Fossilized Dinosaur Feces Reveal Rare Hesperornithid Feathers From 66 Million Years Ago

Paleontologists have discovered remarkably preserved ancient bird feathers inside fossilized dinosaur feces in Montana, offering new clues about bird evolution, predator-prey interactions, and why some birds survived the asteroid impact that caused the mass extinction 66 million years ago.

The fossilized feces, known as a coprolite, were discovered in Montana’s Hell Creek Formation in 2016 by Burke Museum paleontologist David Demmer Jr. The specimen was produced by a large predatory dinosaur, possibly Tyrannosaurus rex or Nanotyrannus.

Inside the coprolite, researchers found multiple feathers, fish scales, and bones belonging to Hesperornithiformes, an extinct group of loon-like diving birds. Because the feathers were preserved alongside the bird remains, scientists concluded that they likely came from the same prey animal.

“Bird fossils are rarely found, and bird feathers are even more rare,” said Professor Gregory Wilson-Mantilla, a researcher at the University of Washington and curator of vertebrate paleontology at the Burke Museum of Natural History and Culture. “This provides very important insight into the evolution of an important aspect of avian biology.”

The fossil also offers rare evidence of predator-prey interactions during the Late Cretaceous, shortly before the asteroid impact that ended the age of non-avian dinosaurs.

Ancient feathers show a mix of primitive and modern features

The discovery represents the first identified Hesperornithiformes feathers. Some of the feathers appear modern and waterproof, while others are smaller, fuzzy, and more primitive, resembling the feathers of early dinosaurs.

Fossilized Hesperornis feathers exposed on the surface of a 66-million-year-old coprolite. Image credit: O’Connor et al., doi: 10.1016/j.cub.2026.08.054.

“The two feathers, including the one exposed on the surface of the coprolite, have features found only in living birds and their immediate ancestors,” said Demmer.

One feather contains a square-shaped rachis, or central shaft, with a sponge-like interior. According to the researchers, no other Mesozoic feather is known to combine these features.

The next oldest known example of this type of spongy feather core comes from the early Eocene of Denmark and is approximately 10 million years younger than the newly discovered Hell Creek feather.

Could feather structure explain which birds survived?

Hesperornithiformes were close relatives of modern birds. Although they lived in aquatic environments, they became extinct during the end-Cretaceous mass extinction, while some other bird lineages survived.

Some paleontologists have proposed that birds living near water may have been more resilient to the environmental changes caused by the asteroid impact. However, the extinction of Hesperornis shows that living near water alone was not enough to guarantee survival.

Researchers now suggest that feather structure and molting behavior may have played an important role in determining which bird groups survived.

“The type of feathers these birds had and/or the way they molted their feathers may be one of the fundamental causes of the selectivity of the end-Cretaceous mass extinction,” said Dr. Jingmai O’Connor, associate curator of fossil reptiles at the Field Museum.

She added that although some diving birds had modern-looking, waterproof feathers, they also possessed small, fuzzy body feathers that retained primitive characteristics.

The study was published in Current Biology.

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Jingmai K. O’Connor et al. “Bird feathers collected from coprolites from the Late Cretaceous period.” Current Biology, published online September 10, 2026. doi: 10.1016/j.cub.2026.08.054.