66-Million-Year-Old Feather Found in Montana Dinosaur Dung

A team led by paleontologist Jingmai O’Connor, associate curator of fossil reptiles at the Field Museum in Chicago, has described the first known feather from a hesperornithiform bird, preserved inside fossilized dinosaur dung from the Hell Creek Formation in Montana. The study was published on September 10, 2026, in the journal Current Biology under the digital identifier 10.1016/j.cub.2026.08.054, and the Field Museum announced the find in a dedicated press release. According to the authors, the discovery offers clues that help explain why birds were the only group of dinosaurs to survive the mass extinction 66 million years ago.

Birds are a specialized branch of dinosaurs with a very long history. The oldest known specimen, Archaeopteryx, is roughly 150 million years old, and birds went on evolving alongside other dinosaurs for nearly 100 million years afterward. Everything changed 66 million years ago, when an asteroid struck Earth and the resulting mass extinction wiped out almost every dinosaur species, including nearly all birds. Only one branch survived: Neornithes, the ancestors of every bird alive today. Hesperornithiform birds, the subject of the new study, were close relatives of that group but did not themselves belong to either Neornithes or the enantiornithines, the most numerous bird group of the Cretaceous. One earlier explanation held that surviving birds made it through because they lived near water. Hesperornithiforms, however, also lived in water and still went extinct, which the study’s authors say suggests that proximity to water alone was not the deciding factor.

How the Fossil Was Found

The specimen was found in 2016 in northeastern Montana by David DeMar Jr., a research associate and collections manager for the Hell Creek Project at the Burke Museum at the University of Washington, while he was collecting fish fossils.

“I was crawling along a rocky outcrop collecting fish fossils when I came across a dark, reddish-brown lump about the size of half a golf ball. I picked it up and examined the surface with a hand lens, and I couldn’t believe what I was seeing: a tiny fossil feather,” DeMar said in the Field Museum’s press release.

The lump was then analyzed in the lab, first by testing its mineral composition and later on a micro-CT scanner at the University of Southern California’s medical campus. Thousands of digital X-ray images were subsequently assembled into a three-dimensional model, which showed that the lump, dated to 66 million years old, contained several feathers, tiny scales from a bony fish and the leg bones of a hesperornithiform bird. Because the feathers and bones were found together, the team concluded that the feathers belonged to that very bird. The animal that swallowed the bird was, in the team’s assessment, a large predatory dinosaur, likely Tyrannosaurus rex or its smaller relative, Nanotyrannus.

What the Feather Reveals About Bird Survival

These are the first feathers ever found from a hesperornithiform bird. Two of the feathers have a square shaft with a spongy core, a combination that, according to DeMar, is not seen in any other known feather from the Mesozoic. The rest of the feathers in the same lump are smaller, downy and more primitive, resembling the plumage of earlier dinosaurs. Taken together, the set sits somewhere between the feathers of enantiornithines and those of modern birds.

“This is, without a doubt, the best feather we’ve ever found from the age of dinosaurs,” O’Connor said in the press release.

Hesperornithiform birds, the team notes, were aquatic and ecologically similar to grebes: most could not fly, and instead used feet adapted for diving to hunt fish underwater. The difference in their plumage, according to O’Connor, may help explain why some Cretaceous birds survived the extinction while others did not.

“We think the types of feathers these birds had, and the way they molted those feathers, could have been one of the key reasons for the selectivity of the mass extinction at the end of the Cretaceous, essentially why some birds went extinct and others survived,” she said.

“It seems that some of the feathers on these diving birds looked modern and were waterproof, but they also had smaller, downy, primitive feathers on their bodies,” O’Connor added.

Body feathers serve to insulate against the cold. If the plumage of hesperornithiforms and enantiornithines did not retain heat as well as that of Neornithes, that could have been, in the team’s interpretation, one of the factors that decided who survived the so-called impact winter that followed the asteroid strike.

The study lists 13 authors in total, with O’Connor as lead author. Twelve other researchers worked on the paper alongside her, including DeMar, paleontologist Nathan Carroll of the Carter County Museum in Ekalaka, who took part in the micro-CT analysis, and professor Greg Wilson Mantilla of the University of Washington and the Burke Museum, who stressed the find’s importance for understanding predator-prey relationships 66 million years ago.

Ten years passed between the discovery of the lump and the published study. Part of the reason, according to O’Connor, is that no one had previously thought to look for feathers inside fossilized dung, since this type of fossil is usually studied for its dietary content rather than for any plumage it might contain.

What the Discovery Does Not Prove

The team’s conclusions rest on a single specimen: one lump of dung and one bird that was eaten. It is the first described example of a hesperornithiform feather, so it cannot be said with certainty that every bird in that group had an identical feather structure. The link between feather type and the ability to survive the impact winter is a hypothesis the team puts forward as a possible explanation, not a proven cause-and-effect relationship, and the authors themselves present it as a direction for further research rather than a final answer to why the Neornithes branch specifically survived the mass extinction 66 million years ago.