By Helen Briggs BBC News science reporter

We may see a challenge to the idea that the oldest dinosaur was bi-pedal and warm-blooded Martin Sander

The family tree of dinosaurs may have to be revised, with the discovery that some could adjust their growth rates.

Until now, most dinosaurs were thought to be warm-blooded, with a steady growth rate independent of environmental factors such as food.

But a study in Science magazine shows that at least one dinosaur came in "little and large" forms.

It raises questions about the descent of dinosaurs from a warm-blooded ancestor, researchers say.

Palaeontologists Martin Sander and Nicole Klein from the University of Bonn, Germany, studied the bones of several specimens of the prosauropod Plateosaurus engelhardti.

'German' dinosaur

The plant-eating dinosaur lived about 200 million years ago in what is now Central Europe. It was one of the first large dinosaurs, with an elephantine body, long neck and long tail.

Martin Sander sampling dinosaur bones with a core drill. (Image: E Premru)

Dinosaur bone growth is marked by rings, rather like the growth rings of trees, which give an indication of how quickly the animals grew.

From their bone structure, experts can also find out how old they were when they stopped growing.

The evidence suggests that some plateosaurs had reached their maximum size by the age of 12, while others were still growing at 27.

The smallest specimen was 4.8m long when fully grown, whereas others reached a giant 10m long.

This variation in growth rate and adult size has not been found in other dinosaurs studied to date, such as Tyrannosaurus.

Common ancestor

The report in Science marks another twist in the long-running debate about how dinosaurs regulated their body temperature.

Polished section of Plateosaurus bone with growth marks. (Image: PM Sander)

Most scientists believe that dinosaurs were warm-blooded and, as such, grew steadily according to a fixed genetic blueprint, rather than relying on warmth and food from the environment.

The plateosaur appears to be an intermediary form, somewhere between cold-blooded reptiles and warm-blooded mammals and birds.

Since the common reptilian ancestor of the dinosaurs, and their closest relatives, the pterosaurs, or flying reptiles, was believed to have been warm-blooded, the Bonn discovery could throw ideas about their evolution into disarray.

According to Martin Sander, the most likely explanation is that warm-bloodedness evolved several times in the history of the dinosaur and was not inherited from a common ancestor.

"My hunch right now is that maybe there was repeated evolution of warm-bloodedness," he told the BBC News website.

Intriguingly, the paper adds weight to emerging evidence challenging the idea that the first dinosaurs ran on two legs and were warm-blooded, he says.

"We may see the beginning of a paradigm shift, a challenge to the idea that the oldest dinosaur was bi-pedal and warm-blooded," Dr Sander explained.

"There are several lines of evidence that the oldest dinosaur may have walked on four legs and maybe was not warm-blooded.