|
Mammals are warm-blooded vertebrates that nurse their young
with milk. They evolved from reptiles. Their body is well insulated,
usually with hair or fur. The right and left sides
of the 4-chambered heart are completely separate.
Class Mammalia: Vertebrates covered with insulation, usually hair or fur (occasionally blubber). Metabolic rates and body temperatures are kept high (homeothermy). Glands in skin secrete sweat and oily secretions (sebum). Young mammals are nursed by their mothers; milk is secreted by mammary glands, derived from sweat glands. Frequent parental care. Normal standing posture keeps the body elevated from the ground, compared to the low-slung posture of amphibians and reptiles. Three tiny ossicles (malleus, incus, stapes) transmit sounds in middle ear. Four-chambered heart has complete separation of oxygen-rich and oxygen-poor blood. Only one bone, the dentary, makes up the lower jaw on each side. A muscular diaphragm is responsible for most breathing movements. A bony hard palate separates nasal cavity from oral cavity, allowing breathing and chewing at the same time. Teeth vary in shape with their position in the mouth and are restricted to only two waves of growth and replacement instead of many. Brain and braincase larger than in reptiles. Origin of mammals: Mammals evolved during Triassic times from mammal-like reptiles (Synapsida). The transition involved changes in the teeth and tooth replacement, the replacement of one jaw hinge (between articular and quadrate bones) with another (between dentary and squamosal bones), and the conversion of the articular and quadrate bones into the malleus and incus. Monotremata: Egg-laying mammals. Example: platypus. Marsupials: Pouched mammals. Examples: kangaroo, opossum, koala. Placental mammals (Eutheria): Mammals in which the fetus is nourished in utero by a placenta. Includes the vast majority of mammals, arranged in over 30 orders, about half extinct and half with living members. Examples: shrews, mice, bats, rats, cows, deer, pigs, dogs, cats, monkeys, humans, whales, horses, elephants, rabbits. Among the many orders of placental mammals are these:
|
| Prev | rev. March 2020 | Next |