17'4 Reptiles

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17.4 Reptiles
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Objectives

• Discuss the evolutionary history of reptiles.
• Describe the characteristics and adaptations of
  reptiles.
• Identify the different groups of reptiles.

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• Lizards are related to the extinct dinosaurs.
• Both are vertebrates with scaly skin adapted to
  life on land.
• Today, lizards live in most temperate and warmer
  areas of the earth.
• Together with snakes, turtles, tortoises,
  alligators, and crocodiles, they make up the
  reptiles.

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History of Reptiles

• Scientists can learn many things about extinct
  animals by studying their fossil remains.
• For example, scientists can infer the kind of
  food an animal ate by the shape of its teeth.

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• The length and thickness of leg bones are clues
  about the body size of an animal.
• Even footprints can provide clues about an
  animal's movement and behavior.
• Using such evidence, scientists have assembled a
  history of reptiles.
• This history began when reptiles' amphibianlike
  ancestors developed adaptations for living their
  entire lives on land.

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• These adaptations included a spongy lung, a body
  covering that reduced water loss and eggs that
  could be laid on land.
• About 300 million years ago, these adaptations
  appeared in a group of ancestral reptiles.

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• These early reptiles evolved in many different
  directions.
• One path of their evolution resulted in the
  dinosaurs.
• But even before the dinosaurs evolved, the
  ancestors of modern turtles, lizards, and snakes
  had appeared.

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Characteristics of Reptiles

• Some people expect snakes to be slimy and
  slippery.
• However, if you touch a snake, you discover that
  the animal is not slimy at all.
• Unlike amphibians, whose outer skin is slimy,
  snakes and other reptiles have a dry, waterproof
  body covering made up of scales.

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• This covering helps these land-dwellers conserve
  water.
• Scales also prevent the exchange of oxygen and
  carbon dioxide across the skin.
• The evolution of a spongy lung, one with a
  greatly increased internal surface area, solved
  the problem of gas exchange on dry land.

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• The switch from skin to lung breathing required
  changes in circulation.
• Amphibians and reptiles have three-chambered
  hearts, which allow some mixing of fresh and
  stale blood.
• Mixing fresh and stale blood isn't suitable for
  the land-dwelling reptile.
• Therefore, reptiles have a partial wall dividing
  their hearts.

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• Reptiles reproduce entirely on land.
• As with other land-dwellers, the fragile egg must
  be fertilized inside the female body.
• When the egg is laid, a tough, leathery eggshell
  keeps the embryo from drying out.
• Special membranes transfer food from the egg
  yolk, remove wastes and take in oxygen.

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• Even though reptiles are ectotherms, most are
  very good at regulating their internal
  temperatures.
• They bask in the sun when the air is too cool and
  seek shade when it is too hot.
• Through these behavioral adaptations, many
  reptiles keep their body temperatures about as
  warm as yours for much of the day.

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Lizards and Snakes

• They may look very different, but lizards and
  snakes are closely related.
• All snakes and most lizards are carnivores.
• Snakes have a type of jaw hinge not found in
  other reptiles.
• It allows snakes to swallow their prey whole by
  increasing the size of their mouths.

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• A major difference between lizards and snakes is
  that snakes don't have legs.
• Snakes have developed ways of moving about
  without appendages.
• But a few snakes still have hipbones and remnants
  of hind legs.
• These are signs that they evolved from reptiles
  that did have legs.

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• Snakes have limited hearing and most have poor
  eyesight.
• They do, however,have keen senses of smell and
  taste.
• Snakes use their tongues to find prey and to
  gather other information about the environment.

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• A snake sticks out its tongue to sample the air
  or the ground.
• The tip of the tongue picks up chemicals.
• The snake then places its tongue into an organ in
  the roof of its mouth.
• This sense organ detects the odors in the air or
  on the ground.

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Turtles and Tortoises

• Members of another group of reptiles all have
  shells.
• They are the turtles and tortoises (TOR tuh
  suhs).
• These reptiles have hardly changed since they
  appeared hundreds of millions of years ago.

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• Their shells have proved to be important
  adaptations.
• When in danger, most turtles and tortoises can
  pull their heads, legs, and tails into their
  shells.

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• The shells of turtles and tortoises form from
  bony plates connected to their ribs and
  vertebrae.
• The shell is covered by a layer of tough
  material.
• This material is responsible for the markings and
  color patterns on the shell.

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• Turtles and tortoises differ in the structures of
  their shells.
• Most turtles have flat, streamlined shells.
• Tortoises have dome-shaped shells.

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• Turtles and tortoises also live in different
  environments.
• Tortoises are land animals.
• Most turtles live in ponds, lakes, rivers, or the
  ocean.
• Turtles that spend most of their lives in water
  must return to land to lay their eggs.

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Alligators and Crocodiles

• Among the reptiles, the closest living relatives
  of the dinosaurs are the alligators and
  crocodiles.
• They are lizardlike in shape and can grow as long
  as a small boat.
• Their backs have large, deep scales.

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• Alligators and crocodiles are very similar in
  appearance.
• However, they can be distinguished by the shapes
  of their heads.
• Crocodiles have a narrow head with a
  triangle-shaped snout.
• Alligators have a broad head with a rounded
  snout.

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• Both alligators and crocodiles live in tropical
  areas and spend most of their time in the water.
• Alligators live mostly in North America and Asia.
  
• Crocodiles inhabit tropical areas of America,
  Africa, Asia, and Australia.

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• Unlike most other reptiles, alligators and
  crocodiles care for their young.
• The female guards her eggs, and often, both male
  and female protect the babies.

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End