A giant new predatory dinosaur that roamed the Earth about 100 million years ago and even rivalled the mighty tyrannosaurus rex has been discovered.
The new species of carnivorous dinosaur - one of the three largest ever discovered in North America - lived alongside and competed with small-bodied tyrannosaurs.
The species, called Siats meekerorum, was the apex predator of its time, and kept tyrannosaurs from assuming top predator roles for millions of years.
Siats is a species of carcharodontosaur, a group of giant meat-eaters that includes some of the largest predatory dinosaurs ever discovered.
Siats is only the second carcharodontosaur ever discovered in North America; Acrocanthosaurus, discovered in 1950, was the first.
"It's been 63 years since a predator of this size has been named from North America," said Lindsay Zanno, a North Carolina State University paleontologist.
Zanno and colleague Peter Makovicky, from Chicago's Field Museum of Natural History, discovered the partial skeleton of the new predator in Utah's Cedar Mountain Formation in 2008.
The recovered specimen belonged to an individual that would have been more than 30 feet long and weighed at least four tons. Despite its giant size, these bones are from a juvenile.
Zanno and Makovicky theorise that an adult Siats might have reached the size of Acrocanthosaurus, meaning the two species vie for the second largest predator ever discovered in North America.
Tyrannosaurus rex, which holds first place, came along 30 million years later and weighed in at more than twice that amount.
Siats terrorised what is now Utah during the Late Cretaceous period - 100 to 66 million years ago.
"Carcharodontosaurs reigned for much longer in North America than we expected," said Zanno.
In fact, Siats fills a gap of more than 30 million years in the fossil record, during which time the top predator role changed hands from carcharodontosaurs in the Early Cretaceous to tyrannosaurs in the Late Cretaceous.
The lack of fossils left paleontologists unsure about when this change happened and if tyrannosaurs outcompeted carcharodontosaurs, or were simply able to assume apex predator roles following carcharodontosaur extinction.
It is now clear that Siats' large size would have prevented smaller tyrannosaurs from taking their place atop the food chain.
"The huge size difference certainly suggests that tyrannosaurs were held in check by carcharodontosaurs, and only evolved into enormous apex predators after the carcharodontosaurs disappeared," said Makovicky.
"Contemporary tyrannosaurs would have been no more than a nuisance to Siats, like jackals at a lion kill. It wasn't until carcharodontosaurs bowed out that the stage could be set for the evolution of T rex," Zanno added.
