Diprotodontia

Diprotodontia (διπρωτός diprotos, meaning "two front" and οδοντος odontos meaning "teeth") is a large order of about 120 marsupial mammals including the kangaroos, wallabies, possums, koala, wombats, and many others. Extinct diprotodonts include the rhinoceros-sized Diprotodon, and Thylacoleo, the so-called "marsupial lion".

Characteristics
Living diprotodonts are almost all herbivores, as were most of those that are now extinct. There are a few insectivore and omnivore diprotodonts, but these seem to have arisen as relatively recent adaptations from the mainstream herbivorous lifestyle. The extinct thylacoleonids ("marsupial lions") are the only known group to have exhibited carnivory on a large scale.

Diprotodonts are restricted to Australasia. The earliest known fossils date to the late Oligocene. However it is certain that their genesis lies earlier than this, as there are large gaps in Australia's fossil record and virtually no fossil record at all in geologically active New Guinea. The great diversity of known Oligocene diprotodonts suggests that the order began to diverge well beforehand.

Many of the largest and least athletic diprotodonts (along with a wide range of other Australian megafauna) became extinct when humans first arrived in Australia about 50,000 years ago. It is possible that this extinction occurred as a direct result of hunting, but was more probably a result of widespread habitat changes brought about by human activities—notably the use of fire.

There are two key anatomical features that, in combination, identify diprotodontia. Members of the order are, first, diprotodont (meaning "two front teeth"): they have a pair of large, procumbent incisors on the lower jaw, a common feature of many early groups of mammals and mammaliforms. The diprotodont jaw is short, usually with three pairs of upper incisors (wombats, like rodents have only one pair), and no lower canines.

The second trait distinguishing diprotodonts is syndactyly, a fusing of the second and third digits of the foot up to the base of the claws which leaves the claws themselves separate. Digit five is usually absent, and digit four is often greatly enlarged.

Syndactyly is not particularly common (though the Australian omnivorous marsupials share it) and is generally posited as an adaptation to assist in climbing. Many modern diprotodonts, however, are strictly terrestrial, and have evolved further adaptations to their feet to better suit this lifestyle. This makes the history of the tree-kangaroos particularly convoluted: it appears that the animals were arboreal at some time in the far distant past, moving afterward to the ground—gaining long kangaroo-like feet in the process—before returning to the trees, where they further developed a shortening and broadening of the hind feet and a novel climbing method.

Fossil record
The earliest known fossil of Diprotodontia dates back to the Late Oligocene (23.03mya-28.4mya), and the earliest identifiable species is Hypsiprymnodon bartholomaii from the Early Miocene.

Classification
Until recently there were only two suborders in Diprotodontia: Vombatiformes which encompassed the wombats and koala and Phalangerida which contained all other families. Kirsch et al. (1997) split the families into three suborders. In addition, the six Phalangeriformes families are split into two superfamilies.

Order DIPROTODONTIA
 * Genus †Brachalletes
 * Genus †Koalemas
 * Genus †Sthenomerus
 * Genus †Nimbadon
 * Family †Maradidae
 * Family †Thylacoleonidae (marsupial lions)
 * Family †Palorchestidae
 * Family †Wynyardiidae
 * Suborder Vombatiformes
 * Family Phascolarctidae: koala (1 species)
 * Family Vombatidae: wombats (3 species)
 * Family †Ilariidae
 * Family †Diprotodontidae
 * Suborder Phalangeriformes
 * Superfamily Phalangeroidea
 * Family Phalangeridae: brushtail possums and cuscuses
 * Family Burramyidae: pygmy possums
 * Superfamily Petauroidea
 * Family Tarsipedidae: honey possum
 * Family Petauridae (striped possum, Leadbeater's possum, yellow-bellied glider, sugar glider, mahogany glider, squirrel glider)
 * Family Pseudocheiridae: ringtailed possums and allies
 * Family Acrobatidae: (feathertail glider and feather-tailed possum)
 * Suborder Macropodiformes
 * Family †Balbaridae : basal quadrupedal kangaroos
 * Family Macropodidae: kangaroos, wallabies and allies
 * Family Potoroidae: bettongs, potaroos and rat-kangaroos
 * Family Hypsiprymnodontidae: musky rat-kangaroo

† means extinct family, genus or species.