"Previously they were thought to be intimately related with modern kangaroos, but we have demonstrated that they're not," he said.
"What this means is that kangaroos are much more diverse than people have given them credit for in the past."
While kangaroos are a dominant feature of today's Australian fauna, 25 million years ago they were only a small part of a much more diverse biosphere, he explained.
"You had a whole range of other animals—giant cow-like things called diprotodontids which were also marsupials and a range of chameleon-sized animals."
There were also big carnivores, Kear said.
"Things like killer possums with teeth like garden shears [and] the ancestors of the modern Tasmanian tiger, these things were probably creeping around, ambushing kangaroos which were sitting there, chomping on their fallen fruit."
(Read related story: "Killer Kangaroo Had Wolf-Like Fangs, Scientists Say" [July 25, 2006].)
Going for the Record
Kear and his team use fossil kangaroos as "climate markers" by comparing their evolution and divergence with evidence of Australia's changing environment, he added.
The team presents its findings in the November issue of the Journal of Paleontology.
Gavin Prideaux, a paleontologist at Flinders University in Adelaide who was not involved in the study, said the rich fossil record of ancient kangaroos made them an ideal species for gauging climate trends.
"You can make inferences based on things like how they ran, whether they were adapted to climbing trees or whether they were better adapted to more open terrain," Prideaux said.
Prehistoric kangaroos' unique dental features can show whether the animals were adapted to a browsing forest environment or a grazing grassland one, he explained.
"The record is filling out more and more. That allows us to make more precise postulations about what happened and when," he said.
A small marsupial living today called the musky rat kangaroo might offer clues about Nambaroo, he added.
"Like Nambaroo, it has a flexible big toe on the hind foot, and it has an ankle joint that is very similar in a lot of ways—sort of midway between a hopping kangaroo and a possum—and Nambaroos probably moved in a very similar way.
"The balbarids were the first group of kangaroos to move into the leaf-eating niche," he said, "but for whatever reason, the hoppers basically usurped them.
"For me, that's the really interesting question: What was it about hopping that conferred such an advantage?"
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