The feather of a bird eaten by a dinosaur 66 million years ago could provide the answer to one of science’s great mysteries. The bird’s unfortunate demise resulted in the best example of a feather ever found from the age of dinosaurs, as it survived in the form of fossilised dung.By studying this feather, researchers have found clues that help explain why birds are the only group of dinosaurs that survived the mass extinction event caused by the impact of a huge asteroid. “It’s such a beautiful, well-preserved feather, from such an unexpected source, and it’s exciting that it could help us answer this huge question in paleontology,” said Jingmai O’Connor, the lead author of the new paper and the associate curator of fossil reptiles at the Field Museum in Chicago.The earliest known bird is Archaeopteryx, from 150 million years ago, and birds continued to evolve and live alongside their fellow dinosaurs for nearly 100 million years. In the aftermath of the Earth’s collision with an asteroid 66 million years ago, nearly all the dinosaurs, including most birds, went extinct.But one group of birds, Neornithes, survived, and every living bird today is one of their descendants.An illustration of a tyrannosaur preying on an ancient diving bird (Andrey Atuchin)Why this one small group survived is one of palaeontology’s biggest questions. “As far as I know, no one has ever thought to look for feathers or to study feathers in coprolites, so this project was really exciting,” Dr O’Connor said. The fossil was found in 2016, when David DeMar Jr, co-author of the paper in the journal Current Biology, was conducting fieldwork in northeastern Montana."I was crawling up a rocky outcrop collecting fish fossils when I came across a dark, reddish-brown nodule about half the size of a golf ball”, he said.“I picked it up and scanned its surface through my hand lens, and that's when I couldn't believe what I was seeing, a tiny fossil feather.“I was cautiously optimistic about its discovery, because feathers had not yet been found in the Hell Creek Formation, even after more than 150 years of prospecting.”The fossil feather preserved in dinosaur dung (Field Museum Chicago)Researchers examined its mineral composition and took CT scans of it.It contained multiple feathers, tiny fish scales from a gar, and leg bones from a hesperonithiform bird. The hesperornithiforms are close cousins of the Neornithes birds that survived the mass extinction and still live today, but they were not part of Neornithes themselves.Some scientists believe that the Neornithes branch of the bird family survived because they lived near water, which helped protect them from the effects of the mass extinction.However, the hesperornithiforms also lived by water, and went extinct, suggesting there is another reason which explains why Neornithes are the only birds that survived.“We think the types of feathers that these birds had, and/or the way they moulted those feathers, may have been one of the underlying causes of the selectivity of the end-Cretaceous mass extinction - essentially, why some birds died out and why others survived,” Dr O’Connor said. “Some of these diving birds’ feathers seem to have been modern-looking and water-proof, but they also had some smaller, fuzzy, primitive body feathers that we associate with dinosaurs and enantiornithines,” she added.Greg Wilson Mantilla, a professor at the University of Washington and co-author of the study, believes that the study’s findings are important because, “we rarely find fossils of birds and even more rarely their feathers, giving us such important insight into the evolution of this key aspect of their biology.“On top of that, these bird feathers found within a large fossilised dinosaur dung give us an incredible window into predator-prey interactions 66 million years ago."