T-REX males used their tiny arms to grip their female dinosaur mates during sex, scientists now believe.
The way the bones join up in a newly found, near perfect skeleton prove the stunted limbs supported strong muscles.
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It means the arms, which have mystified boffins for decades, had a purpose and did not shrink because they were useless.
A team came to the conclusion after studying the fossil of Meraxes gigas, a new species, which grew to 36ft and weighed four tons.
Like T-rex, it had a giant head. But it was also decorated with crests, bumps, furrows and small horns.
The experts say the two dinos can be compared as they were similarly shaped predators, although they evolved to have tiny arms independently.
The fossil was found in Patagonia in Argentina, and lived 90million years ago — around 20million before Tyrannosaurus rex evolved.
It showed complete regions of the skeleton never seen before, such as the arms and legs and, say the scientists, contains a lot of novel information.
Project 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.
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“Sex and balance are most likely.”
Dr Canale, of Ernesto Bachmann Palaeontological Museum in Neuquen, Argentina, 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.”
