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A team of scientists have discovered a new predatory dinosaur which they think might help explain why the T-rex has such small arms.

The skeletal remains of the previously unknown species were dug up in Argentina.

Named the Meraxes gigas, it was about 36 feet (11m) long, about the size of a bus, with a skull 4 feet (1.2m) long - but its arms were only 2 feet long, which is tiny in proportion to its body.

The Tyrannosaurus rex is around 15 feet (4.5 metres) high, approximately 40 feet (12 metre) in length with arms about 3 feet long.

The scientists who have studied the new dinosaur believe the animal's small arms might have actually helped it to survive.

Juan Canale, lead author of the study said to Â鶹¹ÙÍøÊ×Ò³Èë¿Ú: "I'm convinced that those proportionally tiny arms had some sort of function."

"They may have used the arms for reproductive behaviour... or support themselves to stand back up after a break or a fall."

His co-author Peter Makovicky said the dinosaur's arms were "literally half the length of the skull and the animal would not have been able to reach its mouth".

Mr Makovicky said he believed the species' massive heads were the main predatory tool, taking on the functions that arms would have had in smaller species.

The findings are published in the Current Biology journal.

Meraxes gigas - which is named after a dragon in a series of fantasy books - is part of the theropod group of dinosaurs, which walked on two legs, in the carcharodontosaurus - or shark-toothed - family.

Previous theories on why the T-rex had such small arms included suggestions that it was a way to prevent them getting bitten.

The reptiles are believed to have roamed the Earth some 90-100 million years ago.