Mammal classification has been through several iterations since Carl Linnaeus initially defined the class. No classification system is universally accepted; McKenna & Bell (1997) and Wilson & Reader (2005) provide useful recent compendiums.George Gaylord Simpson‘s “Principles of Classification and a Classification of Mammals” (AMNH Bulletin v. 85, 1945) provides systematics of mammal origins and relationships that were universally taught until the end of the 20th century.
- Mammals are primarily terrestrial vertebrates but few like whales and dolphins are aquatic.
- They are the most dominant animals on earth and have the capacity to learn because of their better developed brain.
- Mammals have evolved from Therapsid (mammals-like) reptiles some 220 million years ago in the Triassic period.
- There are two pairs of pentadactyl limbs (each foot bears 5 or fewer toes provided with horny claws, nails or hooves).
- They are variously adapted for walking, running, jumping, climbing, burrowing, swimming, flying etc.
- The skin is glandular (sweat glands, sebum glands etc.) and mostly covered by a horny epidermal exoskeleton of hair and fur.
- The hair on the skin and adipose tissue beneath the skin conserve body heat.
- Mammary glands are well developed in females that produce milk to suckle the young ones.
- sebaceous gland,
- Arrector pili muscle,
- hair follicle,
- sweat gland,
- (not labeled, the bottom layer) — hypodermis, showing round adipocytes
- The mouth is relatively small and has movable lips.
- Buccal cavity has true salivary glands.
- Teeth occur in both the jaws and they are thecodont (developed in sockets) and heterodont (different types of teeth growing in two sets). Tongue is mobile.
- Distinct liver and pancreas are present.
- Alimentary canal is complete and opens by anus.
- Respiration occurs only by lungs which are elastic and spongy.
- They are enclosed in the pleural cavities.
- Rib muscles and diaphragm (a muscular partition separating thoracic cavity from abdominal cavity) play a role in breathing.
- Glottis is guarded by epiglottis.
- The circulatory system is closed.
- The heart is four chambered with two auricles and two ventricles. Heart pumps oxygenated blood to different parts of the body.
- Hepatic portal system is present but renal portal system is RBCS are circular, biconcave and non-nucleated most species.
- They are homeothermic (warm-blooded), which gives them high rate of metabolism and makes them active.
- The brain has large cerebrum and cerebellum.
- There are 12 pairs of cranial nerves and 31 pairs of spinal nerves.
- Optic lobes are divided into four bodies; corpora quadrigemina. Corpus callosum connects the two cerebral hemispheres.
- The olfactory sacs open by the internal nares far back into the pharynx.
- Eyes have movable lids.
- Internal ear has an organ of corti, middle ear has 3 bony ear osciles (malleus, incus and stapes) and external ear generally has a large, fleshy pinna.
- Two bean shaped kidneys are present which are metanephric.
- They are ureotelic (excrete urea).
- Ureters open into the urinary bladder and urine is fluid.
- Sexes are often distinguishable externally (sexual dimorphism). The testes usually descend into scrotal sacs in the adult.
- Male has a copulatory organ called penis.
- Gonoducts lead directly to the exterior and fertilization is internal.
- Mammals are mostly viviparous (directly give birth to the young ones).
- Development occurs in the uterus of the female. Embryo has amnion, allantois and chorion.
- A placenta fixes the fetus to the uterine wall for nourishment, respiration and excretion. Young ones are nourished with milk for some time after birth and are brought up with great love and care.