Mammals
Characteristics
Mammals (formally Mammalia) are members of a class of air-breathing vertebrate animals characterized by the possession of hair, three middle ear bones, and mammary glands functional in mothers with young. A mammary gland is an exocrine gland: an enlarged and modified sweat gland, which gives mammals their name. In addition to milk production, mammals engage in a high level of prenatal care of their offspring. Most mammals also possess sweat glands and specialized teeth, and the largest group of mammals (placentals) have a placenta that feeds the offspring during gestation. Mammals are endothermic (warm-blooded) and their skin is much thicker than that of birds and often has a continuous layer of insulating fat beneath the dermis.
Figure 16. Otter
"Otter" by Michael Ayers, C3BC is licensed under CC BY 3.0
Mammals are generally considered to be more intelligent than other animals. The mammalian brain, with its characteristic neocortex, regulates endothermic and circulatory systems, including a four-chambered heart.
Mammals belong to an early amniote group called synapsids. Synapsids include mammals and all extinct amniotes more closely related to mammals than to reptiles. Early synapsids had a sprawling posture and a small brain, like most early tetrapods. The parasagittal gait characteristic of most mammals appeared gradually in some therapsids capable of sprawling; this characteristic may have appeared in the hind limb before the fore limb.
Monotremes
Monotremes are mammals that lay eggs (Prototheria) instead of giving birth to live young like marsupials (Metatheria) and placental mammals (Eutheria). Like other mammals, monotremes are warm-blooded with a high metabolic rate (though not as high as other mammals), have hair on their bodies, produce milk through mammary glands to feed their young, have a single bone in their lower jaw, and have three middle-ear bones. The key anatomical difference between monotremes and other mammals is the fact that their urinary, defecatory, and reproductive systems all open into a single duct: the cloaca. This structure is very similar to the one found in reptiles. Monotremes and marsupials have a single cloaca (though marsupials also have a separate genital tract), while placental mammal females have separate openings for reproduction, urination, and defecation: the vagina, the urethra, and the anus. Platypuses and Echnidnas are well known examples of monotremes.
Figure 17. Feeding Platypus
"Feeding Platypus" by Brisbane City Council, Wikimedia Commons is licensed under CC BY 2.0
Marsupials
Figure 18. A wild shortbeak echidna.
"A wild shortbeak echidna," by Fir0002/Flagstaffotos, Wikimedia Commons is licensed under CC BY-NC 3.0
Marsupials are an infra class of mammals, characterized by giving birth to relatively undeveloped young. In the absence of soft tissues, such as the pouch and reproductive system, fossil marsupials can be distinguished from placentals by the form of their teeth; primitive marsupials possess four pairs of molar teeth in each jaw, whereas placental mammals never have more than three pairs.
Like other mammals, marsupials develop their young in a uterus and develop a placenta. Pregnant marsupial females develop a kind of yolk sac in their wombs, which delivers nutrients to the embryo. Marsupials give birth at a very early stage of development (about 4-5 weeks); after birth, newborn marsupials crawl up the bodies of their mothers and attach themselves to a nipple, which is located on the underside of the mother either inside a pouch called the marsupium or open to the environment. The offspring are eventually able to leave the marsupium for short periods, returning to it for warmth, protection and nourishment.
Figure 19. Eastern Grey Kangaroo with joey.
" Eastern Grey Kangaroo with joey" by Fir0002/Flagstaffotos, Wikimedia Commons is licensed under CC BY-NC 3.0
Eutherians
Eutheria are a group of mammals consisting of placental mammals plus all extinct mammals that are more closely related to living placentals (such as humans) than to living marsupials (such as kangaroos). They are distinguished from non-eutherians by various features of the feet ankles, jaws and teeth.