Humans might sometimes remind each other to “remember to breathe”, but the process itself seems pretty straightforward. We expand our lungs to let air in, and contract them to push it out. Birds, however, do things differently. Their lungs don't move, instead, they have air sacks outside them that pump air in and out. Birds are the last surviving dinosaurs, so we might expect their lost relatives did the same thing, but one fossil proves that wasn't always the case.
Heterodontosaurus tucki is the oldest known member of the Ornithischians, a venerable dinosaur clade that includes such favorites as Stegosaurus and hadrosaurs. “Heterodontosaurus is a missing link between the ancestors of dinosaurs and the bigger, charismatic species we know,” said University of Minnesota PhD student Viktor Radermacher in a statement.
Lungs and air sacks almost never fossilize, so if we want to know how extinct animals drew breath we need to look to the bones that supported their breathing apparatus. In eLife, Radermacher describes examining a spectacularly well-preserved Heterodontosaurus fossil from South Africa to show it had a different breathing technique, expanding both chest and lower abdomen to get air in and out. He calls this system “pelvic bellows”. The evidence comes in the form of the ribs, which are shaped like paddles and small bones running off them that pulled the lungs in and out.
Radermacher applied the ultra-high powered x-rays of a synchrotron to examine the skeleton in detail, and reconstruct the muscles it supported.
Had Triceratops inherited this trait from their ancestors it would represent a rare case of Jurassic Park having got the science right 28 years before the scientists got there. Sadly, however, the paper notes; "this increased motion of the chest was only possible in more primitive ornithischians. More advanced species lost much of the anatomy that made this motion possible.”
Although birds are theropods, and therefore more closely related to T-Rexes than to ornithiscians, Heterodontosaurus and its descendants had pelvic structures so closely resembling modern birds their name actually means “bird-hipped”. Despite this, it seems the early ornithischian pelvis was nothing like that of birds. Some birds have skeletons that reveal the presence of air sacks but others, such as diving birds, do not, even though the sacks are there. This has left paleontologists struggling to know how to interpret the bodies of extinct species without these structures.
"The takeaway message is that there are many ways to breathe," Radermacher said. "And the really interesting thing about life on Earth is that we all have different strategies to do the same thing, and we've just identified a new strategy of breathing."