A study of the fossil therapod - the group that includes T rex and the vicious velociraptors of Steven Spielberg's Jurassic Park - shows their organs were perfectly adapted to frantic bursts of speed when it was necessary.
The fossil of a baby Scipionyx, which lived 110 million years ago and bore a resemblance to velociraptors, displays within the body cavity a partition separating the heart and lungs from the liver and guts. Scientists believe this acted as a primitive diaphragm, which ventilated the lungs during periods of intense activity.
Nicholas Geist, a dinosaur expert at Oregon State University and member of the team that studied the fossil, said the find has shed new light on the behaviour of the dinosaurs and could help to resolve whether they were cold- blooded, like reptiles, or warm-blooded, like mammals. "The therapod dinosaurs were fast, dangerous animals, certainly not slow or sluggish. They could conserve energy much of the time and then go like hell whenever they wanted to ...
"This fossil is helping to confirm the dinosaurs were ... cold-blooded ... But the extraordinary condition of the fossil allows us to hang some meat on the bones of these animals and bring them back to life a little bit. It's almost like a dinosaur dissection."
Cold-blooded animals in a warm climate can move quickly, Dr Geist added. "Then, if you add in the lung capacity that we're finding for meat-eating dinosaurs, what you have is a turbo-charged reptile. If you could go back in time and saw one of them, that's probably the last thing you'd ever see."
The fossil Scipionyx was found in Italy. Terry Jones, another member of the Oregon team, said: "The baby dinosaur probably died in a ... saltwater marsh that preserved its structure incredibly well. It's like a Rosetta stone for palaeontology, and shows us more about dinosaur biology than we ever knew before." Details of the findings are in Science, journal of the American Association for the Advancement of Science, which yesterday opened its annual meeting in Los Angeles.