At no other time in the history of life on Earth were there animals like the pterosaurs, those flying creatures who truly did rule the sky. They were the first vertebrates to achieve flight, and they did it in a way that was all their own. In addition to very distinctly shaped, thin, hollow bones, pterosaurs had membranous wings which were probably flexible and able to change shape during flight. The actual mechanical details are not completely understood by paleontologists, thus giving rise to multiple opinions.