New "Destroyer" Dinosaur Found, Was T. Rex Relative

A 29-foot-long (9-meter-long) "destroyer" dinosaur once reigned over the Wild West, according to a new study of a fossil T. rex relative found in New Mexico.

Two nearly complete skeletons of the new species, Bistahieversor sealeyi—eversor means "destroyer" in Latin—were discovered in the desolate badlands of New Mexico's Bisti/De-na-zin Wilderness.

A "teenager's" skeleton was found between 1989 and 1990, and an adult was unearthed in 1998, researchers say. The fossils had been on display at the New Mexico Museum of Natural History until recently, so scientists hadn't previously had a chance to study the remains.

Discovering that B. sealeyi is a primitive Tyrannosaurus rex relative—and, like T. rex, part of a group called the tyrannosauroids—is a "big deal," said study co-author Thomas Carr, director of the Carthage College Institute of Paleontology in Kenosha, Wisconsin.

"In and of itself, a relatively complete dinosaur from 75 million years ago in New Mexico is not common," he said. But "it's doubly rare to have a predator like this."

Scant tyrannosauroid teeth and scraps of bone had previously been found in the Southwest. But they all had come from tyrannosauroid species known to live in the northern Rocky Mountain region.

But B. sealeyi is a completely new species, found nowhere else—proving that the Southwest had its own top predator stalking the tropical forests and rivers of the late Cretaceous period.

When Carr first heard a new tyrannosauroid fossil had been found, "I was very excited, because I knew that if it was complete, we would actually finally know tyrannosauroids were living in the Southwest," he said.

What's more, finding the teenaged B. sealeyi skeleton and partial skull gives the scientists "a really unique snapshot of the biological development of this particular dinosaur," he said.

For instance, the team found that a hole above the adult's eyes—one of many air sacs common in tyrannosaur skulls—was not present in the young dinosaur's skeleton.

This suggests that the hole developed in adulthood, he said, although scientists aren't sure what the hole's function might have been.

B. sealeyi also had a deep snout like T. rex, though the two species are not closely related, Carr said.

Deeper—or shorter—snouts may have evolved in concert with a more powerful bite and smaller forearms in tyrannosauroids in western North America andAsia, which were connected during the Cretaceous.

"The main implement of killing was the head, and they needed the power for that," Carr said. 

But for some reason, tyrannosauroids in eastern North America retained the more primitive features of shallow snouts and large arms.

For B. sealeyi to have a deep snout suggests that the adaptation evolved early in tyrannosauroids—opening up new mysteries in tyrannosaur evolution.

"We can answer what happened, and if we're lucky, we can answer how things happened," Carr said, but "we can't answer why."