New dinosaur species had tiny arms like T. rex

The characteristically diminutive arms of Tyrannosaurus rex that have long been the subject of jokes and internet memes may have been more widespread in the Late Cretaceous than first thought.

A newly described dinosaur, uncovered in Patagonia and described by researchers from the US and Argentina, also bears short, two-fingered claws that appear relatively small for its body size – but it’s no relation to the T. rex.

Gualicho shinyae was uncovered in 2007 at the Huincul Formation, a fossil-rich rocky outcrop in Patagonia’s north.

According to Peter Makovicky, curator of dinosaurs at Chicago’s Field Museum and co-author of the paper, Gualicho is a theropod – a group of large carnivorous dinosaurs that walked on two legs and shared characteristics with birds. But Gualicho also represents a mish-mash of familiar traits, making it hard to pin down.

“Gualicho is kind of a mosaic dinosaur – it has features that you normally see in different kinds of theropods,” he says. “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.”

Back to those tiny arms: Gualicho is estimated to have weighed around 450 kilograms, making it a medium-sized predator, but its forearms are comparable in size to those of a human child.

Like T. rex, Gualicho’s forearms bear only a thumb and forefinger. The discovery contributes to mounting evidence that these small limbs evolved separately in a number of species for reasons as yet unknown.

“By learning more about how reduced forelimbs evolved, we may be able to figure out why they evolved,” Makovicky adds.

The paper describes the new species as a close relative to Deltadromeus, a carnivore that hailed from northern Africa.

Both species are characterised by long, slender hind limbs. The relationship contributes to a growing database of similarities between fossils uncovered in the Huincul Formation, and those found in rocks in Africa such as Morocco’s Kem Kem Beds, where Deltadromeus was discovered.

The new species name references the dinosaur’s discoverer, Akiko Shinya from the Field Museum. The name Gualicho is derived from Gualichu, a spirit of Patagonia’s Tehuelche people.

As Shinya recounts: “We found Gualicho at the very end of the expedition. 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 new species is described in the journal PLOS One.