Scientists believe that mammals descended from animal-toothed dinosaurs – a group of ancient reptiles (lived more than 200 million years ago), whose legs were located under the body, and the teeth were subdivided into incisors, canines and molars.
This is evidenced by the similarity of modern mammals with modern reptiles at the early stages of embryonic development.
The kinship of mammals with reptiles is also indicated by the existence of such modern animals that lay eggs with a large amount of nutrients, have developed crow bones, cloaca, and other signs of low organization (the Order of the First Beast, or Oviparous (platypus and echidna)).
The oldest mammals were melanodons, which lived 160 million years ago.
Melanodon was the size of a rat, covered with hair, and had a thin, slightly pubescent tail. His teeth were typical of mammals. He was nocturnal. Probably, he could not only run on the ground, but also climb trees, and, if necessary, even swim.
Ecological groups of mammals
Having appeared on Earth, mammals with a constant body temperature, a highly developed brain, and the ability to feed their cubs with milk, spread widely and settled in territories with different climates.
Their development of adaptations to various environments of life led to the emergence of ground, underground, water and flying groups. Each of these ecological groups includes smaller groups that differ in the adaptations of animals to different habitats (for example, among terrestrial animals there are typically terrestrial, semi-arboreal, and tree-climbing lifestyles).
1. Terrestrial forest animals.
In our forests there are brown bear, squirrel, elk, sable, chipmunk, elk, lynx.
Typical tree-climbing animals include squirrels and some species of martens. On trees they get food, arrange nests for rest and breeding.
Forest animals, leading a semi-terrestrial, semi-woody way of life, feed mainly on the ground, and arrange nests under the roots of trees, in hollows located not high above the ground, in the trunks of fallen trees (sable, chipmunk).
The brown bear, wolverine, badger, elk, roe deer and many other large animals lead the land way of life. They get all the food on the ground. Cubs are born on the forest floor (moose, roe deer), in burrows (wolverine, badger), in dens (brown bear).
2. Terrestrial animals of open spaces (meadows, steppes, deserts).
Large herbivorous mammals (herbivorous ungulates), capable of being in the pasture for a long time, moving long and fast in search of watering places, fleeing to escape from enemies (antelopes, saigas, gazelles, kulans, zebras, wild horses, giraffes). Their cubs rise to their feet immediately after birth and follow their parents.
Ungulates are hunted by predatory animals (lions, steppe wolves, cheetahs).
Groups of small and medium-sized animals (ground squirrels and jerboas) are also referred to as animals of open spaces.
3. Terrestrial animals living in different habitats.
Some animals, for example, the wolf and the fox, live in forests, steppes, deserts, mountains. The composition of food, the methods of obtaining it, the breeding conditions of these animals are different in different habitats. So, wolves living in forests give birth to cubs in dens, and in the desert and tundra they often dig holes.
The group of underground mammals is not numerous. These are moles, mole rats, African mole rats. This ecological group of mammals includes many mice, voles, and shrews. All or a significant part of their life, underground mammals spend in burrows, feeding in the soil layer.
The most typical representative of this ecological group is the common mole. Mole habitats can be found by looking at the moles (heaps of discarded soil). In the soil, moles make a complex system of moves. The mole makes underground passages with its large forepaws turned outward. Acting with them like shovels, the mole loosens the earth and throws it back. During the day, the mole runs several times along its underground passages, picking up worms, insects and their larvae that have got there.
Aquatic and semi-aquatic animals
Whales and dolphins lead an aquatic lifestyle, mainly walruses and seals. Mammals such as muskrat, beaver, river otter live both on land and in water.
Despite their adaptability to living in the aquatic environment, all these animals have retained the main features of mammals – viviparity and feeding the cubs with milk. These animals breathe atmospheric air, rising to the surface to inhale.
Only Bats, such as Bats, are real flying animals. Most of the bat species are found in the tropics and subtropics. In our country, there are red noctresses and common long-eared bat.
The bones of bats are thin and light. On the sternum, like a bird, there is a keel. Powerful pectoral muscles are attached to it, which set the wings in motion. The forelimbs take part in the formation of the wings. Elastic leathery flying membranes cover the bones of the forelimbs and, joining laterally with the trunk and hind limbs, reach the tail.
Bats are crepuscular and nocturnal. Bats orient themselves in space even in the dark and do not collide with obstacles due to echolocation (catching sounds reflected from some objects). Large auricles contribute to this. Along with ordinary sounds, bats are able to emit and hear special, very high, ultrasonic signals that are not perceived by humans. The ultrasound sent forward in the direction of movement is reflected from objects and, returning, is captured by bats.
During the day, bats take refuge in attics, basements, tree hollows and other secluded places. Clinging with their hind legs, they hang upside down. During the day’s rest, the animals’ body temperature drops, respiration and blood circulation slow down.