Pterosaurs were the principal flying creatures during much of the Mesozoic Era (birds
evolved at the end of the Jurassic Period but did not appear to attain great size or numbers
until the Tertiary Period). Some pterosaurs, such as Pteranodon and Quetzalcoatlus, were
the largest flying creatures ever, with wingspans in excess of 12mJ40ft; The front limbs had
evolved into wings, held out mainly by the very elongated bones of the fourth finger. A
pterosaur's body had many weight-saving features, including tube-like, hollow bones - as in
modern birds - so their remains are very rare and fragmentary.