By Steve Johnson

Field Museum scientists have discovered a new "top predator" dinosaur in North America, a significant precursor to Tyrannosaurus rex and an important part of an emerging fossil record for the continent, the museum planned to announce early Friday.

The 4-ton, 30-foot animal was discovered in a region of 100-million-year-old rock in Utah during a museum expedition led by Peter Makovicky, curator of dinosaurs, and Lindsay Zanno, then a postdoctoral fellow at the Field.

Siats meekerorum _ named by the scientists after a man-eating monster of legend from the region's Ute Indian people and the Meekers, a museum donor family from Evanston, Ill. _ helps flesh out what has been a skeletal picture of North American wildlife in the tens of millions of years before T. rex was the dominant predator.

In 2008, Zanno spotted the first of the bones on a hillside in Utah's Cedar Mountain Formation, days before the group's relatively unfruitful expedition was to end and as "morale was low," said Zanno, now director of paleontology at the North Carolina Museum of Natural Sciences.

Friday's publication in Nature Communications of Zanno's and Makovicky's paper, "Neovenatorid theropods are apex predators in the Late Cretaceous of North America," marks the official unveiling of the new species.

"The gist of this paper is about the changing of the guard among the top apex predators," said University of Chicago paleontologist Paul Sereno, who read an advance copy of the paper. And while the couple of dozen bones do not include the ideal full-skull find, it is "very significant," he said.

"It's from a really poorly known time period out West. You need these flagpoles," Sereno said. 'They may not be pretty. You're not going to mount (the Siats bones) on a stick. But it's enough" to further the developing belief that "pre-tyrannosaur giants" had radiated across the globe.

Makovicky elaborated.

"It's more than just a new dinosaur," he said. "It's the first evidence we have of a whole new group of dinosaurs in North America. And it challenges a view that was common maybe a decade or so ago where we thought, 'OK, you know, North America, rising sea levels, the species seem to be very unique to North America and only remotely related to (those on) other continents.' That's slowly being challenged. This is another piece of evidence saying, 'No, dinosaurs were actually distributing themselves pretty widely across the continents.'"

The S. meekerorum announcement follows one earlier this month that Lythronax argestes, a smaller T. rex ancestor from 80 million years ago, had been discovered in southern Utah, proving that such predators had been around some 10 million years earlier than previously known.

Because of its size and the history of top predators elsewhere, Zanno and Makovicky theorize, Siats was atop a food chain that included what their paper calls "small-bodied tyrannosauroids," T. rex's ancestors, which hadn't evolved to the size and dominance they would reach before dinosaurs went extinct 65 million years ago.

Siats, though, isn't related to the tyrannosauroid family but rather to the allosauroids. Its best-known relative might be the Giganotosaurus, found in South America.

Makovicky and Zanno chose the discovery site in Utah's Cedar Mountain Formation, in Emery County in the east-central part of the state, because it gave access to a rarely explored time period. The trade-off is that because the area was a lush, wet environment similar to today's Mississippi River Delta, Makovicky said, skeletons there tend to be less well-preserved.

"The one unfortunate thing is that complete skeletons are very rare in this (area)," Makovicky said. "But the positive side seems to be that when you do find something, there's a very high probability you're dealing with a new species."

c)2013 Chicago Tribune