Tetrapod: Classes, Characteristics, Examples & Evolution


Class Tetrapod (Tetrapoda) has four pairs of limbs. These include:

  1. Class Amphibian
  2. Class Reptiles
  3. Aves (Birds) Class
  4. Class Mammals

1. Class Amphibia (Bony Tetrapod)

The amphibians show the following characteristics:

  1. Skeleton is mostly bony. Different amphibians have different body forms. The tail may or may not be present.
  2. They are tetrapods and have four limbs. Some are limbless (e.g. caecilians). Mostly they have webbed feet.
  3. Skin is smooth and moist. It has many glands. Some glands are poisonous. Pigment cells (chromatophores) are present in the skin. Scales are absent.
  4. Respiration takes place by gills in larvae and by lungs and skin in adults.
  5. The heart is 3-chambered. It has two atria and one ventricle. Sinus venosus and truncus arteriosus are present. There is double circulation in the heart.
  6. Sexes are separate. Fertilization is external. Larval stages represent.
  7. The larva changes into adulthood by metamorphosis. Amphibians are Anamniotes.
  8. Amphibians are cold-blooded (Poikilothermic) animals. They hibernate in winter.

Examples: Frog, Toad, and salamander.

2. Class Reptilia

Reptiles are completely adapted to land. There are certain terrestrial adaptations in reptiles. These adaptations are not found in amphibians. Reptile shows the following features:

  1. They have developed some copulatory organs for internal development.
  2. The shell of the egg is leathery. It resists dryness and prevents injury. They have large yolky eggs.
  3. Reptiles have dry scaly skin. It is an adaptation for terrestrial life.
  4. Reptiles develop protective embryonic membranes amnion (around embryo) allantois (store waste) and Chorion (below shell).
  5. The ventricle of the heart is incompletely partitioned. It supplies more oxygen to the body through blood circulation. The ventricle is completely partitioned into two in crocodiles.
  6. Most reptiles have better-developed limbs. These are well adapted for efficient locomotion.
  7. Reptiles are also cold-blooded (Poikilothermic). They also hibernate in winter.

Evolution of Reptiles

Reptiles have evolved from amphibians by developing the above characteristics. So, reptiles have fully adapted to terrestrial animals. Reptiles flourished throughout the Mesozoic period (225 – 65 million years). The climate of this period was suitable for reptiles. The climate became less favorable in the Tertiary period (65 — 1.8 million years). So most of them become extinct. There were a dozen lines (groups) of reptiles. Only four groups exist today. The reptiles of today have been derived from dinosaurs. These dinosaurs were present in the Jurassic (195 – 136 million years) and the Cretaceous period (135 – 65 million years).

Groups of Living Reptiles

There are three groups of living reptiles:

  • Lizard and snakes
  • Tautra (shpenodon): It is present in New Zealand. It has survived up to today with little change.
  • Crocodiles: These are offshoots (branches) of the stock (group) from which modern birds were derived.

Most of the reptiles live in temperate and tropical zones. They flourish in the tropical zone.

3. Class Aves – Birds

Evolution of Birds

Birds are one of the most interesting animals. Birds, along with mammals form important parts of the animal kingdom. It is believed that both birds and mammals have evolved from different lines (groups) of reptiles.


The earliest known fossil of birds is archaeopteryx. The fossils of two species of archaeopteryx have been discovered in the rocks of the Jurassic period. These fossils show the following characteristics:

  • It is about the size of a crow.
  • Its skull is similar to present-day’s birds.
  • It has bony teeth. These teeth are present in sockets on the jaw.
  • Modern birds do not have teeth.
  • Jaws extend to form beaks.
  • It has a long tail.
  • Each wing has three claws.
  • It has feathers on the body.

Except for feathers, archaeopteryx shows resemblances with the dinosaurs (giant reptiles). Many other fossils of later birds also have teeth. This evidence suggests that the bird has evolved from its reptilian ancestors. Archaeopteryx shows the characters of both reptiles and birds. So it is the connecting link between reptiles and birds.

Characters of Birds (Aves – Tetrapod)

  1. Their body is streamlined and spindle-shaped. The body is divided into four parts, head, neck, trunk (main body), and tail.
  2. These are warm-blooded or homoeothermic.
  3. Limbs are adapted for flying. The forelimbs are modified into wings. The hind limbs are used for perching. They are also used for running like an ostrich.
  4. There is an epidermal exoskeleton of feathers. Legs are covered by scales.
  5. The skeleton is light due to the presence of air spaces in it. It is an adaptation for flying.
  6. The skull has large sockets. Jaws extend to form the horny beaks. Teeth are absent in birds.
  7. The heart has four chambers, two atria, and two ventricles. There is only the right aorta (aortic arch). It curves to the right side and bends backward.
  8. The lungs have extensions called air sacs. These air sacs extend into the bones.
  9. The organ of the voice is called a syrinx. It is present at the lower end of the trachea, near the origin of the bronchi.
  10. The excretory system does not have a bladder. Their urine is semi-solid.
  11. Sexes are separate. Fertilization is internal. They have large-sized eggs with much yolk. Only one ovary and oviduct are functional.
  12. Birds do not have teeth. So they have developed a thick muscular structure, called Gizzard. It is used for crushing food.
  13. Some birds have secondarily lost the power of flight. They are called running birds e.g. Ostrich, Kiwi, etc.

