 Heilmann's extremely thorough approach ensured that his book became a classic in the field, and its conclusions on bird origins, as with most other topics, were accepted by nearly all evolutionary biologists for the next four decades.Clavicles are relatively delicate bones and therefore in danger of being destroyed or at least damaged beyond recognition.Nevertheless, some fossil theropod clavicles had actually been excavated before Heilmann wrote his book but these had been misidentified.