© Jason Brougham & Frank Ippolito8-Horned T. Rex Cousin
The discovery reveals that the fearsome "tyrant lizards," or tyrannosaurids, were much more diverse than thought.
"Instead of [its] big bad boy ... relatives, this one is more like a ballerina," said study co-author Stephen Brusatte, a vertebrate paleontologist at the American Museum of Natural History in New York.
A well-preserved skull and a near-complete skeleton from the new species of eight-horned, long-snouted carnivore - dubbed Alioramus altai - were unearthed in 2001 in Mongolia.
The predator (seen at top in an artist's conception) lived in the hot, lush floodplains of the late Cretaceous, near the end of the age of dinosaurs, roughly 65 million years ago.
The creature had two short horns above each eye and two jutting downward from its cheeks - all four are also seen in T. rex.
Strangely, the beast also had up to two-inch-long (five-centimeter-long) horns sticking out of each cheek, "which have never been seen in any carnivorous dinosaur before," Brusatte said.
Too short for combat, these horns likely served as sexual ornaments to attract females.
Smaller than T. rex, the newfound species also possessed an unusually airy skeleton; lacked a skull built for the strong jaws seen in its larger cousins; and had thinner, steak knife-like teeth (skull pictured in a diagram above).
Diverse Lifestyles
The new findings also suggest tyrannosaurids had more diverse lifestyles than previously thought.
While its larger cousins might have taken down big targets, the newfound dinosaur "probably relied more on speed, agility, and finesse, to go after smaller prey," Brusatte said.
"The very different body types we see now in tyrannosaurids probably allowed the ecosystem to support two closely related species ... just as you have lions and cheetahs close together."
Findings reported online October 5 in the journal Proceedings of the National Academy of Sciences.
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