Written by Will Dunham
(Reuters) – A rare fossil of a teenage Tyrannosaurus rex has been excavated in the Badlands of North Dakota, a noteworthy discovery for the scientific insight it may provide into the life history of the famous dinosaur and the story of the children who found it.
The Denver Museum of Nature and Science announced on Tuesday the discovery of the fossil, nicknamed “Ten Rex,” which will be studied and displayed.
In July 2022, brothers Liam and Jessen Fisher, 7 and 10 at the time, and their cousin, Cayden Madsen, then 9, were hiking and searching for fossils with Sam Fisher, Liam and Jessen's father, on land managed by the United States. The Bureau of Land Management is about 10 miles (16 km) from the town of Marmarth in southwestern North Dakota. Liam and his father noticed a large leg bone sticking out of the ground.
“My dad shouted for Jaysen and Kayden to come, and they came running,” said Liam, now 9. “Dad asked, ‘What is this?’” “This is a dinosaur,” Jessen said.
“I didn't know what kind,” said Jessen, now 12.
Sam Fisher sent a photo to paleontologist Tyler Leeson, a Marmarth native and former high school classmate who is now curator of vertebrate paleontology at the Denver Museum of Nature and Science. Leeson obtained excavation permits, and in July 2023 everyone returned to the site.
At first, the leg appeared to belong to a plant-eating duck-billed dinosaur.
“However, on the first day of the excavation, Jason and I discovered a lower jaw with several large teeth from the tyrannosaur visible,” Leeson said.
“It still gives me chills,” Leeson added.
“I was completely speechless,” said Kayden, now 11.
Tyrannosaurus, which roamed western North America, was one of the largest carnivorous dinosaurs.
This dinosaur appears to have been about 13-15 years old, two-thirds its adult size, 25 feet (7.6 m) long, and 3,500 pounds (1,600 kg) in weight. Tyrannosaurus rex was fully grown at approximately 18-21 years of age. Perhaps the largest known dinosaur, a specimen named Sue at the Field Museum in Chicago, measured 40-1/2 feet (12.3 meters) long.
This individual lived about 67 million years ago, near the end of the Cretaceous period. Tyrannosaurus and the rest of the dinosaurs, except for the descendants of birds, were wiped out 66 million years ago after an asteroid hit Earth.
The fossil comes from the Hell Creek Formation, which “preserves the last dinosaur ecosystem on Earth” just before the mass extinction, Leeson said. Based on the soft sandstone in which it was found, it appears that the animal's body ended up on a sandbar in an ancient river system.
The completion of the skeleton remains unclear because many of the bones are still embedded inside a three-ton piece of rock, which is now being studied at the museum. Leeson said there appeared to be a lot of skull, hip bone and some vertebrae in addition to the leg.
Tyrannosaurus had a massive head and enormous bite force, walked on two legs, and had weak arms with only two toes. Younger tyrannosaurs had a different body type than adults—more agile, faster, and a more streamlined skull—and may have hunted different prey, reducing competition with older ones.
“Juvenile T. rex were more lightly built, with long, thick legs, but without as much mass as adult T. rex,” Leeson said.
Leeson said having small specimens helps reveal the dinosaur's growth rate and body changes during maturation. Only a few of these fossils are available for study. Leeson said this fossil appears to be slightly larger than another fossil of a small tyrannosaur called “Jane” located at the Burpee Museum of Natural History in Rockford, Illinois.
A minority of paleontologists believe that Tyrannosaurus lived alongside a smaller cousin called Nanotyrannus, based on fossils that most paleontologists believe represent juvenile Tyrannosaurus. Leeson said the new fossil could shed light on the issue.
A documentary film crew was present during the excavation, and the film “T. REX” is scheduled to be shown on June 21.
