Eutheria, marsupalia, and monotremata are the three main "infraclasses" of the Mammaliam subclass Theria (Archibald). Of these three infraclasses, eutheria currently boasts the largest membership and monotremata the smallest. Eutherian mammals like bats are characterized by their production of placenta as well as live births. Marsupials like koalas are characterized by live births of immature young that continue their gestation in a pouch located externally on the mother's body; that pouch contains a nipple hidden inside. Unique to all mammals, the monotremes like echidnas give birth to eggs instead of live young but do produce milk. Whereas bats are widely distributed around the globe, monotremes only exist in Australia. There are a few marsupials remaining in the Americas but most have become extinct and the greatest number currently exist in Australia. Eutheria (Placentalia)

Chiroptera (Bat), of which there are 928 species

The only member of the mammal class to truly be able to fly, bats comprise just less than 1000 different species. They are therefore successful as well as unusual. Only rodents (rodentia) boast more individual species than chiroptera. All bats are nocturnal or crepuscular, and most rely on sonar and echolocation for navigation during night flight. A considerable number of bat species roost in caves, and their habitats can be found all over the world except for arctic regions. Most bat species can be found in tropical regions (Wilson and Reeder). Depending on the species, bats feed on insects, nectar, fruit, fish, blood, or small mammals (Wilson and Reeder). Their diets are far more diverse than that of either the koala or the echidna, which have much narrower habitat range.

Elongated hand and finger bones (metacarpal bones and phalanges, respectively) support the wing membrane, called patagium, of the bat. In this key sense, the wings of bats differ significantly from birds, which lack fingers (Simmons and Conway). Also because of this characteristic feature, the scientific name for bats,...

Dietary habits and habitat features determine the physiological characteristics of individual bat species, with for example, longer snouts on nectar feeders, "short, broad faces good for biting rounded fruits," for fruit-eating bats, or larger ears for those relying more on echolocation (Simmons and Conway; Wilson and Reeder). Likewise, the eye sizes of bats differ depending on their need to distinguish patterns in low light situations (Simmons and Conway).
The most recent fossil records found have revealed that flight for chiroptera evolved before echolocation (Randerson). Fossil records for the chiroptera order extend to the early Eocene (Simmons and Conway). However, there is considerable dispute over the evolutionary trajectory of bats. The dispute centers on the debate over the two known extinct clades of bat: Megachiroptera and Microchiroptera, and whether these two clades evolved separately from two independent ancestors, whether they shared a common ancestor, or whether they evolved simultaneously from their own clade (Simmons and Conway). The latter hypothesis seems the most tenable given the recent fossil record discoveries, suggesting "there was only one origin of powered flight in mammals," (Simmons and Conway).

Marsupialia (Metatheria)

Phascolarctos cinereus (Koala)

There are only three extant sub-species of koala, all of which are located in Australia: the Phascolarctos cinereus adustus, which dwells mainly in habitats in northern Queensland, the Phascolarctos cinereus victor from Victoria, and the Phascolarctos cinereus from New South Wales. However, there is some dispute as to whether the latter constitutes a separate sub-species or not ("Taxonomy"). The differences between all three of these sub-species are relatively minor such as thickness of fur, which is itself likely related to the climate conditions in different parts of Australia and the need for thicker fur in colder areas ("Taxonomy"). Therefore, there…