tree kangaroos in australia
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This is because a few shrubs are not enough to support so many herbivores. In this light, it was hard to believe when we discovered in 2008 and 2009 the partial skeletons of two new species of giant tree-kangaroo. They are related to the extinct genus Bohra, which was first named in 1982 on the basis of leg bones found in the Wellington Caves in New South Wales. We used Nullarbor skeletons as a guide to search for individual pieces in museums in order to connect the broken links. We discovered more than 100 teeth and bones belonging to a total of at least seven species of extinct tree-kangaroos. These come from fossil sites extending from southern Victoria to central Australia to the New Guinea Highlands, and range in age from 3.5 million (late Pliocene) to a few hundred thousand years old (middle Pleistocene).
A giant leap forward—and then upward—Anatomical and molecular evidence suggests that among living marsupials, kangaroos are most closely related to possums. Due to large gaps in the Australian fossil record, there is no definitive evidence as to when kangaroo ancestors descended to the forest floor. Similarly, we do not know whether the distinctive “bipedal” jump of locomotion evolved while living in trees or on the ground – but we do know that it became an enduring hallmark of the kangaroo family. They have longer legs than their possum ancestors, and the leg bones are fused together in a way that limits sideways leg movement. Adaptations such as high tendon elasticity and a large fleshy tail make kangaroos the most energy-efficient runners on the planet. The foot bones of tree-kangaroos reveal three stages of evolutionary “reversal” of these adaptations.
The Pliocene species of Bohra evolved a wider heel bone and upper ankle joint, which gave them greater mobility. Later, the Pleistocene species of Bohra developed a smooth joint at the front of that heel bone, giving them the ability to wrap the soles of their feet around tree trunks and limbs. As well as having short legs, modern tree-kangaroos (Dendrolagus) have short hindlimbs, with powerful forelimbs and claws combined for grasping and climbing. They can also walk with their hind legs while climbing, while land-dwelling kangaroos only alternate their hind legs when swimming. Why return to trees? As Australia dried out over the past 10 million years, more open vegetation became widespread. This trend was interrupted by a greenhouse phase 50–35 million years ago.
We speculate that the temporary expansion of forest habitats during this period may have opened up new ecological niches that early tree-kangaroos used to evolve. By the time the climate became drier, tree-kangaroos had become established members of the Australian fauna, with the species having adapted to an expansion of woodland and savannah habitats. As some large monkeys do today, Bohra species probably divided their time between living in trees and on the ground, whereas modern tree-kangaroos spend most of their time in trees. So, although we may now think of the tree-kangaroo as the quintessential rainforest animal, it is because the bohra species that lived in other habitats have become extinct. Despite everything we can learn about evolution from the study of modern species, the fossil record has the potential to turn the entire script upside down with a single discovery.
(Gavin Prideaux, Flinders University and Natalie Warburton, Murdoch University)
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