A Mammal (Class Mammalia) is an endothermic (warm blooded), amniotic, tetrapod, belonging to the (jawed) vertebrate subdivision of chordates. Mammals are characterized by having hair, three middle ear bones (like the human ossicles, for example) and mammary glands for production of milk to nourish their young.
Mammals are thought to have developed around 200 million years ago during the Mesozoic Era, descended from a lineage of so-called "mammal-like reptiles," or synapsids, and were probably small, nocturnal, insect eaters.
Living mammals are divided into three main living groups, Monotremata, Marsupials, and Eutherians (Placental Mammals). A fourth group, Multituberculates are the only major branch of mammals to have become completely extinct, and have no known living descendants.
Evolution and Differentiation
With the extinction of large Dinosaurs at the end of the Cretaceous period ~65 Million Years Ago, mammals underwent a rapid adaptive explosion. That event left the world with practically no larger sized terrestrial animals. Only ten million years later, at the end of the Paleocene, they had occupied a large part of the vacant ecological niches.
Although the time and place of monotreme origin is still largely unknown, they are believed to be the oldest lineage of mammals in part because of their apparently close evolutionary relationship to Reptiles and Birds, indicated by the fact that they lay eggs rather than giving live birth. Echidnas (spiny anteaters) and the duck-billed platypus are the only living monotremes.
Marsupials are thought to have diverged from eutherians about 180 million years ago. They have a brief gestation and give birth to tiny embryonic offspring that complete development while attached to the mothers nipples. The short gestation time is due to having a yolk-type placenta in the mother marsupial. Australia is the home to the largest number of marsupials, where they are the dominant native mammals, but they can also be found in South America, New Zealand, and North America.
Currently the only naturally occurring marsupial in the United States is the opossum, Didelphis marsupialis. In the past, however, marsupials were quite common. During the Mesozoic marsupials were very common in North America; more common, in fact, than placental mammals. They persisted there until the mid- to late-Tertiary.
Commonly referred to as placental mammals, eutherians bear fully developed live young. The term "placental mammals" is somewhat of a misnomer because marsupials also have placentae. The difference is that the placenta of marsupials is very short-lived and does not make as much of a contribution to fetal nourishment as it does in eutherians. They are a rather diverse group, with nearly 4000 described species, mostly rodents and bats. Humans belong to this group as do many of the animals that we are most familiar with: dogs, Cats, horses, elephants, etc.
Eutherians first became common in central Asia during the Upper Cretaceous. Since the end of the Mesozoic, eutherian mammals have been the largest and most common land vertebrates, except in Australia
Multituberculates first appeared in the Late Jurassic, and went extinct in the early Oligocene, with the appearance of true rodents. They get their name from their teeth, which have many cusps, or tubercles arranged in rows. Over 200 species are known, some as small as the tiniest of mice, the largest the size of beavers.
Although not known to many people, they have a 100 million-year fossil history, the longest of any mammalian lineage. These rodent-like mammals were distributed throughout the world, but seem to have eventually been outcompeted by true rodents.
- Biology Concepts and Connections 5th Ed. Campbell, Reece, Taylor and Simon