Around 66 million years ago, a bird was likely consumed by a large carnivorous dinosaur, possibly a Tyrannosaurus rex, leaving behind fossilized evidence that may shed light on why only certain bird lineages survived the mass extinction event that ended the reign of non-avian dinosaurs. Scientists have uncovered fossilized feathers preserved inside coprolite—a piece of fossilized dung—found in the Hell Creek formation in Montana. Alongside the feathers, the coprolite contained fish scales and leg bones belonging to a Hesperornithiform, a group of aquatic diving birds that perished at the close of the Cretaceous period.

This discovery marks the first occasion researchers have studied feathers preserved within fossilized dinosaur droppings. Led by Jingmai O’Connor, a curator at the Field Museum in Chicago, the investigation aims to address a longstanding question in paleontology: why did the Neornithes—the group of modern birds—survive the catastrophic asteroid impact that wiped out most other bird species?

Birds are understood to be a specialized lineage of dinosaurs, with early representatives like Archaeopteryx dating back around 150 million years. As birds evolved throughout the Mesozoic Era, many different groups flourished alongside other dinosaurs. However, the mass extinction event 66 million years ago eliminated nearly all non-avian dinosaurs and many bird species, leaving only the ancestors of today’s birds.

O’Connor and her team suggest that differences in feather structure and molting patterns might have contributed to the survival of certain bird groups. The feathers preserved in the coprolite appear to be a transitional form, intermediate between the more primitive plumage of Cretaceous birds and the highly insulating feathers seen in modern birds. Effective insulation would have been crucial in surviving the “impact winter”—a period following the asteroid impact during which sunlight was blocked by atmospheric debris, leading to severe cooling.

The Hesperornithiform birds, identified through the associated leg bones and fish scales, retained a less developed feather type that may not have provided adequate insulation in the colder post-impact climate. This contrast highlights why some species may have succumbed while others endured.

The specimen was discovered by David DeMar Jr., a researcher at the University of Washington’s Burke Museum, who recognized a tiny fossil feather embedded in the reddish-brown coprolite fragment during routine examination. The exceptional preservation of the feather and its unexpected context offer a new avenue for understanding the selective survival of certain avian species through one of Earth’s most dramatic extinction events.