New Origin Story for Tyrannosaurus Rex Suggested by Fossil

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.