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Class Tetrapod (Tetrapoda) have four pairs of limbs. These include:
The amphibians show following characteristics:
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Examples: Frog, Toad and salamander.
Reptiles are completely adapted on land. There are certain terrestrial adaptations in reptiles. These adaptations are not found in amphibians. Reptile shows following features:
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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).
There are three groups of living reptiles:
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Most of the reptiles live in the temperate and tropical zones. They flourish in the tropical zone.
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:
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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.
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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.
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.
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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).
The mammals show many important characters. These characters are as follow:
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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.
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Examples:
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.
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.
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