Diprotodontia
Vombatiformes
Phalangerida
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Diprotodontia is a large taxon of about 120
marsupial mammals including the
kangaroos,
wallabies,
possums,
Koala,
wombats, and many others. Extinct members include the giant
Diprotodon family, and
Australasia. The earliest known fossils date to the late
Oligocene. However it is certain that their genesis lies earlier than that: there are large gaps in Australia's fossil record and virtually no record at all in geologically active
New Guinea; the great diversity of known Oligocene diprotodonts suggests that they began to diverge from one well beforehand.
Many of the largest and least atheletic diprotodonts (along with a wide range of other Australian
megafauna) became extinct when humans first arrived in Australia about 50,000 years ago, possibly as a direct result of hunting, but more probably as a result of widespread habitat changes brought about by human activity, fire in particular.
There are two key anatomical features that, in combination, identify the diprotodonts. The first of these is that they are
diprotodont: they have a pair of large, procumbent incisors on the lower jaw. This is 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.
Secondly, diprotodonts exhibit
syndactyly: they have the second and third digits of the foot fused together up to the base of the claws, leaving the claws themselves separate. Digit 5 (the hallux) is usually absent, and digit 4 is often greatly enlarged.
Syndactyly is not a particularly common condition (though the
Australian omnivorous marsupials share it) and is usually explained as an adaptation to a climbing lifestyle. Many modern diprotodonts, however, are strictly terrrestrial, and have extensive further adaptations to the feet to suit that lifestyle. This makes the history of the
tree kangaroos particularly convoluted: it appears that they had an aboreal lifestyle at some time in the far distant past, then became adapted to a terrestrial existence, developing their very long kangaroo feet, and finally returned to the trees once more, complete with a shortening and broadening of the hind feet and a novel climbing method.
Order Diprotodontia
