Final answer:
Crocodilians are unique among nonavian reptiles because they can shunt blood, facilitated by their four-chambered heart with specialized structures like the foramen of Panizza.
Step-by-step explanation:
The unique feature of crocodilians among nonavian reptiles is that they have the ability to shunt blood. This is shown by their unique circulatory mechanism where the heart can direct blood away from the lungs and towards the stomach and other organs when they are submerged for long periods. Crocodilians have a four-chambered heart, like birds and mammals, but with special adaptations such as the foramen of Panizza and specialized connective tissue, which allow a certain amount of mixing of oxygenated and deoxygenated blood, and thus, blood can be shunted.