Two front teeth and fused toes define Australasia's marsupial giants
Diprotodontia is the largest living marsupial order, about 155 species including kangaroos, koalas, wombats and possums. Its extinct members ranged from the hippo-sized Diprotodon to Thylacoleo, the marsupial lion. Two traits unite them: a pair of big forward-jutting lower incisors, and two hind toes fused together right up to the claws.
The order's name is Greek for two front teeth. Every member has a pair of large incisors angling forward from the lower jaw, a feature shared with many early mammals. The jaw is compact, typically holding three upper incisor pairs and lacking lower canines, although wombats, like rodents, keep only one upper pair. The second hallmark is syndactyly: the second and third toes of the hind foot are joined as far as the base of the claws, which remain separate, while the fifth toe is usually missing and the fourth often greatly enlarged.
Fused toes are generally thought to have helped ancestral diprotodonts climb, yet many living species are strictly ground dwellers with feet reshaped for that life. Tree-kangaroos have an especially tangled story: their distant forebears apparently lived in trees, came down and acquired long kangaroo feet, then climbed back up, evolving shorter, broader hind feet and a new way of scaling trunks.
Nearly all diprotodonts, living and extinct, eat plants. A few take insects or mixed diets, and potoroos and their relatives are almost unique among vertebrates in living largely on fungi, apparently recent departures from plant-eating. Only the extinct marsupial lions were carnivores on a large scale. The group is confined to Australasia; its oldest fossils date to the Late Oligocene, 23.03 to 28.4 million years ago, but their variety implies an earlier start, hidden by gaps in Australia's fossil beds and an almost empty record in geologically restless New Guinea.
Many of the biggest, least agile species vanished with other megafauna after people reached Australia about 50,000 years ago, more likely through habitat change driven by fire than through hunting alone. Classification once recognised just two suborders, wombats and koala versus everyone else, until Kirsch and colleagues split the order into three in 1997, and kangaroos may nest inside the possum branch. Living families run from the single-species koala and three wombats to gliders, cuscuses and the musky rat-kangaroo, while fossils add extinct marsupial tapirs.
Source: Diprotodontia
