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Like jackals ceding prey to a lion, tyrannosaurs were relegated to the shadows when these guys came thudding past.

About 100 million years ago, at a time when T. rex's smaller ancestors were in the early stages of their evolution, the day's apex predators were a family of giant, fast-moving carnivores called carcharodontosaurus. And if a recent discovery in Eastern Utah is any indication, they are every bit as worthy of your nightmares.

Two scientists announced Friday in the journal Nature Communications that at the Cedar Mountain Formation's Mussentuchit Member (just southwest of the San Rafael Swell) they uncovered bones from one of the three largest dinosaur carnivores ever found in North America. And it was just a kid.

They dated the find to 98 million years ago. The juvenile "Siats meekerorum" was "the major predator on the block," says North Carolina State's Lindsay Zanno, who did her masters and graduate work at the University of Utah and first spotted the animal's hipbone in a hillside in 2008.

Zanno said projections show it would have stretched more than 30 feet and weighed more than 4 tons.

"This guy was a colossal animal," Zanno says. And the only similar-sized carcharodontosaur discovered in North America, the Arcocanthosaurus, is thought to have lived about 10 million years earlier, meaning Siats may have been unrivaled for millions of years.

T. rex, which ruled the range 67 million years ago, probably had a little more bulk. Zanno says that more sophisticated modeling has led scientists to determine that he was almost twice as big as Siats, but that "comparing apples to apples," he was perhaps a mere ton heftier.

The find bolsters suspicions that earlier tyrannosaurs and carcharodontosaurs shared the same ecosystem because both were found in rock dated to the same period, Zanno said.

"This is one of the first times we've found species of both types in the same environment," she said. "We have good evidence that Siats was completely dominating the top predator role."

But for whatever reason, it seems that tyrannosaurs were better suited to rising sea levels and increasing humidity. The supply of Siats' likely meal of choice — long, big-necked dinosaurs — also began to dwindle during this period.

Co-author Peter Makovicky, who works at the Field Museum of Natural History in Chicago, thought to name Siats (pronounced see-otts) after a man-eating monster of Ute tribal lore.

A man who answered a phone number for the Ute Tribe's Bulletin and Public Relations Department took exception, saying "I don't find it too amusing. We're too busy naming dinosaurs, school mascots, after Native Americans."

He declined to give his name.

Informed of his displeasure, Zanno says she wishes in retrospect they had asked the tribe, but "We just thought it was a great fit for our animal. The intention here is good, and we understand that the native peoples were in large part responsible for the protection of these resources."

Twitter: @matthew_piper