Tetrapod: Classes, Characteristics, Examples, and Evolution
Class Tetrapod (Tetrapoda) have four pairs of limbs. These include:
- Class Amphibian
- Class Reptiles
- Aves (Birds) Class
- Class Mammals
1. Class Amphibia (Bony Tetrapod)
The amphibians show following characteristics:
- Skeleton is mostly bony. Different amphibians have different body forms. Tail may or may not present.
- They are tetrapods and have four limbs. Some are limbless (e.g. caecilians). Mostly they have webbed foot.
- Skin is smooth and moist. It has many glands. Some glands are poisonous. Pigments cells (chromatophores) are present in the skin. Scales are absent.
- Respiration takes place by gills in larva and by lungs and skin in adults.
- Heart is 3-chambered. It has two atria and one ventricle. Sinus venosus and truncus arteriosus are present. There is double circulation in heart.
- Sexes are separate. Fertilization is external. Larval stages represent.
- The larva changes into adult by metamorphosis. Amphibians are Anamniotes.
- Amphibians are cold blooded (Poikilothermic) animals. They hibernate in winter.
Examples: Frog, Toad and salamander.
2. Class Reptilia
Reptiles are completely adapted on land. There are certain terrestrial adaptations in reptiles. These adaptations are not found in amphibians. Reptile shows following features:
- They have developed some copulatory organ for internal development.
- The shell of egg is leathery. It resists dryness and prevent from injury. They have large yolky egg.
- Reptiles have dry scaly skin. It is an adaptation for terrestrial life.
- Reptiles develop protective embryonic membranes amnion (around embryo) allantois (store waste) and Chorion (below shell).
- The ventricle of heart is incompletely partitioned. It supplies more oxygen to body through blood circulation. The ventricle is completely partitioned into two in crocodiles.
- Most reptiles have better developed limbs. These are well adapted for efficient locomotion.
- Reptiles are also cold blooded (Poikilothermic). They also hibernate in winter.
Evolution of Reptiles
Reptiles have evolved from amphibians by developing above characteristics. So, reptiles have fully adapted in the terrestrial animals. Reptiles flourished throughout 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 dozen lines (groups) of reptiles. Only four groups exist today. The reptiles of today have been derived from dinosaurs. These dinosaurs were present in Jurassic (195 – 136 million years) and 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 offshoot (branch) of the stock (group) from which modern birds were derived.
Most of the reptiles live in the 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 the different lines (groups) of reptiles.
The earliest known fossil of birds is archaeopteryx. The fossils of two species of archaeopteryx have been discovered from the rocks of the Jurassic period. These fossils show following characters:
- It is about the size of crow.
- Its skull is similar to present day’s birds.
- It has bony teeth. These teeth are present in sockets on jaw.
- The modern birds do not have teeth.
- Jaws extend to form beak.
- It has a long tail.
- Each wing has three claws.
- It has feathers on the body.
Except feathers, archaeopteryx shows resemblances with the dinosaurs (a giant reptile). Many other fossils of later birds also have teeth. These evidences suggest that the bird have evolved from the reptilian ancestors. Archaeopteryx shows the characters of both reptiles and bird. So it is the connecting link between reptiles and birds.
Characters of Birds (Aves – Tetrapod)
- Their body is stream-lined and spindle shaped. Body is divided into four parts, head, neck, trunk (main body) and tail.
- These are warm blooded or homoeothermic.
- Limbs are adapted for flying. The fore-limbs are modified into wings. The hind limbs are used for perching. They are also used for running as in ostrich.
- There is epidermal exoskeleton of feathers. Legs are covered by scales.
- The skeleton is light due to presence of air spaces in it. It is an adaptation for flying.
- The skull has large sockets. Jaws extend to form horny beak. Teeth are absent in birds.
- Heart has four chambers, two atria and two ventricles. There is only right aorta (aortic arch). It curves to right side and bends backward.
- The lungs have extensions called air sacs. These air sacs extend into the bones.
- The organ of voice is called syrinx. It is present at lower end of the trachea, near the origin of bronchi.
- Excretory system does not have a bladder. Their urine is semi- solid.
- Sexes are separate. Fertilization is internal. They have large sized egg with much yolk. Only one ovary and oviduct is functional.
- Birds do not have teeth. So they have developed a thick muscular structure, called Gizzard. It is used for crushing of food.
- 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. Mammal is a group of animals which are nourished by milk from the breast of the mother. It is the highest group in the animal kingdom. They are much advance than all other groups of animals. There is evolution and development of most advanced brain (nervous system) in mammals.
Evolution of Mammals
It is universally accepted that the mammals have evolved from reptilian ancestors called cotylosaurs. This view has been formed on the basis of fossil record. 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 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 characters. They were 50% mammals. Mammals have become dominant in the Cenozoic era (present era).
General Character of Mammals
The mammals show many important characters. These characters are as follow:
- The body of the mammals is covered by hairs.
- There is a muscular diaphragm in the mammals. It separates the thoracic and abdominal cavities. This structure is absent in all other groups.
- The lower jaw has only one large bone. It articulates directly with the skull.
- External ear or pinna is present. There is a chain of three bones in the ear called Incus, Malleus and Stapes.
- Some mammals have two sets of teeth.
- Deciduous teeth: These are milk teeth and formed during early life. They are replaced by permanent teeth.
- Permanent teeth: They come after deciduous teeth and persist throughout life, e.g. Man.
- Mammals have 4 – chambered heart. They have only left aortic arch. (It is right in birds).
- The red blood cells are non-nucleated.
- Mammals are worm blooded or Homoeothermic.
- Mammals have well developed voice apparatus called larynx and epiglottis.
- Most mammals give birth to young.
- Mammals feed their young on milk. Milk is produced by mammary glands of mother.
- 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 reptile and mammals. They provide evidences about the evolution and origin of mammals from reptiles. Certain members of this sub-class like duck bill are aquatic.
- They have thick fur on their body. Hairs are present on this fur.
- The female has mammary gland to feed young.
- They lay eggs.
- They have common opening called cloaca. There are no separate openings for digestive system (anus) and urinogenital system (reproductive + excretory systems).
- Duck bill Platypus: It is adopted for aquatic life. Duck bill has 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 most primitive mammals. They contain an abdominal pouch called Marsupium. It is their characteristic feature. They rear their young in this pouch. The young is borne in immature (incomplete) stage. The mother carries it to its Marsupium. The nipples of the mammary glands of mother are present in the Marsupium. The mother feed the young on milk produce by its mammary glands. Young live in it till it become 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. Complete development of young takes place with the body of mother. The fully developed young are borne. A structure is formed during development in these animals called Placenta. Placenta is used for the nourishment of fetus (developing embryo). Placenta also acts as endocrine gland. It produces certain hormones. So these animals are also called Placental mammals. These mammals show maximum mammalian characters. In some placental mammals, the hairs are modified into scales (pangolin) and spines (porcupine).
Examples: Man, Whale, elephant, horse, rat, bat, dolphin etc.