Tyrannosaur rival S. meekororum ruled the western U.S. in the late Cretaceous

This 30-foot-long allosauroid dinosaur, shown in an artist's rendering, kept the family of Tyrannosaurus rex from assuming the predator crown for millions of years, a study suggests. (Jorge Gonzales / Field Museum)

Meet the rex wrecker, a 3-ton competitor to tyrannosaurs who stoked a family rivalry over millions of years in western North America.

The fossil find in central Utah, dubbed Siats meekerorum, was from the Allosauroid family, weighed around 3 tons, was as long as a boxcar and roamed what now is the intermountain West of the United States around 98 million years ago, according to a study of the find published online Friday in the journal Nature Communications.

S. meekerorum probably competed alongside members of the tyrannosauroid mega-family before it faded away, opening up the evolutionary niche that allowed for new, massive species including Tyrannosaurus rex, the authors suggest.

The novel genus and species helps fill part of a 70-million-year gap in dinosaur history during the late Cretaceous period, a crucial time of local environmental change spurred by an encroaching inland sea that would eventually isolate what now is the western United States, according to North Carolina State University paleontologist Lindsay E. Zanno.

“This is a good case study in how ecosystems change over time, and honestly that’s very poignant for us,” said Zanno, who directs the paleontology laboratory at the North Carolina Museum of Natural Sciences in Raleigh. “This is a time when we see temperatures rising, we see sea levels rising – there’s actually a shallow seaway encroaching onto the continent of North America at this time that will go on to divide North America into several island land masses in the late Cretaceous.”

During that time, the ancestors of T. rex were relatively diminutive, about the size of a Great Dane, and then rapidly evolved into the well-known monsters that are stars of museums and movies.

“We know that the tyrannosaurs were the dominant predators for the last 15-20 million years of the Cretaceous, and prior to that, we know that the precursors to Tyrannosaurus rex were much smaller animals,” said fellow paleontologist and coauthor Peter J. Makovicky, associate curator of dinosaurs at Chicago’s Field Museum of Natural History.

“So just when they went from being mid-sized to very large dominant apex predators is unknown, and why they shifted their ecological role is also something we haven’t had a lot of clarity on,” Makovicky said.

Fossils of tyrannosauroids also have been found in the same central Utah sedimentary deposits explored by the pair, suggesting that S. meekerorum lived alongside its rivals near the lush and humid shoreline of ancient Laramidia during the last 20 million years of the Cretaceous.

The expeditions did not uncover any skulls or teeth, but they collected and studied enough vertebrae, limbs and foot bones from two individuals to establish that they represented a novel genus and species of meat-eating allosauroids.

Paleontologists have scoured the region, near the San Rafael swell and not far from Interstate 70, but have focused on microfossils and small vertebrates, according to Zenno and Makovicky.

“For the most part, they just step over the dinosaurs,” Zenno said. “We’re the first really concerted effort to go back year after year and systematically try to dig up the dinosaurs from this area.”