Two young brothers and their cousin say they were “completely speechless” when they came across a Tyrannosaurus rex bone poking out of the ground while out for a jaunt in the North Dakota badlands.

In July 2022, Liam and Jessin Fisher, then seven and 10, and cousin Kaiden Madsen, then nine, were hiking across a stretch of property owned by the US bureau of land management around Marmarth, a small town with a population of 101 – but also known as the dinosaur capital of North Dakota for its rich deposits of Triceratops and T rex fossils.

The trio, led by the Fisher boys’ father, Sam, hit upon the bone of the teenaged dinosaur which was likely entombed about 67m years ago in the Hell Creek formation, a popular paleontology playground that spans Montana, Wyoming and the Dakotas.

“You just never know what you are going to find out there,” Sam Fisher told the Associated Press when the find was unveiled at the Museum of Nature & Science in Denver, Colorado, on Monday. “You see all kinds of cool rocks and plants and wildlife.”

Liam Fisher said he thought the bone sticking out of the rock was something he described as “chunk-osaurus” – a made-up name for fossil fragments too small to be identifiable. His father shared a picture with a family friend, said Tyler Lyson, the associate curator of vertebrate paleontology who works at the museum.

Lyson initially thought it was a relatively common duckbill dinosaur, also known as hadrosaurs. But during a dig into the sandstone the following year, they did not find the neckbone of a duckbill as they expected – instead, it was something more unexpected.

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“Instead of finding a cervical vertebrae, we found the lower jaw with several teeth sticking out of it,” Lyson said. “And it doesn’t get any more diagnostic than that, seeing these giant tyrannosaurus teeth starring back at you.”

Based on the size of the tibia, experts estimate the dinosaur was 13 to 15 years old when it died and likely weighed about 3,500 lbs (1,587.57kg) – roughly two-thirds of the size of a full-grown adult.

The museum estimates that the T rex fossil will take about a year for crews to fully excavate. A leg, hip, pelvis, a couple of tailbones and part of the skull have so far been found, and it is not yet clear how complete the example is.

Lyson said more than 100 individual T rex fossils have been unearthed, but many are fragmentary. At the museum, he said that it was a “remarkable feeling” to make the discovery and “share the preparation of this fossil with the public”.

The area where the discovery was made is 200 miles (322km) from the Cheyenne River Reservation in South Dakota, where one of the most famous T rex finds was made in 1990.

The massive, mostly intact dinosaur skeleton became known as Sue the T rex and became the centre of a protracted legal battle over ownership rights. Sue, who is more than 40ft long, currently sits in Chicago’s Field Museum.

Marmath is also where Tyler Lyson found the skeleton of an Edmontosaur, a species of duckbill, complete with skin, tendons and bones – and one of only three ever discovered in that condition.

After the children discovered the latest T rex, Jessin Fisher – a fan of the Jurassic Park movies as well as an aspiring paleontologist – offered some advice to other kids, telling them “just to put down their electronics and go out hiking”.

• The Associated Press and Reuters contributed reporting

Source: theguardian.com