Most closely related to modern ducks, the flightless "demon bird" probably resembled an enormous emu or ostrich, although fossil remains are scarce.
A single fossil leg bone from the new, larger find stands as high as his hip, Godthelp says. The bird's entire leg would have towered above his head.
"And I'm not a short chap. I'm well over six foot [two meters]," he said. "So these were big birds, and you can only imagine what some multinational like Kentucky Fried [Chicken] would have done with them."
The fossils come from the Riversleigh fossil beds in northeastern Queensland, an area rich in old river beds that later became riddled with limestone caves.
Acting like pit traps, the caves captured and preserved a wide array of plant and animal species from between 5 million and 20 to 23 million years ago and the environments they lived in.
Such a rich fossil record provides clues about how species evolved in response to climate change in ancient Australia, says Benjamin Kear, who studies ancient kangaroos at the Museum of Southern Australia in Adelaide.
(Read "Weird Australia Rocks Are Earliest Signs of Life, Study Says" [June 7, 2006].)
Riversleigh digs have also uncovered fossils of Earth's oldest known kangaroos and many branches of the kangaroo family tree.
"The beauty of it is they're relatively complete skeletons as well," Kear said. "So you can start looking at things like how hopping evolved, which is quite a novel adaptation for a modern group of animals."
"It's one of the great things about Riversleigh," Godthelp added. "It actually allows us to collect many, many different kinds of things."
Examples from recent years include a number of early crocodile species that were smaller than today's crocs and appear to have been land-based.
It's been suggested that the ancient reptiles may have behaved more like modern-day goannas, or Australian monitor lizards.
The crocs may have climbed trees to escape predators and to leap on prey, scientists theorize.
It's difficult to deduce such behaviors from fossil records alone, however, Godthelp says.
The research team will prepare their new fossils and send them to researchers in Australia and elsewhere for further study.
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