 Fossil evidence also demonstrates that birds and dinosaurs shared features such as hollow, pneumatized bones, gastroliths in the digestive system, nest-building and brooding behaviors.Although the origin of birds has historically been a contentious topic within evolutionary biology, only a few scientists still dispute the dinosaurian origin of birds, suggesting descent from other types of archosaurian reptiles.Within the consensus that supports dinosaurian ancestry, the exact sequence of evolutionary events that gave rise to the early birds within maniraptoran theropods is disputed.