This is called dermal bone, and it helps creatures deal with low oxygen levels. When a turtle or alligator stays under water for a long time, carbon dioxide builds up in their blood. This causes the blood to become acidic. The acidic blood leaches calcium and magnesium ions from bone, neutralizing the acid, and the rough surface of the bone gives more surface area for the blood to interact with. That lets the creature tolerate much more carbon dioxide than they could without that dermal bone. The same is true for fish that live in low oxygen environments. The ions from the bone help them deal with excess carbon dioxide when they exert themselves to catch food or escape from predators.