T. Rex Wasn't The Only Dinosaur With Absurdly Tiny Arms

Newly discovered species evolved them all on its own

An artist's drawing of two *Gualicho shinyae* and an unnamed dinosaur.
An artist's drawing of two Gualicho shinyae and an unnamed dinosaur.Jorge González and Pablo Lara

Tyrannosaurus rex is best known as the carnivorous king of the dinosaurs, with an imposing body, giant head with impressively sharp teeth...and itty-bitty arms.

But T. rex wasn't the only dinosaur with really tiny arms. Many therapods (a group of two-legged carnivorous dinosaurs related to birds) had these small arms, including Carnotaurus which had remarkably wimpy arms. And now another dinosaur can be added to that list.

In a paper published today in PLOS One researchers describe the discovery of a brand new dinosaur that also had inexplicably tiny arms for its size, and wasn't related to T. rex. Instead, this dinosaur, Gualicho shinyae, evolved the tiny arms all on its own.

Gualicho shinyae was probably about the size of a polar bear, but with arms the size of a human child. The dinosaur had two digits (almost a finger and a thumb) just like Tyrannosaurus rex. But despite their similar shapes, it isn't closely related to Tyrannosaurus rex, which lived in North America, and was the size of a bus. But Gualicho shinyae does have some known relatives. It is believed to be closely related to dinosaurs that used to live in Africa.

The researchers hope that by learning more about animals that developed these tiny forearms, they might eventually be able to figure out why that shape evolved in the first place. There are some possible theories including the idea that the limbs might have been used to grip prey, or as part of a mating ritual, but we still don't know for sure.

"Gualicho is kind of a mosaic dinosaur, it has features that you normally see in different kinds of theropods," co-author of the paper Peter Makovicky, The Field Museum's Curator of Dinosaurs, said. "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 fossil was first found in Patagonia in 2007 by researchers with Chicago's Field Museum of Natural History. Akiko Shinya, The Field Museum’s chief fossil preparator was the first to spot the fossil.

“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.”

The fossil is named after Shinya, and Gualicho, a spirit of mischief or devilment in the Tehuelche culture in Patagonia.

An artist's drawing of the anatomy of *Gualicho shinyae*.
An artist's drawing of the anatomy of Gualicho shinyae.Jorge González and Pablo Lara
Akiko Shinya is the Field Museum’s chief fossil preparator, and was the first to spot the fossil. Part of the new dinosaur's name is Shinyae, to honor Shinya's contributions.
Akiko Shinya is the Field Museum’s chief fossil preparator, and was the first to spot the fossil. Part of the new dinosaur's name is Shinyae, to honor Shinya's contributions.Pete Makovicky, The Field Museum
The fossil was discovered in Patagonia.
The fossil was discovered in Patagonia.Pete Makovicky, The Field Museum
Bad Luck
The fossil was named Gualicho after Gualichu, a spirit associated with the Tehuelche people of Patagonia. Gualichu has a reputation for spreading bad luck, which the researchers encountered aplenty on this expedition. The team says they joked about the "curse of Gualichu" when faced with misfortunes like a flipped truck during fieldwork.Pete Makovicky, The Field Museum