Class Mammalia  (V & C fig. 7.195-7.200)

            Although the first mammals date from the early part of the Mesozoic Era, mammals flourished and radiated only after the extinction the dinosaurs and other groups of reptiles that dominated the land, sea, and air during the Mesozoic era.  Mammals are distinguished by the presence of 1) hair, and 2) mammary glands, and 3) three middle ear bones. The amount, position, and appearance of hair, as well as number and size of mammary glands vary among different mammal groups, but these features are present in all mammals.  Here is a classification of extant mammals:

Class Mammalia
            Subclass Prototheria - Monotremes
            Subclass Theria
                        Infraclass Metatheria - Marsupials
                        Infraclass Eutheria - Placentals

(1)  Monotremes

The subclass Prototheria consists of three extant species native to Australia and New Guinea. These animals are peculiar in that, while they are mammals, they lay eggs.

(2)  Marsupials

This group is classified as the infraclass Metatheria within the subclass Theria. In other words, it is the sister group to the placental mammals. Its members are characterized by females having a marsupial pouch in which the young develop, rather than a placenta. By far the most marsupials occur in Australia, where it is the dominant mammal group. Indeed, marsupial species show a parallel series of adaptations to the environment that is seen in placental mammals in other parts of the world. In marsupials, much of the development of young occurs after birth, nourished by milk. In placentals development occurs prior to birth, nourished by a placenta. List the advantages and disadvantages of each system.

(3) Placentals

Classified as the Infraclass Eutheria in subclass Theria, placentals include most mammal species. They have a placenta formed from embryonic membranes and the wall of the uterus that serves as a source of food, water, and oxygen for the embryo. The placental mammals consist of about 18 orders, and we list some of these orders below, to illustrate the diversity of mammals.  Your lab instructor may provide you with an opportunity to make a report on one of these orders.

Examples of orders of mammals: