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 , 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 archosaurthe family that includes crocodiles, dinosaurs, and birdsand 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 dont 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 theyre able to make use of it on both inhalation and exhalation.
Not only can they meet the oxygen demands of flying, says Jingmei OConnor, a vertebrate paleontologist and co-author on the paper, they can fly over the fricken Himalayas. Modern birds are ludicrously over-engineered.
Archeorynchus was a relatively primitive member of the Ornithuromorpha, the group that includes all modern birds. It would have been about the size and shape of a pigeon, OConnor says, with a toothless beak. It lived about 120 million years ago in the forests of China, a subtropical and seasonally chilly environment. The volcanically active region was prey to massive ash falls or occasional burps of poisonous gas that killed fish or dropped dead birds into the lakes.
These rapid burials often preserved soft tissues like plumage, which were what initially attracted OConnor and her colleagues attention. The specimenwhich OConnor described as looking like roadkillspans two slabs, and shows a pin-tail arrangement common in modern birds but previously unknown in fossil ones. (Picture a short, aerodynamic fan with two additional elongated feathers, like that of a or a )
But the team also noted the presence of what looked like soft tissue remains: a pair of white-specked organs in the chest, visible to the naked eye. Stomach contents tend to preserve as a carbonized black smear; livers, with their high iron content, look reddish. These looked a lot like lungs. While writing the paper, OConnor says, she mentioned these organs in passing; the point of the paper was to describe the animals plumage. But after six rounds of review the paper got rejected, she says. The stated reason? Not enough proof that the preserved soft tissues were actually lungs.
So the team got back to work.
Using electron microscopes, the they pored over 12 samples from the suspected lungs, comparing the tissues to 10 samples from elsewhere in the skeleton. The preserved features showed a miniscule honeycomb of tissue. When OConnor contacted John Maina, a specialist in modern-bird respiration, he replied that the fossil appeared to show the subdivided region of tissue responsible for gas exchanges in bird lungs. The tissues looked basically identical to that of living birds, OConnor says.
Archeorhynchus wouldnt be the first fossil species to have preserved lungs: a and a have also been recorded with lungs. "I think that considering what we have, we've put together a really good argument, OConnor says. Nothing is overwhelming evidence, but we're coming at it from several different lines: anatomical, topology, macroscopic and microscopic structure, and also, the fact is that lungs have preserved before in other animals."
Luis Chiappe, a specialist in early birds at the L.A. Museum of Natural History, agrees that O'Connor's argument is strong. I think shes made a very compelling case that those structures are part of the lungs of this animal, he says. (OConnor is one of Chaiappe's grad students.) While he hadnt seen the paper prior to the teams presentation last week at the Society of Vertebrate Paleontology meeting in Albuquerque, he says the preservation on the possible lung tissue looks spectacular. But, he cautions, theres more research to be done. , Corwin Sullivan, a paleontologist at the University of Alberta in Edmonton, Canada, also sounded a note of caution about whether its possible for relatively delicate lung tissues to be preserved in partial three dimensions.
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, OConnor says, its 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.
This is something that we need to be aware of when we're interpreting all these skeletons: Whatever we're estimating is a worst-case scenario," O'Connor says. "They most likely had greater abilities."