Dinosaurs tended to be either massive (sauropods) or quick (ornithomimids), and sometimes even both (carnosaurs). These traits require robust cardiopulmonary systems to circulate adequate oxygen to maintain metabolism. Mammals and avians have four-chambered hearts and muscular diaphragms, while reptiles have neither. While fossil evidence strongly suggests that dinosaurs had four-chambered hearts, they likely lacked diaphragms; clearly, without special adaptation, it is difficult to adequately explain their mass and activity levels, even taking into account the comparatively higher atmospheric oxygen levels of the Mesozoic.

Paleontologists have long noted the hollow bones of many dinosaurs, but did not seriously consider these hollows to serve any other purpose than mass reduction. However, many biologists have started to seriously entertain the notion that dinosaurian respiration was more akin to that of birds than of mammals. In addition to the strong fossil evidence for four-chambered hearts and respiratory air sacs, further evidence suggests nasal turbinates to reduce respiratory water loss (a trait largely shared by birds and mammals).

Evidence strongly suggests a unidirectional respiratory system at least somewhat similar to that of birds, even though researchers have not formed an adequate hypothesis for the mechanism by which breathing took place. A reptilian hepatic piston would be unlikely to move adequate volumes of air through the lungs using mammalian metabolism as a model, but a cardiopulmonary system more reminiscent of that of birds yields better explanatory adequacy while still allowing for the mass and/or activity levels of many families of dinosaur.

However, ultimately this adaptation led to the demise of the dinosaurs, as it allowed them to have more mass for lung space, making them less buoyant, and causing them all to perish in the Flood.