Tyrannosaurus rex has a sibling. And its discovery not only adds a new branch to the dinosaur family tree, but it may also reshape our understanding of how the colossal predator first expanded across western North America.
A team of researchers—including George Washington University’s Alex Pyron, associate professor of biology at the Columbian College of Arts and Sciences, discovered a “sister species” of Tyrannosaurus, as Pyron called it, by analyzing a partial skull found in western New Mexico in 1983 and currently on display at the New Mexico Museum of Natural History & Science (NMMNHS).
The new species—dubbed Tyrannosaurus mcraeensis—is the earliest known relative of T. rex on the continent. Like its famous sibling, it was a carnivore about the size of a double-decker bus. But its discovery suggests that the Tyrannosaurus genus roamed the southwestern United States about 73 million years ago—millions of years before paleontologists previously thought.
“It increases the known biodiversity of these large apex predators in North America as we get up to the Cretaceous extinction,” Pyron said. “Tyrannosaurus mcraeensis is farther south than we thought in North America, and earlier than we knew.”
In addition to GW, the research team included scientists from NMMNHS, the University of Bath (UK), University of Utah, Harrisburg University, Penn State Lehigh Valley and the University of Alberta. The study was published in “Nature Scientific Reports.”
The breakthrough was sparked by a reexamination of the skull and jaw of a horned dinosaur found by boaters in New Mexico’s Elephant Butte Reservoir. The researchers meticulously compared each bone to dozens of T. rex specimens. Like T. rex, Tyrannosaurus mcraeensis was a meat eater that measured up to 40 feet in length and could reach a height of 12 feet at the hips. But the researchers identified small yet significant differences.
“The differences are subtle, but that’s typically the case in closely related species. Evolution slowly causes mutations to build up over millions of years, causing species to look subtly different over time,” said co-author Nick Longrich, a paleontologist and evolutionary biologist from the Milner Centre for Evolution at the University of Bath, in a statement.
GW’s Pyron conducted the evolutionary biogeographic analysis of the species to reconstruct a model of how dinosaurs moved across the earth over millions of years during the Cretaceous period. Using statistical techniques based on factors including bone morphology and plate tectonic history, Pyron identifies key events and stages in evolution, such as when species like Tyrannosaurus moved across continents.
Paleontologists largely believe that species like Tyrannosaurus appeared in Asia about 100 million years ago and migrated to North America across a land bridge through Siberia and Alaska. Fossils uncovered in and around modern-day Montana show that T. rex inhabited North America about 66 million years ago. But the Tyrannosaurus mcraeensis bones in New Mexico date back as far as 73 million years ago—suggesting that the species may have originated in southwestern North America and then later expanded into much of the western portion of the continent.
“We think about T. rex as being the dominant predator roaming the region at the time. But now we’re finding there’s a larger variety of these species,” Pyron said. “We’re seeing that there was actually a denser and more diverse community of dinosaurs in that region—right up to the time the comet hits.”
While the new discovery predates T. rex, the study noted that subtle differences in the jaw bones make it unlikely that Tyrannosaurus mcraeensis was a direct ancestor—raising the possibility that more dinosaur discoveries may be made.
“Biologists don’t like the term missing link, but there’s a large sampling gap,” Pyron said.