(CN) — Scientists studying a dinosaur-era bird fossil found quartz crystals in its stomach, revealing clues about the pigeon-sized bird’s diet.
The fossil bird in question is a specimen of Bohaiornis guoi, part of an early lineage of birds from the Cretaceous from about 120 million years ago. Bohaiornis belonged to a group known as enantiornithines that were once the world’s most common birds.
“I would say it’s some kind of bizarre form of soft tissue preservation that we’ve never seen before,” said Jingmai O’Connor, the associate curator of fossil reptiles at Chicago's Field Museum, in a statement. “Figuring out what’s in this bird’s stomach can help us understand what it ate and what role it played in its ecosystem.”
Sometimes fossils preserve an animal’s stomach contents, revealing what they ate, which in turn can indicate hunting habits and other key facts of an extinct creature’s life. But most of the time paleontologists need to theorize such details for lack of evidence in the fossil record.
A new study published Thursday in the journal Frontiers in Earth Science chronicles the researchers’ efforts to unravel the mystery of what the bird ate.
“They still retain teeth and claws on their hands, but they’re small, about the size of a pigeon, so they’re not particularly terrifying,” O'Connor said.
While scientists over decades have discovered numerous well-preserved enantiornithines, none have included traces of food in their stomachs.
“We can identify the diet and reconstruct the digestive system for all these other groups of birds found in the deposits that record (China’s) Jehol Biota (deposits), except the enantiornithines, even though you have more enantiornithines than any other group,” O’Connor said. “For these guys, we have no specimens or preserved evidence of diet, which is really weird.”
In studying this latest specimen, however, O’Connor and her colleagues at the Institute of Vertebrate Paleontology and Paleoanthropology in China had a clue from a previous study indicating the presence of small rocks in its stomach.
Many birds have a muscular part of the stomach called a gizzard that helps digest food. By swallowing small rocks, birds are able to crush up tough food in their gizzards.
Called gastroliths, such “gizzard stones” are found in various dinosaur and bird fossils that typically ate diets of tough plant materials and seeds. But rocks in a stomach don’t automatically signal that an animal used them to crush food. Some birds of prey swallow rocks to clean their digestive tracts.
“You have to make a differentiation between just a gastrolith and a gastrolith that’s used as a gizzard stone,” O'Connor said.
Researchers extracted a rock sample from Bohaiornis’s stomach and examined it with a scanning electron microscope before exposing the sample to X-rays to determine which wavelengths the rocks absorbed. This helped researchers identify the composition of the sample.
“We found that those pieces of rock that had been called gastroliths were chalcedony crystals,” O'Connor said. “Chalcedony is basically quartz crystals that grow in sedimentary rocks. There hasn't been any evidence of this in the Jehol but there’s plenty of evidence of it within the fossil record where chalcedony crystals will form within a clamshell, or sometimes chalcedony will replace the minerals making up the bones in a fossil.”
In addition, the chalcedony was interconnected in one thin sheet of crystal rather than separate rocks. And the amount of chalcedony was wrong for helping with digestion. The combined evidence indicates that this specimen didn’t contain gastroliths for crushing food or cleaning its stomach.
“We just have this absence of evidence, and paleontologists always say absence of evidence is not evidence of absence,” O'Connor said.
If early Cretaceous enantiornithines used gastroliths, no record has been found.
“We’re always trying to find some evidence, and the specimens that have been suggested to fill this gap just unfortunately don't do it,” O'Connor said. “It’s just part of the paleo game, part of science — constantly correcting. I’m happy when we don't understand things, because it means there’s research to do, it’s exciting.”
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