 They also presented this as a reason for doubting that birds descended from dinosaurs.Critics have claimed that, without avian air sacs, modest improvements in a few aspects of a modern reptile's circulatory and respiratory systems would enable the reptile to achieve 50% to 70% of the oxygen flow of a mammal of similar size, and that lack of avian air sacs would not prevent the development of endothermy.Very few formal rebuttals have been published in scientific journals of Ruben "et al.