Australian scientists say they have discovered the first evidence that an ancestor of the mighty Tyrannosaurus rex once roamed across Australia.

The finding, published today in the journal Science, fills a major gap in the evolutionary history of T rex and overturns the theory the giant predator was a purely northern hemisphere animal.

It also puts a dampener on hopes of finding a unique Australian dinosaur, says Museum Victoria curator of vertebrate palaeontology Dr Tom Rich.

The discovery is based on a pubic bone found about 20 years ago at Dinosaur Cove, 220 kilometres west of Melbourne in Victoria.

It was made after Dr Rich took a number of isolated and unidentified bones overseas for identification.

Conspicuous feature

Lead author Dr Roger Benson, a research fellow in the Department of Earth Sciences at the University of Cambridge, says he instantly recognised one of the bones belonged to a coelurosaur.

Coelurosaurs are the group of mainly small-bodied, predatory dinosaurs that includes birds at one end and tyrannosaurs at the other, Dr Benson says.

He says the identification was initially based on "one conspicuous feature".

Dr Benson says the far end of the pubic bone was expanded into a "boot" shape fore and aft, but was very narrow across.

"Basically, our [the Museum Victoria] pubis is almost identical to that of T rex, only much smaller," Dr Benson said.

The new species, which Dr Rich says would have been about one-third to one-quarter the size of T rex, shares other features with the giant predator, including short arms and powerful jaws.

"It's much more similar to T rex than one other tyrannosaur (Raptorex, from China) of slightly older age than ours," Dr Benson said.

"We know Raptorex had a robust skull and small arms and we know that our new fossil is from a tyrannosaur even more closely related to T rex. Thus it's most likely the general body plan of our new one was similar."

Surge in discovery

Until recently the only known tyrannosaurs were those like T rex - giant predators from Asia and North America that lived about 70 million years ago, just before the Cretaceous mass extinction, says Dr Benson.

However in the past decade a "surge" in discoveries has revealed diverse types and body sizes in the tyrannosauroid family from up to 170 million years back in the Middle to Late Jurassic.

"It's these discoveries, mostly man-sized or smaller, that have filled in the story of tyrannosaur evolution," says Dr Benson.

"Since all discoveries have been from the northern hemisphere, tyrannosaurs have been considered as northern dinosaurs that might have just never made it down into the south.

"The new discovery shows that this is wrong and that 110 million years ago tyrannosaurs were probably global. This poses a question. Why did tyrannosaurs grow to giant size and dominance in the north, but apparently not in the south?"

Dr Rich says the new species of Tyrannosaurus also shows the likelihood of finding a unique Australian dinosaur is low.

"The picture that seems to be emerging is that dinosaurs were more or less cosmopolitan," he says.

"We are getting elements that look like those found in the northern hemisphere and we haven't found the dinosaur equivalent of the koala; we don't seem to have a unique dinosaur."