“My dad hollered for Jessin and Kaiden to come, and they came running up on the butte,” Liam, now 9, recalled this week. “Dad asked, ‘What is this?’ and Jessin said, ‘That’s a dinosaur.’”

Liam lay down next to the bone, which was more than half the length of his body. His dad snapped a photo and sent it to Tyler Lyson, a high school classmate and a paleontologist at the Denver Museum of Nature & Science.

On that summer day in 2022, the family had no idea they had discovered something most kids would only dream about — a Tyrannosaurus rex. And not just a T. rex, Lyson’s team would find upon excavation, but a young one, making it scientifically valuable.

“It’s one in several million,” Lyson told The Washington Post of the chances. “There’s only about five or six juvenile tyrannosaurs from this same interval of time, so it’s quite rare.”

Two years after the boys’ discovery, the fossil is set to go on display June 21 at the Denver Museum of Nature & Science, where the public can watch as paleontologists work on revealing the bones. The museum announced the discovery Monday, alongside a documentary, “T. Rex,” that will play in museums and other venues nationwide.

“‘I can’t believe we just found this,’” Kaiden recalled thinking. When the excavation team later discovered their dinosaur was a T. rex, he said at the museum’s news conference Monday, “I was completely, like, speechless.”

The fossil could contribute to a heated scientific debate over whether there is a single Tyrannosaurus rex species or a larger and a smaller species, paleontologists said. It could also provide clues about how and when the iconic dinosaur got big.

“If we want to know how T. rex goes from a chicken-sized animal to an 8,000-pound monster, we need juveniles to really figure out how it grew,” Lyson said.

The skeleton is incomplete but includes much of a lower leg, hips, pelvis, a large chunk of the skull and some tail vertebrae. The dinosaur’s age at death will be determined in the laboratory but could be around 13 to 15, Lyson said.

Juveniles are “rarer in the fossil record, and anything rare is sought-after information,” said Carl Mehling, a senior specialist in the paleontology division at the American Museum of Natural History, which displays one of the world’s most famous T. rex skeletons.

Before the fossil went public, Jessin, Kaiden and Liam got to name it. Their pick? The Brothers — deciding that the three of them were like brothers, and the dinosaur was now their brother, too. (They briefly named it The Brother, then settled on the plural, Lyson said.)

The best place to find dinosaurs

The Brothers lived about 67 million years ago, roaming an area now known as the Hell Creek Formation. Covering parts of North Dakota, South Dakota, Wyoming and Montana, it’s famous for its fossil-rich landscape and has been intensively studied by scientists.

“It preserves Earth’s last dinosaur ecosystem, right before they went extinct,” Lyson said. “It’s the best place in the world where you can go and find the very last dinosaurs that ever walked around on the planet.”

Once an inland sea bed, the now-dry land holds the Earth’s history in its layers of rock. In some places, a band is visible that marks the meteor strike that is believed to have destroyed the dinosaurs and ended the Cretaceous Period.

“Once you find that layer, that’s when you start to find the dinosaurs,” said Mark Jacobsen, a spokesperson for the Bureau of Land Management district that covers part of Montana, North Dakota and South Dakota.

The new fossil was on those public lands, in a southwestern corner of North Dakota just across the border from Montana. Federal law allows approved institutions to excavate fossils on public land; the museum was granted permits for the project.

The rugged badlands and their secrets drew Lyson as a child growing up in Marmarth, N.D., a tiny town “in the middle of nowhere.” From a young age, he said, he roved the badlands in search of fossils, and in sixth grade, he was allowed to help professors who visited on an expedition.

In 1999, as a high-schooler, Lyson made his own stunning discovery — a dinosaur with its tissue preserved, which became known as Dakota the “Dino Mummy.”

Now, Lyson is watching Jessin, Kaiden and Liam live out a similar story.

“The whole thing just reminded me a lot of me as a kid,” Lyson said. “It’s the full circle. … It’s just super special.”

Excavating a giant

In July, the museum’s large research team — and the Fisher-Madsen family — descended on the butte where The Brothers was waiting.

The researchers originally theorized that the bone might belong to a duckbill dinosaur, a more common specimen. Upon arriving, however, Lyson began to suspect it could be a tyrannosaur.

He and Jessin were digging together when Lyson heard his awl clink against something. He brushed the dirt away, “and out popped a T. rex tooth.” The two brushed over the area and saw three other teeth, lined up in a jaw.

It couldn’t get more definitive than that.

Jessin held the tooth for a moment. Then the pair shouted out their findings.

“Everybody’s high-fiving each other and hooting and hollering and yelling ‘We found a T. rex!’” Lyson recounted.

The boys went wild. The Fishers’s mother, Danielle, began crying, Lyson recalled. The documentary crew, of Giant Screen Films, which had originally come along to get footage of an excavation, captured the moment.

The excavation took 11 days. The last task was the hardest: moving the fossil to Denver.

“There’s so many logistics at play when you’re thinking about getting a dinosaur skeleton off the top of a butte from the middle of nowhere,” said Natalie Toth, the museum’s chief fossil preparator, who helped lead the dig.

The scientists excavated the skeleton in an approximately 6,000-pound, eight-foot-wide chunk of sandstone, which they wrapped in a “jacket” of plaster-soaked burlap and moved onto a wooden frame. On the last day, they carefully rolled it onto a helicopter net.

Lyson was briefly terrified that the fossil was too heavy for the Black Hawk helicopter the team had hired, but after several minutes, the dinosaur was airborne. The helicopter dropped it onto a trailer, and a crane moved it into place. Then The Brothers made the long drive to Denver.

The museum built a new facility to house it, where preparators will fully uncover the skeleton and pull out each bone. It will take about a year, Toth estimated, adding that the skeleton appears to be well preserved.

“I’m really looking forward to seeing what is all contained in this big block of T. rex,” she said.

As for the boys, they were adjusting to the spotlight after Monday’s announcement and “still trying to wrap their heads around” their adventure, Lyson said. The family will travel to Denver for the exhibition’s grand opening.

Asked at the news conference whether they wanted to become paleontologists, Jessin — who once dressed up as Lyson for Halloween — was decisive: “It’s been a lifelong dream of mine,” he said.

Kaiden and Liam were more ambivalent. “It’s fun and all,” Kaiden said, “but I really would [just] do it for fun.”