The image of the hulking T. rex and its comically undersized arms is deeply ingrained in pop culture, but it isn't really fair. They were muscular little appendages well-suited to their evolutionary purpose. The wimpy-armed Carnotaurus is another story entirely.
As you can see in the skeleton up top, Carnotaurus had arms that even T. Rex would snicker at. It wasn't a completely risible dinosaur, in fairness - its name does mean "meat-eating bull", which is pretty cool, and it did have a pair of horns sticking out of its skull to really complete the bullish look. While Tyrannosaurus ruled Asia and North America in the Late Cretaceous, Carnotaurus occupied a similar role in South America.
But there really isn't any getting away from just how ridiculously tiny its arms were. Indeed, some paleontologists had speculated that these were entirely vestigial limbs, a useless reminder of when the dinosaur had working arms. Now paleontologist Javier Ruiz and his colleagues can add a little more to the puzzle of just why Carnotaurus made do with such wimpy arms.
They point out that Carnotaurus probably didn't have completely useless arms - the radius and ulna bones that make up the lower arm appear surprisingly strong and robust, even if they're just a quarter the length of the upper arm. But the hand does appear unusually weak, especially compared to the rest of the arm. And yet the hand still had four fingers, suggesting hands still served some evolutionary purpose or had only relatively recently become vestigial.
This all fits with a broader evolutionary trend of dinosaurs like Carnotaurus, in which arm bones that were strong and powerful in the Jurassic had become borderline vestigial in the Cretaceous. Just why this is remains a mystery, and the tiny size and weak hands of Carnotaurus represent a particular extreme. For now, all we can really say is that Carnotaurus probably takes the prize for the most completely useless arms of any giant dinosaur...but, uh, don't ask me to say that to its face. Its face has horns.