Some researchers have suggested that, based on traces of internal organs found in some fossils, theropod dinosaurs had a respiratory system similar to the highly specialized system of modern crocodilians, with the liver used as a “piston” to pull air in and push it out. These researchers say that such a system would be unlikely to give rise to the kind of breathing system found in birds.
THESE ARE TWO separate questions: How did theropods breath? And how difficult is it to shift from one breathing system to another? The first focuses on the anatomical features present in coelurosaurs and other sorts of theropod dinosaurs. In fact, despite some similarities in side view, crocodilian and theropod hips are dramatically different in detail: In crocs, the spade-shaped pubis bones are on a mobile joint, and are actively moved by muscles to power the liver piston. In theropods the pubis bones are not mobile — and in fact are sometimes fused to the other hip bones — so that they could not operate in the same fashion. It may be that theropods had some form of liver-based piston, but it could not have been the crocodile style.
Furthermore, even the identity of the stain interpreted as the liver in the dinosaur in question is in doubt, as there are no particular anatomical features preserved to indicate it was indeed the animal’s liver.
The second question, whether it is possible to shift from one breathing system to another, is more problematic. No living animals other than birds have the same sort of one-way flow through lungs found in our feathered friends. Therefore, we cannot make direct comparisons with other instances of transitions from one sort of lung system to another.
Vertebrate history contains many instances of dramatic changes in breathing systems — from the lung-gill combination of some primitive bony fish to gill-only breathing in ray-finned fish, lung-only breathing in many land-dwelling vertebrates, skin-breathing in some amphibians and so forth. Claims that a particular anatomical shift is impossible may be overstating the case, given the power of evolution to transform.