'The tipping point': First T. rex mass death site in southern US, found in Utah, strengthens evidence of pack behavior

The Tyrannosaurus rex may not have been as solitary as we believed.

In a groundbreaking discovery of the first T. rex mass death site in the southern U.S., announced Monday by the Utah Bureau of Land Management, scientists found evidence of packlike behavior among the famous ancient predator in the Grand Staircase-Escalante National Monument.

"The new Utah site adds to the growing body of evidence showing that Tyrannosaurs were complex, large predators capable of social behaviors common in many of their living relatives, the birds,” said Dr. Joe Sertich, curator of dinosaurs at the Denver Museum of Nature & Science.

“This discovery should be the tipping point for reconsidering how these top carnivores behaved and hunted across the northern hemisphere during the Cretaceous.”

"Hollywood" specimen, same species as Teratophoneus, discovered approximately two miles north of the "Rainbows and Unicorns Quarry" on Grand Staircase-Escalante National Monument on February 26, 2019. BLM photo courtesy of Dr. Alan Titus.

In the past, paleontologists have long debated whether the huge dinosaurs lived and hunted alone or in groups.

However, with other findings of pack formations in Alberta, Canada, and Montana, the Utah finding may fossilize the belief of a social T. rex.

In the Canadian discovery, 12 individuals found over 20 years ago by Dr. Philip Currie, many scientists doubted T. rexes had the brainpower to organize into anything complex and thought it was an isolated case. Montana's site built upon the social theory, but now this third site may bring more certainty to the idea.

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At the Rainbows and Unicorns site in the Kaiparowits unit of the monument, named for the unbelievable discoveries found there, scientists have been working toward the social dinosaur conclusion since 2014.

“We realized right away this site could potentially be used to test the social tyrannosaur idea. Unfortunately, the site’s ancient history is complicated,” said Dr. Alan Titus, a BLM paleontologist.

A pack of four, possibly five, Teratophoneus T. rexes seemed to have died in a seasonal flood after a slow-burn fire between 66 and 100 million years ago. Turtles, fish, rays, alligators and two other kinds of dinosaurs were also found during the dig.

Later, their bones were exhumed by a flowing river and reburied, making the find more perplexing.

The research of Dr. Celina Suarez, an associate professor of geology at the University of Arkansas, and her former Ph.D. student, Dr. Daigo Yamamura, definitively showed the dinosaurs were moving in a pack.

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"None of the physical evidence conclusively suggested that these organisms came to be fossilized together, so we turned to geochemistry to see if that could help us," Suarez said. "The similarity of rare earth element patterns is highly suggestive that these organisms died and were fossilized together."

Excavation will continue "into the foreseeable future," according to a press release, and will include more research into the T. rex's behavior.

Follow K. Sophie Will on Twitter at @ksophiewill.

This article originally appeared on St. George Spectrum & Daily News: T. rex mass death site, first in southern US, found at Utah monument