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, chiroptera, means “hand wing.” Bats also have fur, not feathers. 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 remains some debate as to whether all koalas occupy the same basic taxonomic group. The koala is therefore almost opposite to the highlyu diversified bat, which boasts almost a thousand different and distinct species with various sizes and features.

Like other members of the marsupial family, koalas give birth to live but completely dependent and underdeveloped young who continue their gestation suckling on a teat inside a pouch on the mother’s body. The term “Phacolarctos” means “pouch bear,” which is why a koala is sometimes referred to as a “koala bear,” but the species is unrelated to true bears. Koalas belong to the order Diprotodontia, which means that two of its toes, located on the hind legs, are fused together. Other features of diprodontia include a single pair of incisors on its lower jaw (“Taxonomy”). Along with wombats, koalas belong to the Vombatiformes family. Koalas have evolved to a highly specialized diet that consists only of eucalyptus tree leaves, a phenomenon that likely occurred as Australia drifted northwards and its rainforests retreated to leave the eucalyptus as the dominant remaining forest tree species in the continent (“Ancient koalas may have been loud and lazy but they didn’t chew gum”). Although the prehistoric species had different diets, they shared in common with contemporary koalas their loud vocalizations used to communicate with each other over long distances and their “lazy” tree-dwelling lifestyle. Koalas have a much louder call than any species of bat or echidna, but like bats, sound is an important part of their ability to communicate and navigate. However, koalas do not use echolocation as bats do.

Koalas can be traced at least to the Miocene. They are “the sole living member of the diprotodontian marsupial family Phascolarctidae,” (“Ancient koalas may have been loud and lazy but they didn’t chew gum”). They are “general scarce in the fossil record,” making it somewhat difficult to understand koala evolution (“Ancient koalas may have been loud and lazy but they didn’t chew gum”). Among living marsupials koalas are most closely related to wombats but the two species diverged some 30-40 million years ago (“Ancient koalas may have been loud and lazy but they didn’t chew gum”).

Monotremata (Prototheria)

Tachyglossus aculeatus (Echidna)

Echidnas are one of two extant monotremes, the other being the platypus. The contemporary monotremes likely evolved from a common ancestor, and diverged around 30 million years ago. There is considerable dispute over monotreme evolution due to a dearth of fossil records, but most likely the echidna and platypus share an ancestor that more closely resembles the latter (Cooper). Relatively scarce now, the monotremes were once the dominant species in Australia until marsupials like the koala took over that distinction (Cooper). Echidnas share more in common with marsupials with regard to geographic distribution than to bats.

There are four species of echidna. Occupying a broad range of habitats in Australia and nearby smaller islands, echidna thrive in a wide range of habitats. Like the platypus, the echidna is unique among mammals in that it lays eggs instead of live young but they do produce milk and share in common with other mammals physiological features like fur or hair (which for the echidna is spiny and unlike the fluffier fur found on most bats and koalas), middle ear bones, high rates of metabolism or warm-bloodedness, and single-boned lower jaws (“Monotremes”). Most echidnas have no teeth but they do have elongated snouts that they use to dig for their insectivore diet. Echidnas tend to be crepuscular or nocturnal, making their feeding habits somewhat similar to bats, particularly among the echidna and bat species that eat mainly insects.

Works Cited

Archibald, J. David. “Eutheria.” Retrieved online: http://www.bio.sdsu.edu/faculty/archibald/Archibald01Eutheria.pdf

“Ancient koalas may have been loud and lazy but they didn’t chew gum.” Science Daily. 1 Dec, 2009. Retrieved online: https://www.sciencedaily.com/releases/2009/12/091218122926.htm

Cooper, Dani. “Echidna Ancestor sam with Platypuses.” ABC Science. Retrieved online: http://www.abc.net.au/science/articles/2009/09/22/2692080.htm

“Monotremes.” Retrieved online: http://www.ucmp.berkeley.edu/mammal/monotreme.html

Randerson, James. “Fossils Solve Mystery of Bat Evolution.” The Guardian. 13 Feb, 2008. Retrieved online: https://www.theguardian.com/science/2008/feb/13/bat.evolution

Simmons, Nancy B. and Conway, Tenley. “Chiroptera.” Tree of Life. Retrieved online: http://tolweb.org/Chiroptera

“Taxonomy.” Australian Koala Foundation. Retrieved online: https://www.savethekoala.com/about-koalas/taxonomy

Wilson and Reeder. Chiroptera. Retrieved online: https://www.utep.edu/leb/pleistnm/taxaMamm/chiroptera.htm

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