How Did Dinosaurs Breathe? A Lung System Mammals Never Managed

A mammalian lung is a dead-end system. Air enters through branching tubes, reaches small sacs where gas is exchanged, and leaves by the same route, which means fresh incoming air mixes with stale air already present and the lung never fully empties. A bird's system works differently. Air passes through the lungs and continues into a set of air sacs distributed through the body, and over two breathing cycles it is moved through the gas exchange tissue in one direction only, so the lung receives fresh air on inhalation and on exhalation. The tissue itself is arranged as fine open tubes rather than blind sacs, and blood flows across the airflow rather than alongside it, which extracts oxygen more completely. The result is markedly better performance at low oxygen concentrations, demonstrated by bar-headed geese crossing the Himalaya at altitudes where a mammal of the same size could not stay conscious, and the whole system is also lighter than an equivalent mammalian one.

Air sacs leave traces in the skeleton, which is how the system can be identified in extinct animals:

• •Pneumatic openings in vertebrae and other bones, where extensions of the air sacs invaded the bone and hollowed it out, leaving distinctive foramina and internal chambers
• •The same pattern in living birds, which allows the fossil structures to be interpreted with confidence rather than by guesswork
• •Its presence in saurischian dinosaurs, including theropods and sauropods, which places the system long before flight
• •Its absence in ornithischian dinosaurs, indicating the feature was not universal among dinosaurs
• •Its presence in pterosaurs, which are not dinosaurs, suggesting either a shared ancestral origin or an independent one
• •Increasing extent through theropod evolution towards birds, with progressively more of the skeleton invaded, which makes the sequence visible across the fossil record

An efficient respiratory system has consequences beyond breathing. It reduces the weight of the skeleton substantially, which matters for any large animal and is part of how sauropods managed their size, and it supplies a mechanism for shedding heat, since air moving through sacs inside the body can carry heat away from a core that has no other route to the surface in a very large animal. It permits high metabolic rates, which the bone microstructure independently supports, since the growth rates recorded in dinosaur bone are far above those of living reptiles and comparable to those of birds and mammals. And it would have allowed activity at oxygen levels lower than today's, which matters because atmospheric oxygen fluctuated considerably through the Mesozoic and was for periods below the modern level, a fact that has been proposed as one reason the bird-style system was advantageous when it appeared. Flight itself came much later, and inherited the system rather than causing it.

The claim rests on more than the bones. Comparative anatomy establishes that in living birds the pneumatic features correspond to specific air sacs, which gives a direct interpretive key. The phylogenetic bracket provides a further check, since dinosaurs sit between crocodilians and birds, and crocodilians turn out to have unidirectional airflow too, which was demonstrated experimentally in the 2010s and was genuinely surprising, since crocodilians have no air sacs. That finding pushed the origin of one-way airflow back further still, to the common ancestor of the whole archosaur group, and it means the system predates dinosaurs rather than being their innovation. Computational modelling of airflow in reconstructed anatomy supports the interpretation. The remaining uncertainties concern the details: how extensive the sacs were in particular groups, how much of the system ornithischians possessed given their lack of skeletal pneumaticity, and how the mechanics of ventilation worked without the flight muscles and sternal movement that birds use.

Air passes through a bird's lungs in one direction into a system of sacs, so fresh air crosses the exchange tissue on both inhalation and exhalation, which is why geese fly over the Himalaya. Air sacs hollow out bones and leave identifiable openings, which places the system in theropods and sauropods long before flight. Crocodilians turned out to have one-way airflow too, pushing the origin back further.