A new species of carnivorous theropod dinosaur has been identified from fossilized remains discovered in the 1990s in northeastern Utah and Wyoming, United States.

A group of Allosaurus jimmadseni attacks a juvenile sauropod dinosaur. Image credit: Todd Marshall.

The newly discovered dinosaur belongs to Allosauridae, a group of two-legged, small to large-bodied carnivorous dinosaurs that lived during the Jurassic and Cretaceous periods.

Nicknamed Allosaurus jimmadseni , the species inhabited the floodplains of western North America during the Jurassic period.

The ancient creature measured 8–9 m (26–29 ft) long and had a mass of around 1.8 metric tons. It had relatively long legs and tail, and long arms with three shaped claws.

At 155 million years old, Allosaurus jimmadseni is the geologically oldest species of Allosaurus predating the better-known Allosaurus fragilis .

The new species is distinguished by a number of unique features, including flow ridges running from above the eyes to the snout and a relatively wide part of the head with a flat surface to the bottom of the upper part below the eyes.

Its ѕkᴜɩɩ was weaker and had less overlapping field of vision than that of the younger Allosaurus fragilis .

“Previously, paleontologists thought there was only one species of Allosaurus in the Jurassic of North America, but this study shows there were two species: the recently described Allosaurus jimmadseni evolved at least 5 million years before its younger cousin, “Allosaurus fragilis ,” said Dr. mагk Loewen, a paleontologist at the Utah Museum of Natural History and the University of Utah.

“The ѕkᴜɩɩ of Allosaurus jimmadseni has a lighter build than its later relative Allosaurus fragilis , suggesting different feeding behavior between the two.”

A cast of the ѕkeɩetoп and ѕkᴜɩɩ of Allosaurus jimmadseni as it was discovered and now on display at Dinosaur National Monument in Utah. Image credit: Dan Chure.

Dr. Loewen and his colleague Dr. Daniel Chure, a certified paleontologist at Dinosaur National Monument, described the new species from two well-preserved skeletons and skulls.

The first specimen was found by Dr. George Engelmann of the University of Nebraska, Omaha, on July 15, 1990 during a contracted paleontological inventory of the Morrison Formation of Dinosaur National Monument in northeastern Utah.

The second specimen was found by the commercial fossil collecting company Siber+Siber, Ltd. at Howe Quarry, Morrison Formation, Wyoming, in 1991.

“Recognizing a new species of dinosaur in rocks that have been intensively investigated for more than 150 years is an extraordinary discovery experience,” Dr. Chure said.

“ Allosaurus jimmadseni is a great example of how much more we have left to learn about the world of dinosaurs.

“Many more interesting fossils await discovery in the Jurassic rocks of the American West.”