Researchers say they have identified a new species of Tyrannosaurus that traces the dino's lineage to North America much earlier than previously thought, and could shed new light on the murky origins of the Tyrannosaurus rex.
T. mcraeensis, the newly named species, is the closest known relative of the T. rex, according to a paper recently published in the scientific journal Nature.
The discovery was the result of a study of incomplete skull and jaw fossils that were collected in southern New Mexico in the 1980s, which now are on display at the New Mexico Museum of Natural History and Science.
"We thought that it was anatomically different enough that it deserved to be a new species," said paleontologist Spencer Lucas, a co-author of the study and a curator of paleontology at the museum.
The previously unknown species had a longer, shallower lower jaw than its cousin, indicating a less forceful bite, according to Lucas. The New Mexican fossils also lack ridges behind the eyes, a distinct feature of the T. rex, that the paleontologist says would have allowed the animals to recognize members of their own species.
The newly identified tyrannosaur is said to predate the T. rex by about 6 to 7 million years, and rivals it in size at about 40 feet long.
T. mcraeensis adds new questions to the ongoing debate over the origins of the T. rex. The T. rex appeared in North America as far back as 68 million years ago, its fossils popping up in modern-day Montana and Wyoming. Until now, however, no close relatives of the species had been recognized in North America, according to the paper.
Some paleontologists believe the Tyrannosaurus migrated to North America from Asia across a land bridge. T. mcraeensis — up to 73 million years old — challenges that origin story, says Lucas.
"This fossil suggests something different may have been going on — that Tyrannosaurus is in North America millions of years earlier than we knew it was here," he said. "Therefore, it's quite possible that Tyrannosaurus originated in North America. And not just in North America, but in the Southwest."
Some in the field are skeptical of the paper's findings. "I hesitate to regard Tyrannosaurus mcraeensis as being distinct from Tyrannosaurus rex," Jared Voris, who studies tyrannosaur biology at the University of Calgary in Canada but wasn't a part of the study, told NewScientist.
Lucas, the co-author, welcomes the counterarguments.
"Hopefully, what will happen now is, the doubters and maybe even the supporters will want to come and study the original fossil to make their own assessment of: How anatomically distinct is it? Do I agree with all the anatomy reported?" he said.
"That's the way the science will progress," he added.
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