NEW YORK, NY.- In the summer of 2022, two boys hiking with their father and a 9-year-old cousin in the North Dakota Badlands came across some large bones poking out of a rock. They had no idea what to make of them.

The father took some photos and sent them to a paleontologist friend. Later, the relatives learned they’d made a staggering discovery: They’d stumbled upon a rare juvenile skeleton of a Tyrannosaurus rex.

Part of the fossil, which measures about 32 inches, is believed to be the tibia, or shin bone, of a 10-foot-tall, 3,500-pound dinosaur that scientists are calling Teen Rex. Only a few such fossils have been discovered worldwide, according to the Denver Museum of Nature & Science, which announced the finding Monday.

The specimen is also the most complete T. rex the museum has ever collected, it said. It is hosting an exhibit and will screen a documentary on the subject later this month.

The friend of the father who identified the fossil, Tyler Lyson, who is the museum’s curator of paleontology, said in a statement that the boys had made an “incredible dinosaur discovery that advances science and deepens our understanding of the natural world.”

After identifying the fossil — found in the Hell Creek Formation, a rocky area south of Marmarth, North Dakota, that dates back more than 65 million years — Lyson led an 11-day excavation that involved removing the overlying rock with a 70-pound jackhammer, picks and shovels. The bones were then carefully excavated, photographed and encased in protective plaster so that they could be safely transported back to the museum. Paleontologists plan to return to the site this year to keep looking for bones, the museum said.

In a video, the brothers, Jessin and Liam Fisher, 9 and 12, and their cousin, Kaiden Madsen, now 11, said that they were busy hiking and exploring when they first came across the bones and had no inkling they could be so special. “I didn’t have a clue,” Jessin says in the video. At first, he added, Lyson believed they belonged to a duck-billed dinosaur.

In a news conference Monday, the three boys agreed that the T. rex was their favorite species of dinosaur. “I was, like, completely speechless,” Kaiden said of how he felt when they finally learned of the significance of their discovery.

Lyson said that he, too, would never forget the moment in the dig when researchers uncovered the lower jaw of the dinosaur, which had several large serrated teeth sticking out of it. He said he knew immediately that it was a T. rex. “It still gives me goose bumps,” he said.

This article originally appeared in The New York Times.