SALT LAKE CITY — The geologically oldest species of the meat-eating dinosaur allosaurus roamed western North America 157 million years ago, with one specimen unearthed at the Dinosaur National Monument in northeastern Utah.

The new species was announced Friday to coincide with a study published in the PeerJ scientific journal detailing distinctive features of Allosaurus jimmadseni and what makes this older version of the allosaurus different than its later relative, Allosaurus fragilis.

Scientists at the Natural History Museum of Utah provided additional information about the new species of allosaurus and displayed casts of the new species’ head to contrast with the species that would replace it through evolution.

“This is one of the most amazing dinosaurs I have seen in my career,” said Mark Loewen, research associate at the Natural History Museum of Utah and associate professor in the Department of Geology and Geophysics at the University of Utah.

Loewen is co-lead author of the study published Friday.

“Previously, paleontologists thought there was only one species of allosaurus in Jurassic North America, but this study shows there were two species — the newly described Allosaurus jimmadseni evolved at least 5 million years earlier than its younger cousin, Allosaurus fragilis,” Loewen said.

The species belongs to the allosauroids, a group of small- to large-bodied, two-legged carnivorous dinosaurs that lived during the Jurassic and Cretaceous periods. Allosaurus jimmadseni possesses several unique features, including a short narrow skull with low facial crests extending from the horns in front of the eyes forward to the nose and a relatively narrow back of the skull, according to researchers.

George Engelmann, of the University of Nebraska, Omaha, discovered the initial skeleton of the new species within Dinosaur National Monument in 1990. It was years later that the headless skeleton was united with its skull by teams at the monument.

Loewen said the skull was found 5 feet away through radiation detection monitors that picked up the uranium properties encapsulated in the fossil through the decay process. He said be believes that is the first time in paleontology that radiation detection has been used to uncover a fossil.

The fully articulated skeleton, he added, was enveloped in a steep rocky precipice that took years to blast out of the mountain and then transport via helicopter to its new home at the museum, which is home to the world’s largest collection of allosaur fossils.

Daniel Chure, retired paleontologist at Dinosaur National Monument and co-lead author of the study, said the find is extremely significant because it peels more layers of history away about the popular dinosaur.

“Recognizing a new species of dinosaur in rocks that have been intensely investigated for over 150 years is an outstanding experience of discovery. Allosaurus jimmadseni is a great example of just how much more we have to learn about the world of dinosaurs. Many more exciting fossils await discovery in the Jurassic rocks of the American West,” Chure said.

Skulls, he stressed, are the most important story teller when it comes to the evolution of dinosaurs.

Allosaur fossils are scattered around the world. Loewen said a close relative of this latest discovery was found in China, and other remains have been found in Switzerland.

The paleontologist said, however, it is hard to compete with Utah.

“Utah is one of the world’s greatest hunting grounds for dinosaurs and the world they lived in,” Loewen said.

More information about this species and others will be showcased Saturday and Sunday at the museum from 10 a.m. to 5 p.m. during DinoFest. Experts will be on hand to answer questions and detail the extinction of dinosaurs.

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Allosaurus jimmadseni is named after Jim Madsen, the state’s first paleontologist who from World War II into the 1990s knew more than anyone about allosaurus and did groundbreaking research on the species, Loewen said.

Madsen died in 2008, but his daughter, Lisa Madsen, was at Friday’s naming event in honor of her father.

“It’s very cool,” she said. “We miss him every day and to have him honored like this by different people from all over is very sweet.”

Madsen said having this new species named after her father will memorialize his work for all time.

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