Wombats Koala Possums Wallabies and Kangaroos: Diprotodontia

Diprotodonts are an order of about 131 species of marsupial mammals that live in Australia, New Guinea, and parts of Indonesia. The order also contains a family of giant diprotodonts that are now extinct. Within this order are some of Australia's best known marsupials, including the kangaroos, koalas, and wombats, as well as some of the least known species such as cuscus and potoroos.

Diprotodonts have evolved to fill almost every terrestrial (land) ecological niche, and as a result, they have evolved special physical features that allow them to live most efficiently in their chosen environment. For example, some tree-dwelling (arboreal) gliding possums have evolved a skin membrane that stretches from wrist to ankle and acts as a parasail, allowing them to stay away from predators, animals that hunt them for food, and conserve energy by "flying" from tree to tree. Wombats have evolved strong claws and short, stocky bodies well suited for digging. Kangaroos have strong hind limbs that allow them to race across open grassland at speeds up to 35 miles (55 kilometers) per hour and to leap distances of up to 30 feet (9 meters). Possums and cuscus have evolved tails that can curve around and grasp a branch (prehensile tails).

As a result of this diversification, the species in this order look very different from one another. However they all share at least two physical characteristics that include them as diprotodonts. All members of this order have two large incisor teeth on the lower jaw. Incisors are front teeth that are modified for cutting. These teeth are also noticeable in more familiar rodents such as beavers and rabbits. Most members of this order also have three pairs of incisors on the upper jaw, and a few species have a second small pair on the lower jaw as well. In addition, members of this order have no canine teeth. Canine teeth are sharp, pointed teeth used for tearing food, and are located between the incisors in the front and the molars (grinding teeth) in the back. Diprotodontia have an empty space where canine teeth usually are located. This pattern of teeth has evolved because most members of this order are herbivores, or plant eaters. They need sharp front teeth to clip off the tough grasses and other plants that make up most of their diet, and they need molars to grind the plants, but they do not need canines to tear their food apart the way carnivores (meat-eaters) do. A few species in this order now eat insects, invertebrates, or flower nectar, but their tooth pattern suggests that at one time during their evolution, they also ate plants.

Besides sharing a common pattern of teeth, all diprotodonts have a condition in their hind limbs called syndactyly (sin-DACK-tuh-lee). Syndactyly means "fused toes." In members of this order, bones of the second and third toe on the hind feet have grown together into a single bone as far down as the claw. However, this fused bone has two separate claws—this twin claw is used for grooming. In many species in this order, the fourth hind toe is enlarged, and the fifth toe is either very small or absent.

On the front limbs of many species, the first two fingers oppose the other three. This means that these fingers, like the thumb on a human hand, can reach across and touch the tip of the other three fingers (unlike, for example, a dog paw or human foot where none of the toes can bend to touch each other). This adaptation is found mainly in species that live in trees, as it helps them grasp branches and climb.

Diprotodonts are marsupials, and like all marsupials they give birth to very poorly developed young after a short pregnancy. The young then attach to teats (nipples) in the mother's pouch and are carried for weeks or months until they mature enough to live independently. All diprotodonts have forward-opening pouches (like the kangaroo) except for wombats and koalas. Wombats are burrowing animals. A backward opening pouch is an advantage when digging, because it will not fill up with dirt. The backward opening pouch of the koala, which lives in trees, may be left over from a time when its ancestors lived on the ground and dug like the wombat.

Diprotodonts have soft fur, and many species have been hunted for their skins. Most species are earth tone colors, grays and browns, but some have quite eye-catching coloration, such as the yellow-footed rock wallaby, whose patches of red, yellow, and white contrast with its gray fur. Diprotodonts range in size from the red kangaroo, which weighs up to 187 pounds (85 kilograms) to the little pygmy possum, which weighs only about a quarter of an ounce (7 grams). In the past, the fossil record shows that there were much larger diprotodonts living in Australia. These animals became extinct about 50,000 years ago when humans first appeared in Australia.