New Origin Story for Tyrannosaurus Rex Recommended by Fossil
A team of researchers has announced the discovery of a new species of Tyrannosaurus from New Mexico, a species that appeared in the fossil record five million to seven million years before the well-known tyrant lizard. Their research, published Thursday in the journal Scientific Reportssuggests a new chapter could be added to Tyrannosaurus rex’s origin story.
When workers at the New Mexico Museum of Natural History and Science collected the partial skull of a large adult tyrannosaur from the state’s Elephant Butte Reservoir in the 1980s, they initially assumed the fossils belonged to T. rex. But when Sebastian Dalman, a paleontologist at the museum, started working on the specimen in 2013, he noticed subtle but consistent differences between the specimen and other T. rex skulls.
Instead of the deep, bone-crushing jaws of an adult T. rex, the reservoir monster’s lower jaw looked slimmer. Its teeth were different and the animal was missing the prominent bone ridge behind the T. rex eye, Mr Dalman said. Scientists estimate the animal was about 36 feet (11 meters) long, about 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, curator of paleontology at the museum and author on the paper. When the fossil was first discovered, researchers initially assumed that the rock layers that produced the fossil – New Mexico’s McRae Formation – belonged to the same period. But the team’s dating of the rocks now suggests that the McRae Formation was 5 to 7 million years older than Hell Creek, and that the specimen found came from an earlier relative.
“Most species don’t exist for more than a million years,” said Dr. Lucas. “The age argument is really matched by the anatomy.”
The researchers argue that this is enough evidence to conclude that the skull belongs to a separate species, which they have named Tyrannosaurus mcraeensis, after the formation in which the specimen was found.
In recent years, some scientists have argued that certain fossils assigned to T. rex actually represented new species — and have faced serious opposition from other scientists. Researchers in the field have generally been reluctant to pick apart such an iconic and well-studied species without overwhelming evidence.
“This will get a lot more attention than the average newly named dinosaur,” said Dave Hone, a paleontologist at Queen Mary University of London, who was not involved in the paper.
But he and other outside researchers have responded more warmly to T. mcraeensis than to other alleged Tyrannosaurus discoveries, noting that the authors have made a reasonable and convincing argument. That the remains appear to be both outside T. rex’s usual range of variation and comfortably separated in time strengthens the argument, says Thomas R. Holtz Jr., a paleontologist at the University of Maryland.
The find also has interesting implications for the timeline of tyrannosaur evolution, said Dr. Holtz.
During the late Cretaceous, Tyrannosaurus relatives spread across what is now western North America, from Alaska to Mexico. During the last few million years of the dinosaurs’ reign, the larger T. rex replaced these genera.
Since the most famous relatives of Tyrannosaurus come from Mongolia, said Dr. Holtz. 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 is clearly more going on than just a move north as we also have the Asian lines,” said Dr. Hone. “There were big tyrannosaurs moving around.”
If Tyrannosaurus did indeed appear in the Southwest, the team suggests, the genus’ gigantic size may have evolved to prey on the enormous herbivores found in the region, such as Alamosaurus, a sauropod that could grow up to 30 meters long, and Sierraceratops, an earlier relative of Triceratops. But what caused the appearance of such a landscape of comparative giants in a specific region of North America remains a mystery.
“I think we need to spend more time looking southwest,” says Nick Longrich, a paleontologist at the University of Bath in England and author of the paper. “There are many poorly studied areas where we will find new things.”