A titanic duel to the death on the banks of a primordial river might have solved one of the greatest riddles of the dinosaur age.
Two monstrous creatures, one a top predator and the other a massive three-horned plant eater, killed each other in a savage battle before being frozen in time, scientists believe.
The carnivore had its skull smashed while teeth were found embedded in the neck of its prey, a Triceratops.
Their fossilised skeletons were found together where the struggle ended 67 million years ago, in a remote and arid region of Montana known as Hell's Creek.
It was only the second time a pair of dinosaurs locked in combat had been unearthed. But what really excited scientists was what the find revealed about the six- to seven-metre long, two-legged meat eater.
At first glance it looked like a smaller version of Tyrannosaurus rex, the apex predator of the Cretaceous era, but there were key differences, in particular its graceful head and large forelimbs.
Scientists believe the fossil provides clear evidence that T. rex shared its habitat with a smaller cousin, Nanotyrannus, in much the same way lions and cheetahs hunt together on the African savannah.
The discovery could end the debate that has raged between experts who believe in Nanotyrannus, and others who say the creature's fragmented fossils belong to T. rex's juvenile offspring.
"This animal had a bizarre, really long neck and a really gracile skull," said Philip Manning, a British member of the research team from the University of Manchester.
"T. rex has distinctive features, one being its very tiny arms. When you look at this specimen you can see the forearms are very large. The hand of Nanotyrannus is huge. Usually during growth the hands start shrinking as an animal gets older.
"This means Nanotyrannus is real. Maybe we're looking at a cheetah of the Cretaceous here."
But just as they appear on the brink of resolving the question, the "beautifully preserved" evidence could be snatched away from them.
There are plans to sell the rock containing the fossils at auction where it could fetch up to $US9 million ($A9.84 million).
Its finders fear it could be snapped up by a rich private owner and lost to science.
