Our Tiny Arms Aren't Useless

Being universally obligate bipeds, theropods often have no need for large arms. Cartoons often depict small-armed theropods as incapable or handicapped, but just because the arms were small doesn't mean they were vestigial. Counterclockwise from top right:

Tyrannosaurus: Its small arms evidently were a compromise for its huge, bone-crushing skull; too much weight at the front end of the body and it would constantly be tipping over. They were still extremely strong, though; strength estimates indicate that it was more than strong enough to lift this doomed ornithomimosaur into reach of its killing jaws.

Rugops: Spoiler alert! Inspired by All Yesterdays. Its arms were no good for hunting, with tiny fingers and no real elbows, but they were very mobile, suggesting they had another function...

Struthio: Totally incapable of flight, it was freed from the burden of giant wings. Its small, fluffy remiges are instead as air brakes for sharp turns, which is useful since it is one of the fastest-ever theropods. (Mesembriornis may have been as fast as a cheetah.)

Alvarezsaurus: An incongruous combination of the body of a typical fast dinosaur and little one-clawed arms oddly similar to an anteater's. It may have used them to dig into trees or mounds and eat termites.