Radioactive dinosaur skull helps researchers make new discovery

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A photo showing the late paleontologist James Madsen Jr assembling a composite skeleton of Allosaurus from the Cleveland-Lloyd Dinosaur Quarry.

SALT LAKE CITY, Utah — Allosaurus was a fierce predator at the top of the dinosaur food chain millions of years before Tyrannosaurus rex claimed its “king of the dinosaurs” title and researchers have now discovered the earliest known species of Allosaurus.

The new species, Allosaurus jimmadseni, has been revealed at the Natural History Museum of Utah, home to the world’s largest collection of Allosaurus fossils. The dinosaur lived in what is now western North America during the Late Jurassic Period between 152 and 157 million years ago. Findings about the new species published Friday in the journal PeerJ.

Allosaurus itself translates from Greek to mean “different reptile.” The species name is in honor of the late Utah State paleontologist James Madsen Jr., known for excavating and studying thousands of Allosaurus bones. He helped recover numerous fossils from the Cleveland-Lloyd Dinosaur Quarry in Utah.

The fossil was initially found by University of Nebraska, Omaha professor George Engelmann at Dinosaur National Monument Park, on the border between Colorado and Utah, in 1990. Only the skeleton was missing a key feature: the skull.

Then, in 1996, University of Utah radiologist Ramal Jones found the skull using a radiation detector, because the skull itself was radioactive.

This isn’t unusual because fossils are in the ground along with radioactive elements found in soil and rocks, making some dinosaur fossils radioactive enough to be picked up by sensitive instruments, according to researchers at the Houston Museum of Natural Science.

It enabled the skull to be recovered, along with the rest of the fossil.

The massive rock containing the skeleton was so heavy that explosives were used to remove it in a 5,952-pound block, which was then flown out by a helicopter. It took seven years for the bones to be prepared and assembled by Dinosaur National Monument employees.

This newly discovered species has unique features compared to the allosauroids that followed. It had a short, narrow skull that was weaker and had less overlapping vision than Allosaurus fragilis, a later Allosaurus and the state fossil of Utah. Fragilis came on the scene five million years later as a younger cousin.

This Allosaurus also had a long tail, legs and arms, along with three sharp claws. Its fearsome face included crests that extended up from horns in front of its eyes down to the nose.

Although some scientists believe there are multiple species of Allosaurus that lived in North America, this study only focuses on two.

“Previously, paleontologists thought there was only one species of Allosaurus in Jurassic North America, but this study shows there were two species,” said Mark Loewen, co-lead study author and research associate at the Natural History Museum of Utah. “The skull of Allosaurus jimmadseni is more lightly built than its later relative Allosaurus fragilis, suggesting a different feeding behavior between the two.”

Allosaurus was first discovered and described in 1877. One of the most well-known fossils is “Big Al,” which researchers previously thought was an Allosaurus fragilis. But they have since determined it belongs to this newly discovered species. Allosauroids have been found at fossil sites in Utah, Colorado and Wyoming.

“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,” said Daniel Chure, co-lead author of the study and retired paleontologist at Dinosaur National Monument.

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