T-Rex males used their little arms to grip female companions during sex.
Male T-Rex's used their little arms to grip females during sex
Scientists believe that the stunted limbs supported strong muscles after studying the skeleton of one of the dinosaurs.
The T-Rex arms have mystified scientists for decades but the study on the fossil - which was discovered in Argentina - proves that the limbs did have a purpose.
The boffins studied the fossil of the Meraxes gigas, which grew to 36 feet and can be compared with a T-Rex as it has a similar shape.
Research leader Dr. Juan Canale said: "The skeleton shows large muscle insertions... so the arm had strong muscles. I am convinced those proportionally tiny arms had a function.
"The harder question is what exactly the functions were. Sex and balance are most likely."
He added: "They may have used the arms for reproductive behaviour such as holding the female during mating or to support themselves to stand back up after a fall."