Mammal like Reptiles therapsida might have given Mammals. It occured at the end of Triassic period of Mesozoic era. In Jurassic period only few mammals existed. In the coenozoic era these animals increased their number and became very complex. This era is called 'age of mammals'. Today nearly 8500 species of mammals are living in the world. Their body is covered by hairs. They show external ears called Pinnae. They are warm blooded organisms. They show mammary glands. They feed their young ones with milk.
GENERAL CHARACTERS OF MAMMALS
In the upper carboniferous period of palaeozoic era Mammal like reptiles are developed. From them early mammal like forms are developed. In the Jurassic period of mesozoic era few mammals are existed. In the coenozoic era different Mammals are developed. Hence this era is called as age of Mammals. The Mammals evolution radiated in many ways and occupied all parts of the globe.
- Mammals are warm blooded animals.
- The body is covered by hairs.
- The skin contains two types of glands. They are Sweat glands and Sebaceous glands. Sweat glands are useful in excretion. Sebaceous glands produce Sebum. This will keep the skin soft & smooth.
- In Mammals mammary glands are present. They are functional in female. They are modified sebaceous glands. But in prototherians they are modified sweat glands.
- External ears are present (Pinnae). The middle ear contains 3 ear ossicles. They are malleus, incus and stapes. The internal ear contains highly coiled hearing organ called organ of Corti or Cochlea.
- External Nostrils open into the internal nostrils. They open into pharynx.
- Dentition in Mammals: The teeth are Heterodont. 4 types of dissimilar teeth are present. (Incissors, Canines,Premolars, molars.) The teeth are diphyodont. (Two sets of teeth are developed). Thecodont teeth are present in sockets of jaw bones.
- The skull has two occipital condyles. Hence it is called dicondylic skull.
- The lower jaw is made by single bone called dentary.
- The, vertebrae are Amphiplatyan type. Each vertebra is composed of one centrum and two epiphyses.
- In the neck of Mammals seven cervical vertebrae are present. In Sirenia has only 6 cervical vertebrae.
- The ribs are double headed, capitulum, Tuberculum.
- The fore and hind limbs bear digits which will not be more than five.
- The body cavity is divisible into two parts, It is because of the presence of a muscular diaphragm. The upper is thoracic cavity and lower abdominal cavity.
- Heart shows 4 chambers.
- Only left aortic arch is present.
- Non-nucleated R.B.C. is present.
- Brain is well developed.
- In brain 4 optic lobes are present. Cerebrum is well developed.
- Metanephric kidney is present.
- In the nephron Henley loop is present.
- Penis is present.
- Male and females are separate.
- They give birth to young ones. But in Monotrems they lay eggs.
- In the uterus the foetus grows. This period is called Gestation period. Foetus is nourished by placenta through placental connection.