A fossil feather preserved inside dinosaur poop could help explain why birds survived the mass extinction. About 66 million years ago, a dinosaur—possibly a T. rex or a Nanotyrannus—ate a bird. The record of that meal, in the form of fossilized poop, has survived to today, preserving the best example of a feather ever found from the age of dinosaurs. By studying this feather, scientists reported in a new study in the journal Current Biology, researchers have found clues that help explain why birds are the only group of dinosaurs that survived the mass extinction 66 million years ago. Birds are a specialized kind of dinosaur, and they've been around for a long time. The earliest known bird is Archaeopteryx, from 150 million years ago, and birds continued to evolve and coexist alongside their fellow dinosaurs for nearly 100 million years. In the aftermath of Earth's collision with an asteroid 66 million years ago, nearly all the dinosaurs, including nearly all the birds, went extinct. But one group of birds, Neornithes, survived, and every living bird today is one of their descendants. One of the biggest mysteries in paleontology is why this one small group of birds survived when all the other birds and all the other dinosaurs died out. The fossil itself came to light in 2016, when David DeMar Jr., a research scientist and Hell Creek Project collections manager at the University of Washington Burke Museum and co-author of the paper, was conducting fieldwork in northeastern Montana. He discovered a dark, reddish-brown nodule about half the size of a golf ball. In the lab, the researchers examined the specimen's mineral composition and took CT scans of it—essentially, thousands of X-rays that are digitally stacked to reveal the contents of an object. What the scans revealed was a fossilized coprolite containing multiple feathers, tiny fish scales from a gar and leg bones from a hesperornithiform bird. Hesperornithiforms were aquatic birds, ecologically similar to loons, and most couldn't fly, instead using their specialized feet to dive into the water to hunt for things like fish. The feathers showed adaptations for being underwater that we see in living aquatic birds. 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 have hypothesized that the Neornithes branch of the bird family survived because they lived near water, and something about this habitat helped buffer them from the effects of the mass extinction. However, the hesperornithiforms also lived by water, and they went extinct. So, there must be another reason to explain why Neornithes are the only birds that survived. The feathers found in the coprolite are the first hesperornithiform feathers ever found, and their features seem to represent a middle ground between the feathers of enantiornithines and modern birds. Some of these diving birds' feathers seem to have been modern-looking and waterproof, but they also had some smaller, fuzzy, primitive body feathers that we associate with dinosaurs and enantiornithines. The feathers on a bird's body help insulate it from the cold. If the hesperornithiforms' and enantiornithines' feathers weren't as good at keeping their bodies warm as Neornithes' feathers, that could have been a major factor in these groups' abilities to survive the impact winter following the asteroid strike 66 million years ago. The study's findings are important because we rarely find fossils of birds and even more rarely their feathers, giving us such important insights into the evolution of this key aspect of their biology. On top of that, these bird feathers found within a large fossilized dinosaur dung give us an incredible window into predator-prey interactions 66 million years ago.