Pterosaurs also had special bones. They were extremely light (even more than bird's wings – some were almost as thin as a piece of paper), and some were almost hollow. Tiny holes in the bones are evidence of air sacs which extended into the vertebrae and limb bones, as it does with birds. Also present were supporting struts which made these bones stronger. With these special bones, even the largest of pterosaurs, "Quetzalcoatlus", probably weighed less than 200 pounds.