Following years of excavation and research, a new species of Allosaurus has been announced in the open-access scientific journal PeerJ. Allosaurus jimmadseni was unveiled at the National History Museum of Utah as the geologically-oldest species of Allosaurus in the world. This species predates its more well-known “younger cousin” Allosaurus fragilis by at least 5 million years.
Roaming the flood plains of western North America during the Late Jurassic Period, the meat-eating Allosaurus jimmadseni was approximately 1,800 kilograms (4,000 pounds) and 8 to 9 meters (26-29 feet) long. In its ecosystem it was the most common and the top predator, due to its relatively long legs and tail, and long arms with three sharp claws.
The “Allosaurus” part of the name translates as “different reptile” whilst the latter part honors the late Utah State Paleontologist James Madsen Jr. who excavated and studied tens of thousands of Allosaurus bones.
Although this paper only recognizes two species of Allosaurus (fragilis and jimmadseni) found in North America, palaeontologists argue that there could be up to 12. Setting this species apart from other members of the two-legged carnivorous Allosaurus genus, who lived during the Jurassic and Cretaceous period, relies on an examination of its head.
“The skull of Allosaurus jimmadseni is more lightly built than its later relative Allosaurus fragilis, suggesting a different feeding behavior between the two,” explained co-lead author Mark Loewen, research associate at the Natural History Museum of Utah. Other unique features of the new species include a facial crest stretching down its short narrow skull from the horns in front of the eyes to the nose, as well as a flat surface under the eyes.
The first specimen of Allosaurus jimmadseni was discovered at one of the most fertile source of dinosaur fossils in North America, the Morrison Formation of Dinosaur National Monument. On July 15, 1990, George Engelmann of the University of Nebraska was carrying out a contracted palaeontological inventory at the site when he came across the headless skeleton. During the excavation of the site which followed, explosives had to be used to detach the 2,700-kilogram (5,952 pounds) skeleton block from the surrounding rock before a helicopter was used to fly it out.
The missing head of the dinosaur was reunited with its body six years later after Ramal Jones of the University of Utah used a Geiger counter to locate the radioactive skull. This study provides a complete description of every bone of the Allosaurus jimmadseni skull uncovered and a comparison to the skulls of other carnivorous dinosaurs.
"Recognizing a new species of dinosaur in rocks that have been intensely investigated for over 150 years is an outstanding experience of discovery,” said Daniel Chure, retired palaeontologist at Dinosaur National Monument and co-lead author of the study. “Many more exciting fossils await discovery in the Jurassic rocks of the American West."