Silly Little Arms Evolved at Least Twice in Dinosaurs

Nature is a bit like a film director or an artist. If you look carefully at its entire body of work, the same motifs emerge again and again: bioluminescence. The Golden Ratio. And, apparently, lumbering animals with ridiculously tiny arms. Scientists say Tyrannosaurus rex and the newly discovered species Gualicho shinyae developed their absurd forelimbs separately at different points in history. Their report was published today in the journal PLOS One. 

Akiko Shinya is chief fossil preparator at The Field Museum in Chicago. In 2007, she and her colleagues were rummaging around the fossil-dense rocks in Argentina’s Huincul Formation. The trip was going terribly—so terribly that some members of the party had begun joking that they’d been cursed by the local goddess Gualichu, bringer of misfortune. As the expedition drew to a close, Shinya and her colleague Peter Makovicky were losing hope. Speaking in a press statement, Shinya remembered that “ ... Pete joked, ‘It’s the last day, you’d better find something good!’” Right after he said that, Shinya says, she uncovered some very unusual fossilized bones. “I could tell right away that it was good.”

Good, yes. Easily identifiable? No. The team had not uncovered an entire skeleton (a very rare find), but they had found enough pieces to determine that the owner of the bones was pretty darn weird.

Image credit: © Jorge González and Pablo Lara

In life, the theropod dinosaur would have been about the size of a polar bear, with strong legs, powerful jaws, and leeeeetle teeny arms.

The newcomer is “kind of a mosaic dinosaur,” Makovicky said. “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.”

And yes, T. rex is the new dinosaur’s cousin, but only in the way that Yao Ming and Julianne Moore are yours. The two species descend from completely different branches of the theropod family tree. In fact, G. shinyae appears to occupy its own branch entirely. The researchers argue in their paper that the dinosaur’s unique jumble of traits means that it is not only its own species but also its own genus. They named their find Gualicho shinyae, after the bothersome goddess who got in their way and the determined researcher who overcame her.

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