A newly discovered 4-ton predatory dinosaur dominated Earth long before T-Rex appeared on the scene.

Calculations suggest that Siats meekerorum was at least 30 feet long, making it the third largest predator ever found in North America. But that’s a conservative estimate because the bones found were from a juvenile, and a full-grown Siats (pronounced see-atch) may have been even bigger, says paleontologist Lindsay Zanno of the North Carolina Museum of Natural Science, who discovered the species.

“This thing is gigantic,” Zanno said. “There’s simply nothing even close in this ecosystem to the size of this animal that could’ve been interpreted as an apex predator.”

The giant dinosaur was discovered in 2008 when Zanno and her colleague Peter Makovicky of Chicago’s Field Museum of Natural History saw fragments of black fossilized bones poking out of the hills in Utah’s Cedar Mountain Formation. They named the dinosaur Siats meekerorum after man-eating monsters in Ute tribal legend and the Meeker family, which has supported early career paleontologists at the museum. The results of the study were published today in Nature Communications.

Despite a similar physical appearance to a tyrannosaur, Siats is a type of carcharodontosaur. They are more slender than* Tyrannosaurus rex* and have three digits instead of two.

The discovery helps fill in a gap in dinosaur history of nearly 60 million years. The first carcharodontosaur, Acrocanthosaurus, was discovered in 1950 and roamed the land 120 million years ago. T-rex is known to have dominated the late Cretaceous period 60 million years ago. But little was known about the dinosaurs in between.

“It’s not only a new cool dinosaur, but from my perspective, it alters our picture of the entire ecosystem of that time,” said Peter Roopnarine, an ancient ecologies expert and curator at the California Academy of Sciences.

Before the latest discovery, the period between Acrocanthosaurus and Tyrannosaurus rex was an ecological blank except for some smaller carnivores and tyrannosaurs.

“I would argue then that the presence of these much larger carnivores than the tyrannosaurs at this time suggests we have a really full developed energetic ecosystem,” said Roopnarine. “The top was already in place. There were just different groups occupying it.”

Credit: Lindsay Zanno / Artist: Julio Lacerda )

The discovery of Siats, though, means that the carcharodontosaur family ruled the ecosystem 20 million years longer than originally thought, says vertebrate paleontologist Michael D’Emic of Stonybrook University. This is also the first North American carcharodontosaur found outside of Wyoming, Oklahoma and Texas.

“It extends the range of this group both in space and in time so we learn some more about this enigmatic group of dinosaurs from this new discovery,” D’Emic said.

At some point between 98 million and 80 million years ago, Siats likely went extinct and the tyrannosaurids became the apex predators. But at the time they co-existed, the tyrannosaurids would’ve been bullied by Siats and probably tried to steer clear of them because of the size difference, Zanno says.

Earlier this month another dinosaur was discovered that lived during the 120 million to 60 million year gap. Lythronax argestes (below), which translates to “king of gore,” was an ancestor to T-rex. The 80 million-year-old fossil was found by a team led by Mark Loewen of the University of Utah in the Grand Staircase-Escalante National Monument in south-central Utah. It is the oldest known relative of T-rex and likely upset Siats as top predator.

But that still leaves several big gaps in dinosaur evolution. “We’re trying to flesh out what the entire dinosaurian ecosystem was at that time,” Zanno said. “Siats is just the first.”