Diprotodontia

• 1. Introduction

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  Diprotodontia is the largest living order of marsupials, and it holds about 155 species. Kangaroos and wallabies belong here. So do possums, the koala, and wombats. But the order once held far stranger residents. One was Diprotodon, an animal that grew to the size of a hippopotamus. Another was Thylacoleo, nicknamed the marsupial lion. This single order stretches from a thumb-sized glider to a herbivore that weighed as much as a car. It carries the only marsupials known to have hunted as large-scale carnivores. It also carries a few that abandoned plants for fungus. What links a koala to a kangaroo, and a giant wombat to a honey possum? The answer hides in two teeth and a pair of fused toes. Why did the heaviest members vanish so suddenly? And how did one branch climb down from the trees, only to climb back up again? The story begins with what these animals eat.

• 2. Diet And Diversity

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  Living diprotodonts are almost all herbivores, and most of the extinct ones ate plants too. A few exceptions break the pattern. Some are insectivorous, and some are omnivorous. The Potoroidae stand out more sharply. They are almost unique among vertebrates in being largely fungivorous, living chiefly on fungus. This habit appears to be a recent twist rather than an ancient one. It seems to have arisen as a relatively recent adaptation from the mainstream herbivorous lifestyle. Against this backdrop of grazers and browsers, one group did something no other diprotodont managed. The extinct thylacoleonids, the so-called marsupial lions, are the only known group to have exhibited carnivory on a large scale. Diet, then, was a line that most members never crossed.

• 3. A Continent Apart

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  Diprotodonts are restricted to Australasia, and their fossil trail is frustratingly thin. The earliest known fossils date to the late Oligocene. Their true origin certainly lies earlier than that. Large gaps run through Australia's fossil record. New Guinea is worse still, with virtually no fossil record at all, a consequence of being geologically active. Yet the few Oligocene fossils that survive already show great diversity. That range of forms suggests the order began to diverge well before the Oligocene. To explain so many distinct animals so early, their split must reach deeper into the past than the rocks reveal. The bones that do exist place the dawn of the group between 23.03 and 28.4 million years ago.

• 4. Two Teeth, Fused Toes

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  Diprotodont means two front teeth, a name built from the Ancient Greek words for two, first, and tooth. Members of the order carry a pair of large, procumbent incisors on the lower jaw. Those forward-jutting front teeth echo a common feature of many early groups of mammals and mammaliforms. The jaw itself is short. It usually holds three pairs of upper incisors, though wombats, like rodents, have only one pair. There are no lower canines. The second defining trait lies in the foot. It is called syndactyly, a fusing of the second and third digits up to the base of the claws. The claws themselves stay separate. Digit five is usually absent. Digit four is often greatly enlarged. These two features, working together, are what identify the order at all.

• 5. Feet That Changed Their Minds

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  Syndactyly is not particularly common, though the Australian omnivorous marsupials share it. It is generally posited as an adaptation to assist in climbing. That explanation runs into a problem. Many modern diprotodonts are strictly terrestrial, living entirely on the ground. They have evolved further adaptations to their feet to better suit that life. The tree-kangaroos carry the most convoluted history of all. It appears they were arboreal in the far distant past. Then they moved to the ground, gaining long, kangaroo-like feet in the process. Then they returned to the trees. There they developed a shortening and broadening of the hind feet and a novel climbing method. A foot built for branches became a foot for open country, then bent back toward branches once more.

• 6. The Great Vanishing

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  Humans first arrived in Australia about 50,000 years ago, and the largest diprotodonts did not survive the encounter. Many of the biggest and least athletic members of the order disappeared around that time. They fell alongside a wide range of other Australian megafauna. The cause is not settled. Their extinction possibly occurred as a direct result of hunting. The more probable explanation points elsewhere. It lies in widespread habitat changes brought about by human activities, notably the use of fire. The animals that thrived in the old landscape could not hold on as that landscape was reshaped. The survivors of the order are the smaller, nimbler kin of the giants that were lost.

• 7. Sorting The Family Tree

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  Until recently, Diprotodontia held only two recognized suborders. Vombatiformes covered the wombats and the koala. Phalangerida held everything else. That arrangement gave way to a finer one. Kirsch and colleagues, in 1997, split the families into three suborders. Within that scheme, the six Phalangeriformes families are split into two superfamilies, Petauroidea and Phalangeroidea. The Macropodiformes are probably nested within the Phalangeriformes. Whether they sit closest to Phalangeroidea or to Petauroidea is still debated. The living families fan out widely. Macropodiformes gathers the kangaroos, wallabies, bettongs, potoroos, and the musky rat-kangaroo. Petauroidea holds the honey possum, the sugar glider, Leadbeater's possum, and ring-tailed possums. Phalangeroidea holds the pygmy possums and the brushtail possums and cuscuses. Vombatiformes keeps the koala, a single species, and the wombats, of which three species remain. Among the extinct branches sit the Palorchestidae, the marsupial tapirs, a name that hints at how much this order once contained.