to move away from bodies of water and become larger. They soon became the most important land vertebrates.

Synapsids and Sauropsids

By about 320 million years ago, early amniotes had diverged into two groups, called synapsids and sauropsids. Synapsids were amniotes that eventually gave rise to mammals. Sauropsids were amniotes that evolved into reptiles, dinosaurs, and birds. The two groups of amniotes differed in their skulls. The earliest known reptile, pictured in Figure below dates back about 315 million years.

Earliest Reptile: Hylonomus. The earliest known reptile is given the genus name Hylonomus. It was about 20 to 30 centimeters (8 to 12 inches) long, lived in swamps, and ate insects and other small invertebrates.

At first, synapsids were more successful than sauropsids. They became the most common vertebrates on land. However, during the Permian mass extinction 245 million years ago, most synapsids went extinct. Their niches were taken over by sauropsids, which had been relatively unimportant until then. This is called the “Triassic takeover.”

Rise and Fall of the Dinosaurs

By the middle of the Triassic about 225 million years ago, sauropsids had evolved into dinosaurs. Dinosaurs became increasingly important throughout the rest of the Mesozoic Era, as they radiated to fill most terrestrial niches. This is why the Mesozoic Era is called the “Age of the Dinosaurs.” During the next mass extinction, which occurred at the end of the Mesozoic Era, all of the dinosaurs went extinct. Many other reptiles survived, however, and they eventually gave rise to modern reptiles.

Evolution of Modern Reptile Orders

Figure below shows a traditional phylogenetic tree of living reptiles. Based on this tree, some of the earliest reptiles to diverge were ancestors of turtles. The first turtle-like reptiles are thought to have evolved about 250 million years ago. Ancestral crocodilians evolved at least 220 million years ago. Tuataras may have diverged from squamates (snakes and lizards) not long after that. Finally, lizards and snakes went their separate ways about 150 million years ago.

Traditional Reptile Phylogenetic Tree. This phylogenetic tree is based on physical traits of living and fossil reptiles. Trees based on DNA comparisons may differ from the traditional tree and from each other, depending on the DNA sequences used. Reptile evolution is currently an area of intense research and constant revision.

Ecology of Reptiles

Today, reptiles live in a wide range of habitats. They can be found on every continent except Antarctica. Many turtles live in the ocean, while others live in freshwater or on land. Lizards are all terrestrial, but their habitats may range from deserts to rainforests, and from underground burrows to the tops of trees. Most snakes are terrestrial and live in a wide range of habitats, but some snakes are aquatic. Crocodilians live in and around swamps or bodies of freshwater or salt water.

Reptile Diets

What reptiles eat is also very diverse, but the majority of reptiles are carnivores. Large reptiles such as crocodilians are the top predators in their ecosystems, preying on birds, fish, deer, turtles, and sometimes domestic livestock. Their powerful jaws can crush bones and even turtle shells. Smaller reptiles—including tuataras, snakes, and many lizards—are also important predators, preying on insects, frogs, birds, and small mammals such as mice.

Most terrestrial turtles are herbivores. They graze on grasses, leaves, flowers, and fruits. Marine turtles and some species of lizards are omnivores, feeding on plants as well as insects, worms, amphibians, and small fish.

Reptiles at Risk

Many species of reptiles, especially marine reptiles, are at risk of extinction. Some are threatened by habitat loss. For example, many beaches where turtles lay their eggs have been taken over and developed by people. Other marine reptiles have been over-hunted by humans. Marine turtles and their eggs are still eaten in some countries despite being protected species.

Some reptiles are preyed upon by non-native species introduced by humans. For example, marine iguanas on the Galápagos Islands are threatened by dogs and cats that people have brought to the islands. The iguanas are slow and tame and have no adaptations to these new predators.

Lesson Summary

  • Reptiles are a class of ectothermic, tetrapod vertebrates. They have several adaptations for living on dry land, such as tough keratin scales and efficient lungs for breathing air. They also have a three-chambered heart and relatively well-developed brain.
  • Most reptiles reproduce sexually and have internal fertilization. Their eggs are amniotic, so they can be laid on land instead of in water. Reptiles do not have a larval stage, and their hatchlings are relatively mature. Reptile parents provide little if any care to their young.
  • There are more than 8,200 living species of reptiles, and they are placed in four orders: Crocodilia, which includes crocodiles and alligators; Sphenodontia, or tuataras; Squamata, which includes lizards and snakes; and Testudines, such as turtles and tortoises.
  • The earliest amniotes appeared about 350 million years ago, and the earliest reptiles evolved from a sauropsid ancestor by about 315 million years ago. Dinosaurs evolved around 225 million years ago and dominated animal life on land until 65 million years ago, when they all went extinct. Other reptiles survived and evolved into the classes of reptiles that exist today.
  • Reptiles can be found on every continent except Antarctica. They may live in terrestrial, freshwater, or marine habitats. Most reptiles are carnivores, and large reptiles are the top predators in their ecosystems. Many species of reptiles, especially marine reptiles, are at risk of extinction.

Lesson Review Questions


1. Describe reptile scales and the functions they serve.

2. What is a diaphragm? What does it do?

3. Describe two senses that reptiles may use to locate prey.

4. Outline the structure and function of an amniotic egg.

5. Identify amniotes called synapsids and sauropsids.

6. Give a brief overview of reptile evolution.

Apply Concepts

7. Pretend you are a reptile such as a lizard. Explain how you might stay warm on a cold day.

Think Critically

8. Compare and contrast crocodilians with other orders of reptiles.

9. Explain why reptiles were able to replace amphibians as the dominant land vertebrates.

Points to Consider

Birds evolved from a reptile ancestor but they are very different from reptiles today. Birds are also the most numerous tetrapod vertebrates.

  • What are some traits that differ in birds and modern reptiles?
  • What traits might explain why birds have been so successful?

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