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T. rex lookalike suggests that tiny arms developed for a purpose

By Josh Gabbatiss

13 July 2016

Artist's impression of two T. rex chasing smaller dinosaurs

Who needs big arms when you have big teeth?

Jorge González and Pablo Lara

While Tyrannosaurus rex was stalking prey some 68 million years ago in what is now North America, down south an unrelated predator was trying out a similar look.

A new dinosaur species, Gualicho shinyae, unearthed in Argentina’s Huincul formation bears a mosaic of features and looks a bit like an Allosaurus, a subgroup of two-legged, bird-like theropods. Its most striking feature, however, is its tiny arms that look like those of a T. rex — even though the two species lived on separate continents.

The Huincul formation is renowned for remarkable dinosaur fossils, including Argentinosaurus – perhaps the largest dinosaur ever. But nothing quite like Gualicho had ever been found here.

Gualicho has anatomical traits that you see in very disparate groups of theropod dinosaurs,” says Pete Makovicky of The Field Museum in Chicago, who described this “mosaic” dinosaur. “It appears to be closest to Allosaurus, but it doesn’t quite fit comfortably.”

Armed and dangerous

The team estimates Gualicho was between 7 and 8 metres long and weighed roughly the same as a polar bear, but its arms were only the size of a human child’s.

The functionality of T. rex’s tiny arms has long been a source of debate, with theories ranging from grasping onto a partner during mating to holding prey in place to deliver a death blow. Many experts still think that the arms served no useful function, but the fact that Tyrannosaurus and Gualicho have both independently evolved the same highly reduced forelimbs with two fingers, despite occupying different branches of the dinosaur family tree, suggests otherwise.

“It seems to be something that’s adaptive in lineages of dominant predators,” says Makovicky. If a feature appears separately multiple times, it becomes less and less likely that it evolved accidentally.

Thomas Carr of Carthage College, Wisconsin, agrees that these were not useless appendages. “Sure, they only have short arms and two fingers, but it’s clear with Tyrannosaurs at least that those arms were powerful,” he says. For Tyrannosaurus, though, the real power would have been in its enormous skull, which it probably used to hunt meaning it didn’t need big arms. It’s possible that Gualicho had a similar hunting strategy, which led to it also having small arms.

Journal reference: PLOS One, DOI: 10.1371/journal.pone.0157793

Read more: Stunning fossils: Dinosaur death match

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