For decades, scientists thought that the two holes in the roof of the dinosaur's skull aided jaw movement.
The fossils are 15 million years older than any others belonging to the tyrannosauroid family that have been found in North America.
"This is a one in 100 million specimen," a University of Kansas fossil expert said.
Steer clear of the short-arm jokes if you want to avoid their wrath.
The strange mixture of features in Chilesaurus has perplexed scientists since its discovery was announced in 2015.
The apex predator probably couldn't even walk very fast, forcing a rethink of the fearsome dinosaur's hunting technique.
The dinosaur's teeth could generate 431,000 pounds of force per square inch.
Among the finds was an incredibly rare 70-million-year-old dinosaur skull.