New dinosaur had 'T. rex arms' that evolved independently

Scientists aren't sure why Tyrannosaurus rex grew such small, stubby arms, but the short, stunted, two-clawed limbs were useful enough to evolve multiples times.

A new dinosaur species uncovered in Patagonia had similar arms and claws, despite only a distant relationship to T. rex.

Like T. rex, Gualicho shinyae was a member of the Therapoda suborder. Theropods are bird-like dinosaurs with two legs, hollow bones and feathers. It turns out, several species boasted small arms.

Because G. shinyae is part of a different arm of the family tree than T. rex, researchers know its arms evolved independently.

Specifically Gualicho shinyae is part of the Allosauridae family. It weighed roughly 1,000 pounds and looked a lot like a leggy, small-armed carnivorous dinosaur found in Africa called Deltadromeus.

"Gualicho is kind of a mosaic dinosaur, it has features that you normally see in different kinds of theropods," Peter Makovicky, curator of dinosaurs at Chicago's Field Museum, said in a news release. "It's really unusual -- it's different from the other carnivorous dinosaurs found in the same rock formation, and it doesn't fit neatly into any category."

The new species is described in the journal PLOS ONE. The dinosaur is named for Akiko Shinya, the Field Museum's chief fossil preparator.

"We found Gualicho at the very end of the expedition," Shinya said. "Pete joked, 'It's the last day, you'd better find something good!' And then I almost immediately was like, 'Pete, I found something.' I could tell right away that it was good."