Dinosaurs as big as buses or five-story buildings would not be possible if their bones were dense and heavy like ours. Like present-day birds, dinosaurs had hollow bones with inner structures known as air sacs, which made their skeletons lighter and less dense. These structures were apparently so advantageous that they emerged at least three times during the evolution of dinosaurs and pterosaurs (flying reptiles), according to a study described in Scientific Reports. Read More