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Mammalia:
The mammals are the class of vertebrate animals characterized by the production of milk in females for the nourishment of young, from mammary glands present on most species and specialized skin glands in monotremes that seep or ooze milk; the presence of hair or fur; specialized teeth; three minute bones within the ear; the presence of a neocortex region in the brain; and endothermic or "warm-blooded" bodies. The brain regulates endothermic and circulatory systems, including a four-chambered heart. Mammals encompass some 5,500 species (including humans), distributed in about 1,200 genera, 152 families and up to 46 orders, though this varies with the classification scheme. Phylogenetically, Mammalia is defined as all descendants of the most recent common ancestor of monotremes (e.g., echidnas and platypuses) and therian mammals (marsupials and placentals). Mammal Anatomy:
Skeletal system: The vast majority of mammals have seven cervical vertebrae (bones in the neck), including bats, giraffes, whales, and humans. The few exceptions include the manatee and the two-toed sloth, which each have only six cervical vertebrae, and the three-toed sloth with nine cervical vertebrae. Respiratory system: The lungs of mammals have a spongy texture and are honeycombed with epithelium having a much larger surface area in total than the outer surface area of the lung itself. The lungs of humans are typical of this type of lung. The environment of the lung is very moist, which makes it a hospitable environment for bacteria. Many respiratory illnesses are the result of bacterial or viral infection of the lungs. Breathing is largely driven by the muscular diaphragm at the bottom of the thorax. Contraction of the diaphragm vertically expands the cavity in which the lung is enclosed. Relaxation of the diaphragm has the opposite effect. The rib cage itself is also able to expand and contract to some degree, through the action of other respiratory and accessory respiratory muscles. As a result, air is sucked into or expelled out of the lungs, always moving down its pressure gradient. This type of lung is known as a bellows lung as it resembles a blacksmith's bellows. Air enters through the oral and nasal cavities; it flows through the larynx and into the trachea, which branches out into bronchi. In humans, it is the two main bronchi (produced by the bifurcation of the trachea) that enter the roots of the lungs. The bronchi continue to divide within the lung, and after multiple generations of divisions, give rise to bronchioles. Eventually the bronchial tree ends in alveolar sacs, composed of alveoli. Alveoli are essentially tiny sacs in close contact with blood filled capillaries. Here oxygen from the air diffuses into the blood, where it is carried by hemoglobin, and carried via pulmonary veins towards the heart. Deoxygenated blood from the heart travels via the pulmonary artery to the lungs for oxidation.
Circulatory system: The mammalian heart has four chambers: the right atrium, right ventricle, left atrium, and left ventricle. Atria are for receiving blood; ventricles are for pumping blood to the lungs and body. The ventricles are larger than the atria and their walls are thick, because muscular walls are needed to forcefully pump the blood from the heart to the body and lungs. Deoxygenated blood from the body enters the right atrium, which pumps it to the right ventricle. The right ventricle pumps blood to the lungs, where carbon dioxide diffuses out, and oxygen diffuses in. From the lungs, oxygenated blood enters the left atrium, where it is pumped to the left ventricle (the largest and strongest of the 4 chambers), which pumps it out to the rest of the body, including the heart's own blood supply.
Head and brain: All mammalian brains possess a neocortex which is a brain region that is unique to mammals.