4.     Class Mammalia – Mammals

The name mammal was given by Linnaeus. A mammal is a group of animals that are nourished by milk from the breast of the mother. It is the highest group in the animal kingdom. They are much more advance than all other groups of animals. There is evolution and development of the most advanced brain (nervous system) in mammals.

Evolution of Mammals

It is universally accepted that mammals have evolved from reptilian ancestors called cotylosaurs. This view has been formed on the basis of fossil records. Mammals have hard bones. These bones are preserved as fossils. So the fossil record of mammals is easily available. While the bones of birds are soft.

So these are not mostly preserved. The ancestor of the mammals lived simultaneously with reptiles during the Jurassic period. These ancestors are called mammal-like reptiles. Some of these ancestors had the size of mice and lived on trees. One of these early reptiles was varanope.

Its fossil was found in Texas (USA). Probably, five groups of such mammal-like reptiles developed mammalian characteristics. They were 50% mammals. Mammals have become dominant in the Cenozoic era (present era).

General Character of Mammals

The mammals show many important characteristics. These characters are as follows:

  1. The body of the mammals is covered by hair.
  2. There is a muscular diaphragm in mammals. It separates the thoracic and abdominal cavities. This structure is absent in all other groups.
  3. The lower jaw has only one large bone. It articulates directly with the skull.
  4. External ear or pinna is present. There is a chain of three bones in the ear called the Incus, Malleus, and Stapes.
  5. Some mammals have two sets of teeth.
  • Deciduous teeth: These are milk teeth formed during early life. They are replaced by permanent teeth.
  • Permanent teeth: They come after deciduous teeth and persist throughout life, e.g. Man.
  1. Mammals have 4 – chambered hearts. They have only the left aortic arch. (It is right in birds).
  2. The red blood cells are non-nucleated.
  3. Mammals are warm-blooded or Homoeothermic.
  4. Mammals have well-developed voice apparatus called the larynx and epiglottis.
  5. Most mammals give birth to young.
  6. Mammals feed their young on milk. Milk is produced by the mammary glands of the mother.
  7. Classification: Mammals are classified into three sub-classes:
  • Prototheria: Egg-lying mammals.
  • Metatheria: Pouched Mammals. Eutheria: Placental mammals including man.

Sub-Class Prototheria (Mammals – Tetrapod)

They have characteristics of both reptiles and mammals. So they form connecting link between reptiles and mammals. They provide evidence about the evolution and origin of mammals from reptiles. Certain members of this sub-class like the duck bill are aquatic.

Mammalian Characters

  1. They have thick fur on their body. Hairs are present on this fur.
  2. The female has a mammary gland to feed young.

Reptilian Characters

  1. They lay eggs.
  2. They have a common opening called the cloaca. There are no separate openings for the digestive system (anus) and urinogenital system (reproductive + excretory systems).


  • Duckbill Platypus: It is adapted for aquatic life. Duckbill has a duck-like bill. It has webbed toes. It has thick fur on its body.
  • Echidna: It is also called spiny anteaters. It has spines on its body and it eats ants.

Sub Class Metatheria (Mammals – Tetrapod)

These are the most primitive mammals. They contain an abdominal pouch called Marsupium. It is their characteristic feature. They rear their young in this pouch. The young are born in the immature (incomplete) stage. The mother carries it to its Marsupium. The nipples of the mammary glands of the mother are present in the Marsupium. The mother feeds the young on milk produced by its mammary glands. Young live in it till it becomes mature. These mammals are also called marsupial or punched mammals.

Examples: Opossum, Kangaroo, and Tasmanian wolf. These animals are present only in Australia and America.

Sub-Class Eutheria (Mammals – Tetrapod)

This subclass includes placental mammals. The complete development of the young takes place with the body of the mother. The fully developed young are borne. A structure is formed during development in these animals called Placenta. The placenta is used for the nourishment of the fetus (developing embryo). Placenta also acts as an endocrine gland. It produces certain hormones. So these animals are also called Placental mammals. These mammals show maximum mammalian characteristics. In some placental mammals, the hairs are modified into scales (pangolin) and spines (porcupine).

Examples: Man, Whale, elephant, horse, rat, bat, dolphin, etc.


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