To date, mammals are considered to be one of the most advanced classes. This group is characterized by a number of important features. According to well-known studies, in the modern world there are more than five thousand representatives of the class. Although it is worth noting that this figure is very approximate, since scientists are still searching for new species.
Mammals and their distribution
Mammals are considered to be completely unique representatives of the fauna. They are common throughout the planet.
Representatives of this class can be found in almost all major biotopes of the Earth. These animals perfectly exist in the hot, dry desert climate, in the highlands. They can be seen in the polar regions and tropical forests. They are common even in the aquatic environment.
Perhaps the only territories that are not populated by mammals are the internal territories of Antarctica and the deepwater part of the world's oceans.
Interestingly, the area of distribution of single-pass animals (for example, platypus) is clearly limited to the territories of New Guinea and Australia. Marsupials are common on the islands of Oceania, as well as in Australia and the Americas. It is also interesting that only some placental mammals lived on the Australian continent until the first settlers arrived there.
Mammals: Class Features
As already mentioned, this group of animals has a number of unique characteristics that indicate a high level of organization:
- Almost mammals are viviparous.
- Females feed their cubs with milk produced in specific glands (by the way, thanks to this fact, mammals got their name).
- Representatives of the class have a specific hairline, active sebaceous and sweat glands.
- Mammals are characterized by the so-called homeothermia, or warm-bloodedness - their body temperature does not depend on the conditions and temperature of the environment.
- A particularly important feature of this group of animals is a well-developed brain, in particular, its final part. It is here that the center is located, thanks to which representatives are prone to very complex forms of behavior.
- In mammals, the auricle, the external ear canal, as well as all three auditory ossicles, which, by the way, are modified bones of the lower jaw, are formed.
- Representatives of the class are characterized by the presence of a developed four-chamber heart and the left aortic arch, which ensure the separation of blood into venous and arterial (for example, in reptiles venous and arterial blood mixes in the heart).
- A distinctive feature is the alveolar structure of the lungs.
- In addition, there are differences in the structure of blood cells - the erythrocytes of these animals do not contain a nucleus, only respiratory pigment.
- Mammals have seven vertebrae in the cervical region.
- Representatives of this class are characterized by a variety of teeth (the presence of incisors, canines, molars). In this case, the teeth with the help of the roots are immersed in the bone tissue of the jaw.
The origin of mammals
Scientists believe that the ancestors of modern mammals were reptiles that appeared in the Paleozoic. These animals still retain some similarities with amphibians, in particular, the presence of skin glands and, possibly, the double occipital condyle.
Particularly close relatives of modern species are considered to be beetles. Representatives of this group were characterized by the presence of different teeth (molars, incisors and fangs). The upper and lower jaws were connected by a special bone, which in mammals turned into an auditory ossicle. But the representatives of the animals were extinct in the Upper Triassic.
Already in the Jurassic period a separation of live-bearing species arose from those representatives who still laid eggs.
In fact, it is impossible to clearly formulate the theory of the origin of mammals, because to this day paleontologists and other scientists are conducting active research in this area.