Fossils, in the popular imagination, preserve hard matter: bones, trackways in the rock, or perhaps even the occasional impressions of feathers in an exceptional skeleton. But when the remnants of soft tissues can be detected, they can unlock a realm of information missing if you only have a skeleton.
Take the late Cretaceous bird Archaeorhynchus spathula, previously known from four specimens with well-preserved bones: useful, but not about to set the world on fire. According to a paper newly published in PNAS, though, the fifth one is the kicker: Not only does it preserve extensive plumage, it appears to show a set of preserved lungs. If confirmed, they would be the first such set from a fossil archosaur—the family that includes crocodiles, dinosaurs, and birds—and suggest that the modern style of bird respiration evolved much earlier than suspected.
Birds have the most complex (and most efficient) respiratory system of any living vertebrate, which they need in order to handle the incredibly difficult task of flying. While mammal lungs pump oxygen in and carbon dioxide out, birds developed a different system. Their lungs don’t expand and contract: Instead, they use a network of air sacs that act like bellows, pulling oxygen through the lungs in such a way that they’re able to make use of it on both inhalation and exhalation.
If these are indeed lungs, however, their presence suggests that the pulmonary specializations that help birds take in oxygen were present as early as 128 million years ago, when the skeletal traces of respiration looked primitive. Since paleontologists usually only have skeletons to work off of, O’Connor says, it’s easy to assume that the soft tissue goes in step with the skeleton. But the lungs add to the growing evidence that skeletal adaptations actually lag behind soft tissue ones, and that the traits key to modern birds' success (digestion and breathing, for example) might have appeared quite a bit earlier than previously suspected.