But many palaeontologists now believe dinosaur s generated their own body heat, allowing them to lead more active lives. The metabolism debate is hard to resolve without access to soft tissues, which are hardly ever preserved. Arguments for and against the two views have relied on circumstantial evidence from microscopic analysis of bone structure. Birds and crocodiles, the closest surviving relatives of dinosaur s, differ sharply in activity levels. The right ventricle pumps oxygen-depleted blood from the body to the lungs, while the left ventricle takes the oxygenated blood from the lungs and pumps it through a single aorta to the body.
Crocodiles are less active.
Blood from the left ventricle is carried away by a pair of parallel aortas. This is an improvement over the three-chambered hearts of lizards, but limits them to brief bursts of activity. The ribcage of the new fossil, a previously obscure animal called Thescelosaurus, was unusually well preserved when found in a dry river bed in northwestern South Dakota. Sand had filled the cavity, so the bones were not flattened. Mike Hammer, the Oregon fossil collector who found it, suspected a hard, iron-rich lump inside the ribcage might be the heart, and a doctor he showed it to suggested that a CT scan might show its structure.