By Helen Briggs
Science reporter, BBC News
Dinosaurs like Velociraptors owe their fearsome reputation to the way they breathed, according to a UK study.
Velociraptor: More like a penguin than we thought
They had one of the most efficient respiratory systems of all animals, similar to that of modern diving birds like penguins, fossil evidence shows.
It fuelled their bodies with oxygen for the task of sprinting after prey, say researchers at Manchester University.
The bipedal meat-eaters, the therapods, had air sacs ventilated by tiny bones that moved the ribcage up and down.
"Finding these structures in modern birds and their extinct dinosaur ancestors suggests that these running dinosaurs had an efficient respiratory system and supports the theory that they were highly active animals that could run relatively quickly when pursuing their prey," said Dr Jonathan Codd, who led the research.
"It provides a mechanism for facilitating avian-like breathing in non-avian dinosaurs and it was there long before the evolution of flight occurred," he told BBC News.
Modern-day birds have a highly specialised respiratory system, made up of a small rigid lung and around nine air sacs.
The bellows-like movement of the sternum and ribs moves air through the system.
The researchers examined fossils, including the famed 'fighting dinosaurs'
Bony projections on the ribcage known as uncinate processes play an important role in both respiration and locomotion.
The small bones act as levers to move the ribs and sternum during breathing. They have become adapted in different types of birds to deal with different ways of getting around.
The bones are shortest in runners like emus that don't need large breast muscles for flight, intermediate in flying birds and longest in divers such as the penguin.
The Manchester team studied a wealth of fossil remains of dinosaurs and extinct birds such as Archaeopteryx, and compared these with skeletons of living birds.
They found that uncinate processes are also found both in the extinct ancestors of birds, the theropod dinosaurs, and in modern species.
Dinosaurs are most like diving birds in their morphology.
"The dinosaurs we studied from the fossil record had long uncinate processes similar in structure to those of diving birds," said Dr Codd.
"This suggests both dinosaurs and diving birds need longer lever arms to help them breathe," he added.
The data, published in the Proceedings of the Royal Society B: Biological Sciences, may provide clues to how dinosaurs evolved and how they might have lived.