The discovery of an unusual 'evil' dinosaur in Patagonia is helping shed light on one of the biggest mysteries in palaeontology – why T-Rex had such small arms. Gualichoshinyae was found to have tiny T-Rex arms despite being on a different family tree to the Cretaceous period's most fearsome predator. This suggests that these unusual limbs did not come from a common short-armed ancestor, but evolved independently, indicating they served some sort of purpose.

Gualicho was discovered in Patagonia's fossil-rich Huincul Formation. It was named after an evil spirit from the Tehuelche culture – Gaulicho was feared and blamed for all bad things that happened.

Scientists from Argentina and the US came across the partial skeleton in 2007. A study describing the species has now been published in the journal PLOS One. The dinosaur was a medium to large theropod – a group that includes T-Rex, Velociraptor and Allosaurus. It would have weighed about 450kg, which is about the same as a polar bear. Yet its arms were about the same size as a human child's.

Akiko Shinya, who the dinosaur is named after, with the fossil Pete Makovicky, The Field Museum

Peter Makovicky, from Chicago's Field Museum and one of the study authors, said: "Gualicho is kind of a mosaic dinosaur, it has features that you normally see in different kinds of theropods. 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."

Gualicho looked most like Deltadromeus, a carnivorous dinosaur from Africa with long legs and slim arms. This is the species the researchers believe Gualicho is most closely related to.

While this species ancestry and its relations remain ambiguous, the authors say it is adding more evidence to the idea that small arms evolved independently many times among theropod dinosaurs, challenging the idea they came from a common ancestor long before. "By learning more about how reduced forelimbs evolved, we may be able to figure out why they evolved," Makovicky said.

Gualichoshinyae t-rex arms
Fossils from Gualichoshinyae Apesteguia et al