Diprotodontia (pronounced /daɪˌproʊtɵˈdɒnʃⁱə/; Greek, meaning "two front 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".
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 3 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 5 is usually absent, and digit 4 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.
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