A feather preserved in fossilized dinosaur feces, known as coprolites, has given scientists new clues about the mass extinction that occurred at the end of the Cretaceous period, 66 million years ago. The feather was discovered in 2016 in the Hell Creek Formation in eastern Montana by David DeMar Jr., a researcher at the University of Washington. It was found almost intact inside a coprolite, which is formed from dinosaur waste. This feather is more than 66 million years old and may have belonged to a diving bird that was eaten by a large carnivorous dinosaur. The discovery was reported by the media outlet Ecoticias and published in September in the scientific journal Current Biology. The research team, led by Jingmai O'Connor, a curator at the Field Museum in Chicago, used a highly precise scanner to examine the coprolite without damaging it. According to Nate Carroll, a paleontologist at the Carter County Museum, "Every hour of data processing revealed a new feather, a new scale, a new bone, in 3D." The analysis showed that the coprolite contained not only a feather but also tiny scales and bones from the legs of a bird from the Hesperornithidae family. These were aquatic birds that used their feet to dive and catch fish. The fish scales found in the coprolite likely came from the bird's last meal. In turn, this bird became prey to a theropod dinosaur, possibly a Tyrannosaurus rex or a Nanotyrannus. Gregory Wilson Mantilla, a biology professor at the University of Washington, noted that the discovery of this coprolite provides "an incredible perspective on predator-prey interactions 66 million years ago." The structure of the feathers is particularly significant. Two of them have a square column with a spongy and light center, a feature found only in modern birds and their direct ancestors. According to the researchers, no other feather from the dinosaur era shows this combination. The closest comparable specimen comes from fossils found in Denmark, dating to the Lower Eocene, about 10 million years later. However, the plumage of this bird was not entirely modern. "Some feathers of these diving birds seem to have had a modern appearance and were waterproof, but they also had small, primitive downy feathers on their bodies," explains Jingmai O'Connor. This mixture of feather types has led the scientific team to propose a hypothesis. When the asteroid struck Earth 66 million years ago, the planet experienced what scientists call an "impact winter"—a period of darkness and cold that lasted long enough to cause the extinction of many species. The Hesperornithes lived in aquatic habitats similar to those of some bird lineages. Yet, they disappeared. Jingmai O'Connor suggests that their ancient plumage may not have provided as effective insulation as the soft down of modern birds, such as that which protects a duck from the cold on a frozen lake. "We think that the type of feathers these birds possessed, or their molting pattern, could be one of the underlying causes of the selectivity of the mass extinction at the end of the Cretaceous," the researcher states. However, caution is needed, as the feather is a single piece of evidence. The idea of the survival of certain species remains, at this stage, a hypothesis rather than a definitive explanation.