PTERODACTYLS means "Winged fingers". These dinosaurs were a group of flying reptiles that ranged in size from having a wingspan of a few inches to over 40 feet. They were lightly built with hollow bones, long, curved necks, long skulls, and small bodies. They had large brains and good eyesight. Some had short tails, some had no tails. Some had light-weight, bony head-crests, some did not. The crest may have acted as a rudder when flying or may have been a sexual characteristic. Some pterodactyloids had fur on their bodies. Their wings were covered by a leathery membrane. This thin but tough membrane stretched between its body, the top of its legs and its elongated fourth fingers, forming the structure of the wing. Claws protruded from the other fingers. Pterodactyloids could flap their wings and fly with power, but the largest ones probably relied on updrafts to help in flying.