Mammals appeared more than 200 million years ago. On the external signs they resembled modern mice voles and shrews and had the small sizes. All animals relating to mammals warm-blooded, feed with milk of the cubs and breathe oxygen. All animals have seven cervical vertebras.
Subclasses of mammals
Contrary to popular belief, the birth of live cubs is not the general sign for all animals. On the contrary, on this sign the scientists distinguish subclasses from animals: yaytsekladushchy, marsupials and placentary.
Yaytsekladushchy mammals are the most primitive of recent mammals. Their reproduction happens the same as at birds and reptiles, by means of postponement of eggs. The bright representative of this subclass is the duck-bill.
The main difference of marsupial mammals from other animals consists in development of their germs. At first the germ is in mother's body, then the tiny helpless being is born. After the birth the cub moves to a bag to mother, is attached to a nipple with milk and already there continues the development. The most known marsupials animal are kangaroos and a koala. Placentary mammals are animals who have a placenta – special body in which the fruit till the birth is and develops. This way of development of a germ is considered the most perfect. Dogs, cats, tigers, lions, dolphins and people treat placentary mammals.
Main signs of mammals
Existence of indumentum and feeding of cubs milk belong to the main characteristics of mammals distinguishing them from fishes, birds and Amphibia. All females of this group of animals have on a body mammary glands which are after the delivery filled with nutritious liquid which the young individual eats subsequently. It is also possible to refer existence of the diaphragm separating lungs and heart from a digestive tract, and the lower jaw consisting of one bone to number of other distinctive signs of mammals. But the most important sign of mammals is the advanced brain and the flexible system of behavior. It means that in the same conditions two individuals of the same look can behave differently, relying on past experience and reflections. It becomes clear why in a class of mammals the type of Homo Sapiens – the Homo sapiens appeared.
Appearance of mammals
According to scientific research, the most ancient mammals appeared during an era of dinosaurs. Then they had the small sizes and resembled brutish reptiles. As scientists-archeologists work with fossils, at their order there are only skeletons of ancient animals because of what it is difficult to establish precisely at what stage of the development these animals acquired wool and began to raise cubs milk. It is at the moment considered to be that the first mammals appeared about 200 million years ago.