Australia: The Land Where Time Began

A biography of the Australian continent 

Riversleigh Kangaroos

Before the demise the vast Australian rainforests, such as at Riversleigh, there were few if any grasslands and associated grazers like kangaroos. The widespread, diverse kangaroos are a new phenomenon that extends back in time no more than about 4.5 million years, when the ancestors of a number of lines such as the nail-tail wallaby, swamp wallaby and the euro. Alongside them were many other related marsupials. Kangaroos only begin to enter the fossil record some time near the start of the Pleistocene. The appearance of the grazing niche seems to have made its first appearance sometime in the Late Miocene, so the grazers couldn't have evolved before that time.

At present there are 2 kangaroo families, the Protoroidae, including forms such as the rat kangaroos, that are usually omnivorous, and the Macropodidae, normal kangaroos, that are herbivorous. The Banded Hare-wallaby might belong to a third group as distinct as the other 2, based on studies in molecular biology and palaeontology.

The Rat-kangaroos flourished in the earliest known Riversleigh rainforests, coming in a wide variety of forms, from omnivores to leaf-eaters, and even carnivores, about 12 million years before the arrival in the fossil record of the grazing kangaroos. These rat-kangaroos can be divided into at least 4 main groups:

  • hypsipyrmnodontines-an extant member being the Musky Rat-kangaroo from the tropical rainforests of the Atherton Tablelands.
  • potoroines-bettongs and potoroos are living survivors of this group.
  • balungamayines-a huge radiation of forms that are now extinct.
  • propleopines-the large, powerful carnivorous kangaroos, all now extinct.

The propleopines differed most widely from extant kangaroos. One of these, Ekaltadeta ima from the Oligocene-Miocene, standing 1.5 m tall, is the only known member of its genus. It is believed to be ancestral to the Propleopus, another group of carnivorous kangaroos that have been found at many sites in eastern Australia, and possibly existed elsewhere on the continent. Propleopus oscillans that reached up to 2 m tall, is known to have survived until the Late Pleistocene, so would have been living when the first Aboriginal People arrived in Australia.

The balungamayines have a mixture of characteristics that makes it difficult to allocate them to a particular group with confidence. In overall anatomy they resemble rat kangaroos most closely, but they have lophodont teeth as do the macropodid kangaroos. Balungamaya delicata was the first member of the balungamayines to be found.

Members of the Macropodidae are the modern kangaroos that dominate the vast grasslands of Australia. Included in the group, are forest-living browsers such as the forest wallabies of New Guinea, and the tree kangaroos.

There are 3 subfamilies of Macropodidae:

  • macropodines-includes many living kangaroos but none found in the local faunas from the Oligocene-Miocene at Riversleigh.
  • sthenurines-include 1 living species, the Banded Hare-wallaby, and giant kangaroos from the Pliocene and Pleistocene.
  • balbarines-the only extinct kangaroos from Riversleigh and Bullock Creek.

By the time of the formation of the deposits at Rackhams's Roost site at Riversleigh, the balbarines had been replaced by other groups, mostly by the macropodines and it is believed possibly by the sthenurines, but evidence of the latter is lacking.

By the formation of the Terrace Site, the agile wallaby, Macropus agils, a member of the macropodine had appeared, seemingly the only living species found at the site.

Balbaroo fangaroo, a new species of balbarineekangaroo from Riversleigh. Several characters were present in the type specimen partial skulls such askangaroo from Riversleigh. Several characters were present in the type specimen partial skulls such as squamosa-frontal contact on the-frontal contact on the neurocraniummwalls, are argued in the paper, below, to bewalls, are argued in the paper, below, to be pleisomorphicforfor Macropodoidea. The skulls display hypertrophied upper canines, a feature never previously seen in kangaroos. It is believed to be a case of convergent evolution between these and eutherian ungulates.. The skulls display hypertrophied upper canines, a feature never previously seen in kangaroos. It is believed to be a case of convergent evolution between these and eutherian ungulates.

Links

Journal of Paleontology

Sources & Further reading

  1. Michael Archer, Suzanne J. Hand & Henk Godthelp, Australia's Lost World: Riversleigh, world heritage Site, Reed New Holland
Author: M. H. Monroe
Email:  admin@austhrutime.com
Last Updated 25/02/2011

 

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                                                                                           Author: M.H.Monroe  Email: admin@austhrutime.com     Sources & Further reading