(CNN) – Hidden in a limestone quarry near the border between Belgium and the Netherlands, researchers found small bone fragments sticking up out of a rock that could fit in their palms. They could tell that the delicate, light bones belonged to a bird.
It wasn't until they used X-ray CT scans that they realized a nearly complete bird skull was encased in the rock itself -- and it was 66.7 million years old. The dinosaurs went extinct 66 million years ago when an asteroid slammed into Earth.
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As they studied the skull, the researchers realized it was similar to modern chickens and ducks. And that means it not only lived and evolved while dinosaurs walked the Earth -- it also survived and even thrived as the dinosaurs went extinct.
They'd found the oldest modern bird fossil to date, which they dubbed "Wonderchicken." Their study announcing the discovery of the bird published Wednesday in the journal Nature.
"The moment I first saw what was beneath the rock was the most exciting moment of my scientific career," said Daniel Field, study author and vertebrate paleontologist at the University of Cambridge's Department of Earth Sciences. "This is one of the best-preserved fossil bird skulls of any age, from anywhere in the world. We almost had to pinch ourselves when we saw it, knowing that it was from such an important time in Earth's history."
Wonderchicken is also known as Asteriornis maastrichtensis. Asteria was the Greek Titan goddess of falling stars.
"We thought it was an appropriate name for a creature that lived just before the end-Cretaceous asteroid impact," said Daniel Ksepka, study co-author and curator of science at the Bruce Museum in Greenwich, Connecticut. "In Greek mythology, Asteria transforms herself into a quail, and we believe Asteriornis was close to the common ancestor that today includes quails, as well as chickens and ducks."
Modern ducks and chickens are in a group known as Galloanserae, and the researchers regard Asteriornis as a weird kind of combination between a chicken and a duck.
Based on the leg bone fragments they found, Wonderchicken had fairly long legs. Parts of Belgium were tropical 66.7 million years ago, when giant marine reptiles swam in the oceans and Tyrannosaurus rex stalked its prey on land.
The researchers believe Wonderchicken was a kind of tropical shorebird. They found the fossil amongst marine sediments. And this chicken-duck mash-up weighed just a little over half a pound.
Understanding the lineage of modern birds, of which there are nearly 11,000 species, is difficult because delicate bird bones rarely survive in the fossil record.
"The origins of living bird diversity are shrouded in mystery -- other than knowing that modern birds arose at some point towards the end of the age of dinosaurs, we have very little fossil evidence of them until after the asteroid hit," said Albert Chen, study co-author and a PhD student based at the University of Cambridge. "This fossil provides our earliest direct glimpse of what modern birds were like during the initial stages of their evolutionary history."
But the researchers wouldn't have fully understood the significance of what they found if it weren't for CT scans.
"The ability to CT scan fossils, like we can at the Cambridge Biotomography Centre, has completely transformed how we study palaeontology in the 21st century," Field said.
Field and study co-author Juan Benito, a PhD student at Cambridge, discovered the skull together while conducting the CT scans.
"Finding the skull blew my mind," Benito said. "Without these cutting-edge scans, we never would have known that we were holding the oldest modern bird skull in the world."
This has potential to facilitate future discoveries of even older modern birds that can help shed light on how, when and where modern birds first originated, Field said.
Finding the bird fossil in Europe, and the northern hemisphere, made it an even more unique discovery.
"The late Cretaceous fossil record of birds from Europe is extremely sparse," said John Jagt, study co-author and staff member at the Natuurhistorische Museum Maastricht in the Netherlands. "The discovery of Asteriornis provides some of the first evidence that Europe was a key area in the early evolutionary history of modern birds."
If that's the case, our diverse bird species today may have evolved from this early example of crown bird. Crown birds are the common ancestors of all known birds.
"This fossil tells us that early on, at least some modern birds were fairly small-bodied, ground-dwelling birds that lived near the seashore," Field said. "Asteriornis now gives us a search image for future fossil discoveries -- hopefully it ushers in a new era of fossil finds that help clarify how, when and where modern birds first evolved."
But the question remains. Why did this bird, and other potential birds, survive the asteroid impact?
The study authors said they believe these survivors had a special suite of features that helped them outlast the dinosaurs.
"This bird seems to have been a small-bodied, predominantly non-tree-dwelling, flying bird, with a generalized diet," Field said. "We believe that all of these features were probably important for enabling the survivorship of modern birds across the end-Cretaceous mass extinction event, with respect to their non-avian dinosaur relatives."
Previously, Field published a study about the birds who survived in the wake of the mass extinction event. They were small ground-dwellers as well.
Field and his colleagues will study other rocks, including the end of the dinosaurs and the recovery of other species, to piece together more of the modern bird timeline.
The Wonderchicken fossil will be on display at the Cambridge Sedgwick Museum of Earth Sciences, in its own exhibit called “Dawn of the Wonderchicken.” The museum is currently closed due to the novel coronavirus pandemic.