3 boys helped find a young T. rex in North Dakota. Research complicates what experts know about it

Liam Fisher was 7 when he spotted a bone jutting from the North Dakota badlands and figured it was “chunk-osaurus,” a made-up name for fossil bits too small to identify. It was a young Tyrannosaurus rex.

Liam and his father, Sam Fisher, spotted the bone in July 2022 while hiking on federal land near Marmarth, the AP reported. With them were Liam’s brother, Jessin, then 10, and their cousin Kaiden Madsen, then 9. The animal had been buried about 67 million years earlier in the Hell Creek Formation, a fossil-rich layer described by the North Dakota Geological Survey.

The Denver Museum of Nature & Science calls the fossil “Teen Rex.” Tyler Lyson, the museum’s senior curator of vertebrate paleontology, has said it is one of only four young T. rex fossils found worldwide, CBS News reported. How do scientists know a dinosaur died as a teenager, and is every small tyrannosaur a young T. rex? Museum accounts, guidance from the federal land agency and two peer-reviewed studies suggest the first answer is still an estimate and the second is back in dispute.

How did three kids find a T. rex in North Dakota?

They spotted a bone on public land, sent a photo to a paleontologist and helped dig it out a year later.

Mr. Fisher photographed the bone and sent it to Mr. Lyson, a family friend who went to high school with him. Mr. Lyson told the museum’s Catalyst magazine that from the photos he thought the family had found a duck-billed dinosaur, “the second most common dinosaur in the Hell Creek Formation.”

The fossil sat on land managed by the Bureau of Land Management. Mr. Lyson obtained a federal permit, ND2023-0084, before his team returned with the family in the summer of 2023.

Digging alongside Jessin, Mr. Lyson expected to reach a neck bone. “Instead of finding a cervical vertebrae, we found the lower jaw with several teeth sticking out of it,” Mr. Lyson said, according to the AP. “And it doesn’t get any more diagnostic than that.”

The dig took 11 days, Mr. Lyson told Catalyst. The skeleton had been buried along a sandbar in an ancient river. He said the crew used picks, shovels and a 70-pound jackhammer to clear the rock above it, along with awls and brushes to trace the bones. The finished field jacket, a shell of plaster and burlap around the bones and rock, weighed about 6,000 pounds. A Black Hawk helicopter lifted it onto a trailer bound for Denver.

The museum says the block holds about 30% of the skeleton. Mr. Lyson said in 2024 that the team had found a leg, hip, pelvis, a couple of tailbones and a large part of the skull, the AP reported. The museum estimates the animal was about 25 feet long and 10 feet tall and weighed roughly 3,500 pounds. It adds that the exact weight “is subject to ongoing research and analysis.”

Can you keep a dinosaur fossil you find on public land?

Not on BLM land. Dinosaur fossils collected there remain public property, the agency says.

Because the skeleton came from BLM land, it had to go to a federal repository such as the Denver museum. There it is “held in trust for the American people in perpetuity,” Mr. Lyson said.

BLM rules make it illegal to dig, remove or collect vertebrate fossils such as dinosaurs on the agency’s land without a research permit. The agency asks anyone who finds a fossil they think is unique to “leave it in place, take pictures and record its location, and report it to a local BLM office.”

How do scientists know Teen Rex was a teenager?

So far the age comes mostly from the size of its shinbone, a method Mr. Lyson says is less reliable than looking inside the bone.

The museum estimated the animal was 13 to 15 years old by plotting the length of its tibia, or shinbone, on a Tyrannosaurus growth curve, it said in a blog post quoted by KDVR. In June 2024, Mr. Lyson gave CBS News an estimate of 12 to 14 years.

“It’s important to note that using size is not the best way to determine the age of an animal,” Mr. Lyson told Catalyst. The more rigorous method, he said, is to examine limb bones for growth lines “similar to tree rings.” The museum has said it will determine an exact age by “carefully removing a portion of the bone and looking at its cross-section.” None of the museum’s press releases through October 2026 report the results.

A 2020 study published in Science Advances used that technique on two small tyrannosaurs from Montana nicknamed Jane and Petey. Researchers led by Holly Woodward of Oklahoma State University Center for Health Sciences counted annual growth rings in leg-bone slices and concluded the animals were 13 and 15 years old. Ms. Woodward’s team also found that a young T. rex grew less in years when food was scarce. That suggests size alone can mislead about age.

Is Teen Rex a T. rex or a Nanotyrannus?

The Denver museum identifies it as a young T. rex, but a 2025 study has reopened the question of whether small tyrannosaurs are young T. rex at all.

The 2020 researchers concluded that Jane and Petey were juvenile T. rex, not a pygmy relative called Nanotyrannus. A 2025 study published in Nature by Lindsay Zanno of North Carolina State University and the North Carolina Museum of Natural Sciences, and James Napoli, reached a different conclusion. The study described a nearly mature tyrannosaur skeleton from the Hell Creek Formation in Montana. It concluded that Nanotyrannus was a separate genus that lived alongside T. rex and included two species.

One of those species is Jane. The study identified the fossil as a new species, Nanotyrannus lethaeus, according to the Burpee Museum of Natural History in Rockford, Illinois, which houses it. Ms. Zanno was also a co-author of the 2020 study. The 2025 authors wrote that their results “prompt a re-evaluation of dozens of existing hypotheses” built on growth patterns they called “currently indefensible.”

The Nature study’s summary does not mention the North Dakota skeleton. The Denver museum’s exhibit page describes it as a “rare adolescent T. rex fossil.”

That makes detailed measurements, anatomical comparisons and bone-slice tests more important than ever. A small tyrannosaur is not automatically a young T. rex, and the fossil that began as “chunk-osaurus” still has evidence to give up.