Fossil evidence of the soft internal parts of dinosaurs, including the heart, is sadly lacking. But based on indirect evidence—and one heart from a dinosaur called “Willo”—paleontologists have extrapolated that dinosaurs had a divided heart capable of keeping the blood at two different pressures.

Dinosaur bone tissue also shows evidence of blood vessels. Therefore, a heart was necessary to drive the circulatory system, sending blood around the body. Dinosaurs with extremely long necks (such as the large sauropods) and those with heads held upright needed a high blood pressure. This would allow the blood to pump all the way to the brain when the animal was reaching for food. But such a blood system would have too high a pressure to safely circulate to the lungs for oxygenation. Thus, scientists believe dinosaurs probably had a divided heart capable of supplying blood at two different pressures into two separate circulatory systems.