Fearsome Dinosaur With Ludicrously Tiny Arms Means T. Rex Wasn't Alone

Artists' impression of two G. shinyae hunting down their prey. Jorge González and Pablo Lara

Despite its fearsomeness, the Tyrannosaurus rex was somewhat of an awkward, clumsy dinosaur. If it fell forwards while running, its tiny arms would not only be useless in helping it stand up again, but it would hit the ground so hard it’d crush its own skull. In fact, its silly arms are one of its defining features, and a new PLOS ONE study has revealed that this bizarre trait evolved independently in other dinosaurs.

A newly-excavated dinosaur from Patagonia has been found with a pair of ludicrously tiny arms ending in short, two-fingered claws. This sizeable theropod was a carnivorous, bipedal, bird-like dinosaur, meaning that it’s somewhat related to the T. rex.

However, it’s technically part of the Allosauridae group – a completely different branch of the family tree. This means that little forelimbs didn’t evolve from a short-armed common ancestor, but arose separately several times in a process known as convergent evolution.

Gualicho [shinyae] is kind of a mosaic dinosaur, it has features that you normally see in different kinds of theropods,” corresponding author Peter Makovicky, The Field Museum's Curator of Dinosaurs, said in a statement. “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.”

Dating back to the Late Cretaceous, the final chapter in the story of the non-avian dinosaurs, this 90-million-year-old beast would have been a fearsome predator, hunting down prey with its powerful legs and crushing them with razor sharp teeth-lined jaws. It was about 1.8 meters (6 feet) tall, 7.6 meters (25 feet) long, and weighed about 1 tonne (1.1 tons). It also predated the appearance of the famous T. rex by about 23 million years.

Just like the T. rex’s, the true function of its arms – actually the size of a human child’s arms – remains decidedly unclear. Some have hypothesized that they were used as part of mating practices, but there really isn’t enough evidence to confidently conclude anything at this point.

A sketch of the new beast. Only the white-shaded parts were found, so around three-quarters of the skeleton had to be inferred. Jorge González and Pablo Lara/PLOS ONE

This new fossil discovery adds credence to the idea that there was some sort of evolutionary pressure on large carnivorous theropods to reduce their arm length. When creatures evolve, if they gain a new morphological advantage they have to normally lose something else from their body plan – new features cannot simply keep adding to the original.

When human ancestors began to evolve larger brains, for example, they also lost a lot of muscle mass at the same time. The same idea applies to these carnivorous beasts – by losing any useful arms, they may have gained something elsewhere, perhaps increased muscle mass in their legs. In any case, there appeared to be no real practical use for their arms, as they were perfectly adapted to hunting down their prey without them.

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

The genus of G. shinyae is named after Gualichu, a spirit revered by Patagonia’s Tehuelche people. The team of paleontologists decided to name their fossilized monster after it after they joked about the string of bad luck, which they dubbed the “curse of Gualichu”, that befell them during their expedition. In one particular incident, their truck hit a severe bump in the road and rolled over – although no-one was injured, at least.

The curse strikes again. Credit: Pete Makovicky

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