Skin: Mammals have integumentary systems made up of three layers: the outermost epidermis, the dermis, and the hypodermis. This characteristic is not unique to mammals, since it is found in all vertebrates. The epidermis is typically ten to thirty cells thick, its main function being to provide a waterproof layer. Its outermost cells are constantly lost; its bottommost cells are constantly dividing and pushing upward. The middle layer, the dermis, is fifteen to forty times thicker than the epidermis. The dermis is made up of many components such as bony structures and blood vessels. The hypodermis is made up of adipose tissue. Its job is to store lipids, and to provide cushioning and insulation. The thickness of this layer varies widely from species to species. No mammals are known to have hair that is naturally blue or green in color. Some cetaceans, along with the mandrills appear to have shades of blue skin. Many mammals are indicated as having blue hair or fur, but in all known cases, it has been found to be a shade of grey. The two-toed sloth can seem to have green fur, but this color is caused by algae growths. Reproduction: Most mammals give birth to live young, but a few (the monotremes) lay eggs. Live birth also occurs in some non-mammalian species, such as guppies and hammerhead sharks; thus it is not a distinguishing characteristic of mammals. Although all mammals are endothermic, so are birds, and so this too is not a defining feature. A characteristic of mammals is that they have mammary glands, a defining feature present only in mammals. The monotremes branched from other mammals early on, and do not have nipples, but they do have mammary glands. Most mammals are terrestrial, but some are aquatic, including sirenia (manatees and dugongs) and the cetaceans (dolphins and whales). Whales are the largest of all animals. There are semi-aquatic species such as seals which come to land to breed but spend most of the time in water. Flight: True flight has been observed only once in mammals, the bats; mammals such as flying squirrels and flying lemurs are more accurately classified as gliding mammals. Origin: Mammals belong among the amniotes, and in particular to a group called the synapsids, which are distinguished by the shape of their skulls, having a single hole on each side where jaw muscles attach, called a temporal fenestra. In comparison, dinosaurs, birds, and most reptiles are diapsids, with two temporal fenestrae on each side of the skull; and turtles, with no temporal fenestra, are anapsids. From early synapsids came the first mammal precursors, therapsids, and more specifically the eucynodonts, 220 million years ago (mya) during the Triassic period. From the earliest synapsids (such as Archaeothyris), their temporal fenestra expanded as synapsids evolved. In cynodonts, the temporal fenestra is much larger than the pelycosaurs and the primitive therapsids. From cynodonts to mammals, the temporal fenestra has been modified, now no longer a hole. The erect posture (unlike reptiles and pelycosaurs whose posture was sprawling) evolved in the Middle Permian by therapsids. The secondary palate also evolved by therapsids at the same time (the therocephalians had both of these traits). Mammalian hair also evolved in the Middle Permian, probably evolved from scales. Pre-mammalian ears began evolving in the late Permian to early Triassic to their current state, as three tiny bones (incus, malleus, and stapes) inside the skull; accompanied by the transformation of the lower jaw into a single bone. Other animals, including reptiles and pre-mammalian synapsids and therapsids, have several bones in the lower jaw, some of which are used for hearing; and a single ear-bone in the skull, the stapes. This transition is evidence of mammalian evolution from reptilian beginnings: from a single ear bone, and several lower jaw bones (for example the sailback pelycosaur, Dimetrodon) to progressively smaller "hearing jaw bones" (for example the cynodont, Probainognathus), and finally (possibly with Morganucodon, but definitely with Hadrocodium), true mammals with three ear bones in the skull and a single lower jaw bone. Hence pelycosaurs and cynodonts are sometimes called "mammal-like reptiles", but this is strictly incorrect as these two are not reptiles but synapsids. The earliest-known species seen to be mammal-like is the megazostrodon, which evolved toward the end of the Triassic period. It had evolved to posses fur, be warm-blooded, and was the first animal to have mammary glands, the defining feature of mammals. It is also believed to be nocturnal, for which a warm-blooded nature is necessary. However, it still bore young by laying reptile-like leathery eggs. During the Mesozoic Era , mammals diversified into four main groups: multituberculates (Allotherium), monotremes, marsupials, and placentals. Multituberculates went extinct during the Oligocene, about 30 million years ago, but the three other mammal groups are all represented today. Most early mammals remained small and shrew-like throughout the Mesozoic, but rapidly developed into larger more diverse forms following the Cretaceous-Tertiary extinction event 65 mya. The names "Prototheria", "Metatheria" and "Eutheria" expressed the theory that Placentalia were descendants of Marsupialia, which were in turn descendants of Monotremata, but this theory has been refuted. However, Eutheria and Metatheria are often used in paleontology, especially with regards to mammals of the Mesozoic.
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