http://www.newsweek.com/t-rex-arms-evolved-sex-meraxes-1722901

The characteristic small arms of the T. rex might have been used to help them in the throes of passion.

Scientists in Argentina have discovered a new species of dinosaur predator that has similar little arms to a T. rex, which they think it may have used to have sex, leading them to suggest that T. rex evolved them for the same reasons.