The thigh bone of a tyrannosaur was found in Idaho — and it’s tinier than you think
Millions of years ago, Idaho may have been home to a notorious dinosaur’s predecessor.
A fossil unearthed in Southeastern Idaho by an Idaho State University paleontologist was identified as the thigh bone of a Tyrannosaurus rex’s ancestor in a new Journal of Paleontology paper published last month.
As the first and only tyrannosaur bone found in the state, the discovery helps paint a richer picture of the world nearly 100 million years ago during the Cretaceous period, a slice of time still hazy to researchers.
L.J. Krumenacker, a geosciences adjunct professor at Idaho State University and affiliate curator at the Idaho Museum of Natural History, found the fossil a few years ago in Eastern Idaho’s Bonneville County, on land managed by the U.S. Forest Service. The discovery site, located near the Idaho-Wyoming border, is part of the Wayan Formation, a nearly-100 million year old body of rock made of mudstone and siltsone.
The Cretaceous period lasted from about 146 million years ago to about 66 million years ago. The fossil, which is the oldest Cretaceous-age bone from North America, will be permanently housed at the Idaho Museum of Natural History in Pocatello as part of the museum’s research collections, available for scientists to research.
This predator was tinier than the infamous T. rex. At about 110 pounds, it was the size of a pony, or three to four tubas, Krumenacker said. It was also not fully grown and at least 5 years old at the time of its death.
“The T. rex is always used as a quintessential monster, but when you find something like this, I think it’s more relatable,” Krumenacker said. “It shows that these were animals, not just some scary monster, but an animal like you have today.”
The specimen instantly stood out to him. It was different from the dinosaurs usually found in Idaho. Dinosaurs previously found in the state were commonly from another genus, oryctodromeus, known for digging burrows and living underground.
What tyrannosaur bone in Idaho reveals
The tyrannosaur bone was “very hollow, which is typical for predators,” Krumenacker said. “It immediately struck me that we’ve got a predator that would have been eating the poor little ‘ryctos,” he joked, in reference to the oryctodromeus.
To confirm the specimen’s origins, Krumenacker and his colleagues examined specific details of the thigh bone, such as the shape of the femoral head, muscle attachments, and placement of blood vessels. While some tyrannosaur teeth have been found in Idaho, the bone supplied more information that let researchers know who close relatives might be, Krumenacker said.
The researchers determined that the animal the bone came from predated the Tyrannosaurus rex by 30 million to 35 million years, a discovery that sheds light on how the makeup of dinosaur-age ecosystems changed over time. These tiny tyrannosaurs were not dominant predators, but “little guys in the background,” Krumenacker said.
Next year, a reconstruction of the new tyrannosaur will be included in an exhibit about Idaho’s dinosaurs at the Idaho Museum of Natural History. The reconstruction will be modeled after similar little tyrannosaurs found in Utah and New Mexico, for which more complete skeletons exist.
Idaho isn’t known for its dinosaurs. And neighboring states like Wyoming, Montana, and Utah have more rocks that preserve dinosaur fossils, Brandon Peecook, a paleontology curator at the state’s Museum of Natural History and paleobiology assistant professor at Idaho State University, told the Statesman.
But other fossils of life in Idaho — such as plants, mammals, crocodiles, and turtles — helped researchers reconstruct what the ancient ecosystem would have looked like.
Dinosaur time, and all of earth history, is humongous, Peecook said. There’s a saying among paleontologists, that a Tyrannosaurus rex lived closer in time to us today than a T. rex lived to a Stegosaurus.
Not many rocks reveal dinosaur fossils from the Cretaceous period. The Idaho fossil record is still “one of our best windows” into what the world was like nearly 100 million years ago, Peecook said — a window Idaho can provide.
This story was originally published June 14, 2022 at 3:00 AM with the headline "The thigh bone of a tyrannosaur was found in Idaho — and it’s tinier than you think."
