Tyrannosaurus rex may have been king of the dinosaurs, but it was also president of the itty-bitty forelimb committee.

      Paleontologists aren’t quite sure why the massive killing machine had such tiny arms, but what they do know is that it wasn’t alone.

      There is Gualicho shinyae, a newly discovered dinosaur from Argentina that was also a member of the Late Cretaceous period’s puny-arm club. It weighed around a thousand pounds and stood about six feet tall. For all that size, its arms were as small as a little kid’s.

      Like T-rex it was a carnivorous, two-legged creature called a theropod. But what makes it interesting to paleontologists is that it isn’t directly related to T-rex. Rather, Gualicho shinyae evolved its stubby arms completely independently, according to a new study.

      The new dinosaur belonged to a different type of theropods called the allosaurs, which usually had strong, proportionally sized arms. It’s the only known member of the group to have feeble arms with two fingers. Along with the tyrannosaurs, and a second dinosaur group known as the abelisaurs, Gualicho shinyae is one of the few theropods with tiny arms that paleontologists have found.

      “It begins to tell us a little bit about the story of how and why animals like T-rex and other theropods reduce their forearms,” said Peter Makovicky, a paleontologist with the Field Museum in Chicago, and an author of a paper published Wednesday in the journal PLOS One.

      He said that the earliest ancestors of tyrannosaurs lived some 165 million years ago and had long arms with three digits. But by the time the creatures made it to the Late Cretaceous period, about 80 million years ago, they lost a finger and their arms became shorter.

      Although Gualicho shinyae comes from a different line of dinosaurs, studying its wimpy arms may provide some insight into the evolutionary pressures that might cause a giant like T-rex to end up with tiny forelimbs.

      “We don’t actually know what would have triggered a reduction in the forelimb in each individual lineage,” he said. “But obviously there was some adaptive advantage because we see it multiple times in different lineages of theropods”