ALBUQUERQUE — Fossilized lungs found preserved along with an ancient bird may breathe new life into studies of early avian respiration. If confirmed as lungs, the find marks the first time that researchers have spotted the respiratory organs in a bird fossil.
Scientists have previously described four fossils of Archaeorhynchus spathula, an early beaked and feathered bird that lived about 120 million years ago. But unlike those discoveries, a newly described fifth specimen contains significant traces of plumage, and, even more startling, the probable remnants of a pair of lungs, researchers say.
Vertebrate paleontologist Jingmai O’Connor and colleagues reported the findings October 18 at the Society for Vertebrate Paleontology’s annual meeting. The results were also published online October 22 in the Proceedings of the National Academy of Sciences.
About the size of a thrush, Archaeorhynchus was among the earliest ornithuromorphs, the lineage that led to modern birds. It was probably an herbivore, as all known fossils of the creature contain preserved gastroliths, or gizzard stones which some animals use to help grind food, in the belly. The new fossil was found in northeastern China and is part of the Jehol Biota. That wealth of well-preserved fossils dates to between 133 million and 120 million years ago and includes numerous feathered dinosaurs as well as birds.
It’s rare for soft tissues to survive the pressure, heat and chemical transformation of fossilization. But scientists are increasingly reporting finding fossilized feathers, skin and even bits of brain preserved along with bones (SN: 4/1/17, p. 32; SN: 11/26/16, p. 9). Fossilized lungs are also not completely unheard of: The petrified organs have been observed preserved with a 125-million-year-old mammal fossil (SN Online: 10/21/15) and with a 35-million-year-old salamander fossil.
“But we’re arguing that this is the first lung tissue preservation that is anatomically informative,” said O’Connor, of the Institute of Vertebrate Paleontology and Paleoanthropology in Beijing.
That’s because modern birds have a highly specialized and efficient respiratory system that allows them to absorb enough oxygen to meet the demands of powered flight. Unlike mammalian lungs that are elastic and pump air in and out, bird lungs don’t change size when the bird breathes. Instead, several air sacs connected to the lungs act like a bellows to draw the air in through the lungs. The lungs themselves contain highly subdivided tissue with tiny air capillaries that are responsible for the transfer of oxygen and carbon dioxide gases.
The new Archaeorhynchus fossil surprisingly contains many of the same structures, the team announced. That suggests that these important respiratory adaptations were present very early in the modern bird lineage.
The mirror image of the main fossil slab, this counterslab shows more clearly the purported lungs of the ancient bird (outlined in dashed lines at top). A third dashed line region (just below) represents the stomach and contains numerous little stones, or gastroliths.
Like many fossils, the new Archaeorhynchus specimen is split into two halves, a main slab and its mirror image called a counterslab. Both slabs have unusual features. The chest cavity on the main slab contains an odd, white speckly region. And the counterslab has two nearly symmetrical lobe-shaped regions corresponding to the speckly white region. The lobes’ position, and that there were two of them, suggests the possibility of lungs, O’Connor said. The structures were unlikely to be stomach contents, which usually appear black and carbonized in fossils. And while the liver can have two lobes in birds, it tends to appear reddish due to its high iron content.
To get a closer look at the microscopic structures of possible tissues in the speckly region, O’Connor and her team used scanning electron microscopy to study 22 samples from the fossil, including 12 from the purported lung tissue. The images revealed a highly subdivided region of tissue known as the parenchymatous region, which contained many closely packed air cells. That structure is similar to the air capillaries found in small modern birds, O’Connor noted.
The fossilized feathers, meanwhile, suggest that the bird had a pointed tail known as a pintail, something previously unknown in birds that lived during the Mesozoic Era but found in modern birds such as the pintail duck. And like the other Archaeorhynchus fossils, the team found gastroliths in the bird’s stomach — more than 100 of them.
Still, other researchers aren’t convinced that the preserved material represents lungs, in part because it’s not clear how the lungs would survive fossilization. “I think Jingmai has made about the best possible case with the materials and results she has,” says Corwin Sullivan, a vertebrate paleontologist at the University of Alberta in Edmonton, Canada. “I’d very much like it to be true, but there are a few too many question marks about how such tissues could have been preserved,” he says.
Sullivan notes that the material seems not to have been completely compressed into two-dimensions during fossilization — which would be particularly surprising for relatively delicate tissue like a lung. “But then I don’t know what a fossilized bird lung would look like. None of us do.” Fossil lungs, he says, should be on the list of possibilities of what the structures are.
“I knew that a lot of people would be rightfully skeptical” about the lungs, O’Connor said. But bird lungs, though seemingly delicate, contain some sturdy structures that may have helped the tissues become preserved, or perhaps the chemical environment in which the creature died was particularly conducive to preservation. But these are all just hypotheses, O’Connor said. The team is planning future research to address this and other questions about how this bird breathed.
As surprising as finding fossilized bird lungs would be, any other possible explanations for the material would be equally unusual, Sullivan says. For example, he suggests that it might be related to the gastroliths in some way. “I wonder if they could have partially dissolved, spilled forward into the body cavity or something unusual like that. Whatever is happening, it’s very strange and very interesting.”
Editor’s note: This story was updated October 24, 2018, to correct the publication date of the PNAS paper. The study was published online October 22, not October 18.