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A team of researchers have announced the discovery of a new species of Tyrannosaurus from New Mexico, one that appeared in the fossil record five million to seven million years before the familiar tyrant lizard. Their research, published Thursday in the journal Scientific Reports, suggests a new chapter could be added to the origin story of Tyrannosaurus rex.

When staff from the New Mexico Museum of Natural History and Science collected the partial skull of a large adult tyrannosaurus from the Elephant Butte Reservoir in the state in the 1980s, they initially assumed the fossils belonged to T. rex. But when Sebastian Dalman, a paleontologist at the museum, began working on the specimen in 2013, he noticed subtle but consistent differences between it and other T. rex skulls.

Rather than the deep bone-crushing jaws of an adult T. rex, the lower jaw of the reservoir specimen looked more slender. Its teeth were different, and the animal lacked the prominent ridge of bone found behind T. rex’s eye, Mr. Dalman said. Scientists estimate that the animal was approximately 39 feet long, around the same length as an adult T. rex.

T. rex fossils are believed to be 66 million to 68 million years old, the period recorded in the Hell Creek Formation of the Plains states, said Spencer Lucas, paleontology curator at the museum and an author on the paper. When the fossil was initially discovered, researchers initially assumed the rock layers that produced it — the McRae Formation of New Mexico — belonged to the same period. But the team’s dating of the rocks now suggests that the McRae Formation was 5 million to 7 million years older than Hell Creek, and that the specimen they found came from an earlier relative.

“Most species are not around for more than a million years,” Dr. Lucas said. “The age argument really is matched by the anatomy.”

The researchers argue that this is enough evidence to conclude that the skull belongs to a distinct species, which they’ve named Tyrannosaurus mcraeensis after the formation where the specimen was found.

Over the past few years, some scientists have argued that certain fossils assigned to T. rex actually represented new species — and have faced serious pushback from other scholars. Researchers in the field have generally been reluctant to split apart such an iconic and well-studied species without overwhelming evidence.

“This is going to get a lot more scrutiny than the average newly named dinosaur,” said Dave Hone, a paleontologist at Queen Mary University of London who was not involved with the paper.

But he and other outside researchers have reacted more warmly to T. mcraeensis than to other purported Tyrannosaur discoveries, noting that the authors have made a reasonable and compelling case. That the remains seem both outside the usual range of variation for T. rex and comfortably separated in time strengthens the argument, said Thomas R. Holtz Jr., a paleontologist at the University of Maryland.

The find also has interesting implications for the timeline of tyrannosaurs’ evolution, Dr. Holtz said.

Throughout the late Cretaceous period, Tyrannosaurus relatives ranged across what is today western North America, from Alaska to Mexico. By the last few million years of the dinosaurs’ reign, however, the larger T. rex replaced these lineages.

Since the closest-known relatives of Tyrannosaurus are from Mongolia, Dr. Holtz said, some researchers argued that Tyrannosaurus represented a specific group that crossed a land bridge from Asia. But the new discovery suggests that the Tyrannosaurus lineage appeared in North America five million to seven million years earlier than expected, and may have originated in the Southwest before spreading north.

There’s obviously more going on than just a move north given that we have the Asian lineages too,” Dr. Hone said. “Big tyrannosaurs were getting around.”

If Tyrannosaurus did appear in the Southwest, the team suggests, the genus’s giant size may have evolved to prey on the enormous herbivores found in the region, like Alamosaurus, a sauropod that grew up to 100 feet long, and Sierraceratops, an earlier relative of Triceratops. But what caused such a landscape of comparative giants to appear in a specific region of North America remains a mystery.

“I think we need to spend more time looking at the Southwest,” said Nick Longrich, a paleontologist at the University of Bath in England and an author on the paper. “There are a lot of poorly studied areas where we’re going to find new things.”


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