Dino Heart.

Hunting for fossils (preserved remnants of animals or plants) demands grueling hours digging up dirt. But when expert dino hunter Michael Hammer unearthed the skeleton of a Thescelosaurus (a small plant-eating dinosaur) in South Dakota, the tedious task revealed a "heart-stopping" find.

After examining an X-ray scan of the fossilized dinosaur, Hammer and his team discovered that dirt in the dino's rib cage contained a fossilized four-chambered heart--an organ found in warm-blooded animals but rarely in reptiles. "It's an astonishing dead ringer for the heart of a mammal or bird today," says Dale Russell, a paleontologist (fossil scientist) at North Carolina State University in Raleigh. The dino's high-performance heart was capable of pumping enough oxygen-rich blood to power a super-active lifestyle. Scientists always thought dinos possessed three-chambered hearts like some slow and sluggish reptiles.

The mammalian and avian (birdlike) four-chambered heart pumps cold blood to the lungs and warm blood to the rest of the body. Warm-blooded animals use their metabolism (conversion of food into heat and energy) to regulate body temperature. Reptiles, however, are cold-blooded and possess three-chambered hearts that pump cold blood to both the lungs and the body; their body temperature changes according to their external surroundings.

The astonishing find sheds light on the age-old question: Were dinosaurs warm-blooded cousins to modern birds? It's more likely than ever, Hammer thinks. And that comes from the heart.