The remains are those of a baby Scipionyx samniticus. The creature lived about 110 million years ago and bore some similarity to a velociraptor - the frightening, killer dinosaur featured in Stephen Spielberg's film, Jurassic Park.

Nicholas Geist: They were fast and dangerous

The quality of the fossil is so good, say the researchers from Oregon State University (ORSTU), that is has provided an unprecedented view of the creature's biology and given us a good indication of why the dinosaurs ruled the Earth for so long.

The Scipionyx was a "turbocharged reptile," says Nicholas Geist, a paleobiologist at ORSTU.

"If you could go back in time and see one of them, that's probably the last thing you'd ever see."

Rosetta stone

The Scipionyx was discovered in Italy just a few years ago. The skeleton is intact and there are remnants of the liver, large intestine, windpipe and even muscles.

"The baby dinosaur probably died in a shallow, still, saltwater marsh that preserved its structure incredibly well. It's like a Rosetta stone for paleontology, and shows us more about dinosaur biology than we ever knew before," says ORSTU colleague Terry Jones.

The ORSTU analysis of the fossil remains reveals an animal that had the best of both worlds. Like other cold-blooded creatures, it had a low metabolic rate while at rest, which is an excellent strategy for conserving energy.

But its enhanced lung ventilation capacity gave it the potential for the type of aggressive, extended activity typical of birds and mammals.

"These theropod dinosaurs were fast, dangerous animals," Geist says, "certainly not slow or sluggish. They could conserve energy much of the time and then go like hell whenever they wanted to. That might go a long way towards explaining why they were able to dominate mammals for 150 million years."

Bird theory

The research, published in the journal Science, also puts a question mark against the popular theory that dinosaurs might have been the ancestors of birds.

The lungs and other structures found in this dinosaur fossil bear little similarity to that of modern birds, the team say.

The internal organs are clearly visible

They point to the clear presence of a complete separation of the body cavity into two parts, one containing the lungs and heart, and the other holding the liver and guts.

This sort of body cavity partitioning is only seen in living animals that use an active diaphragm to help ventilate their lungs, such as mammals and crocodilians. The liver of these carnivorous dinosaurs was pulled back by large muscles causing it to act like a piston.

This "hepatic piston" mechanism probably enhanced the capacity for high levels of oxygen exchange - and the fast-paced activity associated with it - that could have matched some mammals.

"This type of physiology would provide some metabolic advantages unlike that of any animal still alive today," Jones says. "But for various reasons it only works well in a warm, equitable climate, which most of the world had during the age of dinosaurs."

Lively debate

The ORSTU position on the dinosaur-bird link is sure to provoke a strong reaction.

Proponents of the link point to the recent discovery in China's Liaoning Province of a Protarchaeopteryx robusta and a Caudipteryx zoui. The Caudipteryx, in particular, displayed the features of a theropod dinosaur but had feathers over its body.

Caudipteryx zoui: Features like a bird and a theropod

They will also argue that bird-like lungs could have evolved much later.

Bernice Wuethrich from the National Museum of Natural History in Washington DC, writing in a Science commentary, says there are also questions about the reliability of the Scipionyx fossil itself.

She quotes paleontologist Phil Currie of the Royal Tyrell Museum in Alberta, Canada. He says: "You can't take a squashed specimen and interpret the position and shape of any soft organ inside."