The remains of a juvenile T. rex in North Dakota were stumbled upon not by a group of skilled paleontologists, but rather by three young boys whose goal that day was to simply go for a hike.

Sam Fisher, 45, took his sons Jessin, 12; Liam, 9; and their 11-year-old cousin, Kaiden Madsen, on a hike in the North Dakota Badlands in July 2022, according to the Denver Museum of Nature and Science. While hiking, the trio of "young fossil hunters" discovered the rare T. rex fossil, the museum said in a news release.

Once the remains were found, Sam Fisher called Tyler Lyson, his former high school classmate and the museum's curator of paleontology. Lyson returned to the site with the boys and his team of paleontologists in summer 2023 to excavate the fossil, according to the museum. It took a year to retrieve the fossil because Lyson had to get an excavation permit from the land's manager.

"By going outside and embracing their passions and the thrill of discovery, these boys have made an incredible dinosaur discovery that advances science and deepens our understanding of the natural world,” Lyson said in the release. "I'm excited for Museum guests to dig into the 'Teen Rex Discovery' experience." 

How big was the teenage T. rex?

Lyson and his group of paleontologists were able to determine the fossil belonged to a juvenile T. rex by analyzing the bone structure, growth patterns and bone histology, among other techniques, to come up with the age of the dinosaur at the time of its death, the museum announced.

"While histology has not yet been done, but will be, we estimated the size of teenage T. Rex based on the length of the specimen’s tibia (shin bone)," the museum said on a Q&A page about an upcoming documentary for the discovery. "The tibia from our specimen measures 82 cm, compared to that of a full-grown T. rex with a 112 cm long tibia suggests that our specimen likely belongs to a teenager."

The paleontologists believe the "Teen Rex" weighed 3,500 pounds, stood 10 feet tall and was 25 feet long at the time of its death, according to a museum Q&A page. The exact weight of the fossil will be determined through ongoing research and analysis.

In comparison, a fully grown adult T. rex could reach lengths of up to 40 feet or more and weigh up to 8,000 pounds, the page said.

What can the teenage T. rex fossil teach paleontologists?

The teenage T. rex fossil is rare because usually, the T. rex fossils that are found are from older and larger animals, according to the museum. The teenage skeleton will allow researchers to study the growth and development of the T. rex species during a "crucial phase" of its life, the Q&A page said.

"The story of Teen Rex and the three kids, Jessin, Liam and Kaiden, underscores the power of curiosity, determination, and the pursuit of knowledge. It serves as a reminder that anyone, regardless of age, can make a significant impact by following their passions and embracing the excitement of discovery," according to the museum.

Excavation of the teenage T. rex filmed, will premiere at museum

The discovery of the fossil was videotaped and cut into a new 40-minute documentary titled "T.REX," the museum said. The film will be showing on June 21 on the museum's giant screen Infinity Theater.

"With state-of-the-art CGI and cutting-edge paleontological insights, this giant screen movie offers an unprecedented journey into the world of T. rex and its fellow Cretaceous carnivores," according to the release.

A temporary display for the juvenile T. rex fossil called "Discovering Teen Rex" will also be open to the public on June 21, but at 1:30 p.m., the museum said.