Physical characteristics

Diprotodont marsupials are united by two important characteristics that belie their great divergence in size and overall form. The first concerns dentition (the arrangement of teeth) in the lower jaw. Koalas, wombats, kangaroos, and possums all have only two developed incisor teeth at the front of the lower jaw. These are large and often project forwards as an adaptation to cropping vegetation. A second pair of very small incisors is present in some species, but there are no lower canines, just a gap between the incisors and the cheek teeth. This arrangement is known as diprotodonty (literally translated this means "two first teeth")—hence the ordinal name Diprotodontia.

The second major unifying characteristic of diprotodonts is syndactyly. This means "fused toes," and refers to the structure of the hind feet, the second and third digits of which are always fused together forming a strange-looking double toe with two claws. The twin claws of the fused digits are retained and in most species serve the useful function of a grooming comb for removing caked dirt or other debris clinging to the animal's fur. Another shared feature of many diprotodonts is the arrangement of digits on the front paws. In most climbing species, the first two fingers oppose the other three, allowing the animals to maintain a firm grip of branches and stems. A notable exception to this rule is the brushtailed possum, whose forepaws are more like tiny nimble hands.

Members of the suborder Vombatiformes are set apart from other diprotodonts, being rather squat and heavy, with a pouch the opens to the rear. In burrowing wombats this prevents the pouch filling up with soil, whereas in koalas it is a rather inconvenient arrangement inherited from non-climbing ancestors. Other diprotodonts, members of the suborder Phalangerida, have a more athletic build with a long tail. In many possums the tail is prehensile. It may be thin as in po-toroos or muscular as in large kangaroos, very long and brushy (as in Leadbeater's possum, Gymnobelideus leadbeateri) or virtually naked. Phalangerids have retained the forward opening pouch of their ancestors as a secure means of transporting young while climbing or hopping. In members of the super-family Macropodoidea (kangaroos, wallabies, and rat-kanga-

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