Siats meekerorum: New Carnivorous Dinosaur Species Kept T. Rex in Check

Paleontologists have discovered a huge new apex predator dinosaur that lived alongside and likely competed with early tyranosaurs around 98 million years ago.

Siats meekerorum, as the new species has been dubbed, takes its name from a cannibalistic man-eating monster from tribal legends of the Ute people who live chiefly in Colorado and Utah. Siats (prounced SEE-atch) is one of the three largest carnivorous dinosaurs ever discovered in North America. As a species of carcharodontosaur, Siats comes from a line of distinguished predators. The carcharodontosaur group of dinosaurs includes some of the largest predatory carnivore dinosaurs ever discovered.

Only one other carcharodontosaur, the Acrocanthosaurus, has been discovered in North America. Discovered more than half a century ago, Acrocanthosaurus lived about 10 million years before its newfound relative.

"It's been 63 years since a predator of this size has been named from North America," said Lindsay Zanno, a paleontologist at North Carolina State University and the North Carolina Museum of Natural Sciences.

"You can't imagine how thrilled we were to see the bones of this behemoth poking out of the hillside," said Zanno, who is the lead author of a Nature Communications paper describing the new species.

Zanno, along with Peter Makovicky, from Chicago's Field Museum of Natural History, discovered the dinosaur bones in Utah's Cedar Mountain Formation in 2008. The dinosaur's species name "meekerorum" is an homage to the Meeker family, who provided support to Zanno and other young paleontologists.

The Siats meekerorum fossils found in Utah do not compose a complete skeleton, but based on what was found, the paleontologists estimate the carnivore would have been more than 30 feet long and weighed at least 4 tons.

But the Siats specimen is believed to be a juvenile, which would mean in adulthood the dinosaur could have been as large as the Acrocanthosaurus, which is the second largest predator ever found in North America, behind only the Tyranosaurus rex.

Paleontologists say Siats terrorized what is now Utah during the Late Cretaceous period (100 million years ago to 66 million years ago). The top meat-eating predator during this time was previously unknown.

"Carcharodontosaurs reigned for much longer in North America than we expected," Zanno said, adding that the find fills a 30 million year gap in the fossil record prior to the Tyranosaurs rex ascending the throne as the land's top predator. But due to a lack of a more comprehensive collection of fossils, it is unclear whether the T. rex outcompeted Siats to become to top predator or if it took the helm simply because Siats went extinct. Because Siats was such an enormous creature, smaller tyranosaurs would not have been able to usurp it as the top predator of the time, the paleontologists said.

"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," Makovicky said.

Zanno added: "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."

The paleontologists added that in addition to the Siats bones, the dig site also yielded two other new dinosaur species, which the researchers intend to introduce soon.

"Stay tuned," Zanno said. "There are a lot more cool critters where Siats came from."