Which group of reptiles is unique for having a four-chambered heart?

The reptile group unique for having a four-chambered heart is crocodilians. This group includes crocodiles, alligators, caimans and gharials, whose hearts have two atria and two ventricles, much like those of birds and mammals.

Most living reptiles have a three-chambered heart, in which the ventricles are not completely separated. Crocodilians are the major exception. Their unusually advanced circulation separates oxygen-rich and oxygen-poor blood more effectively, supporting their large bodies and active, semi-aquatic lifestyles.

Crocodilian circulation has another unusual feature: the left and right aortas connect through the Foramen of Panizza. Specialized valves can redirect blood away from the lungs during diving. This does not mean crocodilians are mammals or birds; they remain reptiles, but their cardiovascular system is exceptionally specialized.

A common mix-up is assuming turtles, snakes or lizards also have four chambers because they can be active predators. They generally retain the typical reptilian three-chambered arrangement, while crocodilians stand out anatomically.