91av

PALAEONTOLOGISTS have found what appears to be a fossilised dinosaur heart in
the chest cavity of a 300-kilogram plant-eating beast that died 66 million years
ago.

The discovery may help resolve a long-running debate over dinosaurs’
metabolism. Early researchers thought dinosaurs were relatively inactive
“cold-blooded” creatures, like modern reptiles and crocodiles. But many
palaeontologists now believe dinosaurs generated their own body heat, allowing
them to lead more active lives. The fossilised heart resembles that of modern
birds and mammals, suggesting the dinosaur was “a pretty active fellow”, says
Dale Russell of North Carolina State University in Raleigh.

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 dinosaurs, differ
sharply in activity levels. Birds have active metabolisms based on
four-chambered hearts that separate oxygenated and oxygen-depleted blood. 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. Their four-chambered hearts contain a valve that
can shunt oxygen-depleted blood around the lungs when they dive. 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.

The CT images showed “two ventricular-like cavities and a cardiac structure
of the correct volume for an animal weighing 300 kilograms”, Russell reports.
Crucially, the aorta is bird-like. “We have this one aorta that originates and
arches back from the left ventricle,” says Mike Stoskopf, a veterinarian at
North Carolina State University. This points to an active, bird-like metabolism
for the dinosaurs. However, John Ruben of Oregon State University in Corvallis
warns that a second aorta may not have been preserved, like the pulmonary arch,
which is also missing.

More fossils should resolve the question. Bob Bakker, a Colorado
palaeontologist who champions the idea of active dinosaurs—believes the
odds of finding them are good. Heart muscles are tough, and should last longer
than other organs, he says. “I bet we traditional palaeontologists have dug
through a bunch of hearts in the past, not knowing these variations in the sand
were cardiac fossils.”