Like a scene from the 19th century, US fossil hunters have used Clydesdale horses to remove a rare giant dinosaur bone from a gully in the mid-west desert.

Key points: The rare upper arm bone of a Brachiosaurus was excavated using horses in Utah

The rare upper arm bone of a Brachiosaurus was excavated using horses in Utah Similar fossils have only been found twice before

Similar fossils have only been found twice before The discovery adds pieces to the puzzle of what Brachiosaurus really looked like

The 2-metre-long upper arm bone — among the largest ever found — belonged to one of the tallest dinosaurs that walked the planet, a Brachiosaurus.

This long-necked herbivore of Jurassic Park fame would have weighed in around 30 tonnes and walked the land about 150 million years ago.

The bone was so big it was taller than anyone on the team that collected it.

"For me it's really exciting because the last time somebody found a humerus of a Brachiosaurus was about 60 years ago," said palaeontologist John Foster, of the Utah Field House of Natural History State Park Museum, where the bone will go on display on Thursday.

"A lot of us figured we'd just never see one in the field. It was really neat to be able to dig one up unexpectedly like that."

This shows how the bone compares in size to a human. ( Supplied: Brian Engh )

The researchers unearthed the most complete Brachiosaurus upper arm bone yet in the famous US fossil-hunting ground called the Morrison Formation, in Utah.

The right humerus was also found alongside other bones including several rib fragments, as well as fossil plants.

"We also found the matching left humerus in pieces just a few feet away," anatomist Mathew Wedel of the Western University of Health Sciences, wrote in an email.

The upper arm bones from this type of dinosaur have only been found twice before — originally in 1900 then in 1955.

"Despite its public popularity ... Brachiosaurus is extremely rare," Dr Wedel wrote.

"Brachiosaurus was previously known from only about 10 very incomplete skeletons. Ours makes 11."

And it may also be the oldest, the researchers added, given where the fossils were found.

"We are particularly excited because this Brachiosaurus was found very near several other identifiable sauropod sites, petrified logs, and plant fragments ... in the lower layers of the Morrison Formation, which pushes these lineages back by several million years," Dr Wedel wrote.

Palaeoartist Brian Engh next to the bone he found, which is partially covered in a plaster and hessian 'jacket' to keep it together. ( Supplied: Brian Engh )

The bone was originally discovered in May 2019 at a site on Utah State Park land by the palaeoartist on the team Brian Engh.

But it took until October before the scientists got the permits to remove the bone from the rugged terrain.

And then there was the small question of how they were going to get it out in a hurry before the winter rains washed it away.

Dr Forster said there was no way a motor vehicle could get into the gully and there was not enough time to raise funds for a helicopter.

"We had to find another way to get it out."

By October, they had come up with a plan: to enlist the help of some Clydesdale horses owned by locals Wes and Resha Bartlett.

The bone was found on the side of a steep gully. ( Supplied: Brian Engh )

But before they moved the bone the team had to make sure it didn't fall apart.

"If it flexed it would break up into a million pieces," said Dr Foster.

So the team encased the fossil and the surrounding rock with a "jacket" of plaster and hessian, splinted lengthwise with two pieces of wood.

They then had to wrangle the whole package, weighing around 450 kilograms, down to the bottom of the gully where the horses were used to drag it out to more level ground.

"This was definitely the most complicated thing I've ever done," said Dr Foster.

Palaeontologist Rebecca Hunt-Foster of the Dinosaur National Monument was one of the scientific leaders involved in the find.

Australian dinosaur expert Stephen Poropat of Swinburne University agreed the discovery was very rare.

"Interesting find, and very significant."

He said the fossil hunters would have used the unique shape of a Brachiosaurus humerus to identify it, as well as its size.

"Looking at the pictures it's typical."

Dr Poropat said all Brachiosaurus skeletons were incomplete.

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"Often in the media what is presented as Brachiosaurus is based a lot on Giraffatitan [a closely related African genus of dinosaur]," Dr Poropat said.

"The more fossils we find, the better understanding we'll get of what Brachiosaurus was like as an animal — its anatomy and where it sits on the dinosaur family tree, its behaviour and its ecology."

Palaeontologist Matt Herne from Flinders University agreed the rare find would help fill gaps in our understanding.

"It adds another piece to the puzzle."

He said the modern technique of biomechanics, which relies on CT scanning and reconstruction of muscles, was helping scientists put together what the dinosaur looked like in the absence of complete skeletons.

Dr Herne said there was a "revivalist" feeling about the recovery of the fossil from the rugged mid-west desert, reminiscent of the Great Dinosaur Rush — also known as the Bone Wars — of the 19th century, when competing fossil hunters turned up many new species of dinosaurs.

"It's wonderful country," he said. "The landscape surrounds you, and images of the wild west and those days of the great dinosaur discoveries are there in the environment."

The Bartlett family and their two horses, Darla and Molly, who helped retrieve the fossil. ( Supplied: Brian Engh )

"I guess that's what they're trying to relive — they're trying to live the dream. It's great."

"These sorts of discoveries highlight also the thrill and importance of finding large sauropods in Australia," he added.

Huge dinosaur footprints in places like Broome and Winton show large dinosaurs were also all over Australia at one time.

