Fossilised feathers found in dinosaur dung offer clues to avian survival

A 66-million-year-old pile of dinosaur dung has yielded an extraordinary secret: the exceptionally preserved feathers of an ancient diving bird, offering new clues into why some species survived the asteroid strike that wiped out the dinosaurs.

The fossilised dropping, known as a coprolite, was discovered in Montana’s Hell Creek Formation. Analysis of the specimen, published in the journal Current Biology, reveals it was left by a massive predator—likely a Tyrannosaurus rex or Nanotyrannus—that had recently dined on a loon-like aquatic bird called a hesperornithiform.

What did scientists find in the dinosaur dung?

Inside the fossil, researchers found fish scales, bird bones, and several distinct feathers. These are the first hesperornithiform feathers ever identified, and they reveal an unexpected evolutionary halfway house.

Two of the feathers show highly advanced characteristics that, until now, were only associated with modern birds. They feature a square shaft with a spongy centre, designed to help with waterproofing. This pushes the record for this specific feather structure back by 10 million years; the previous oldest example was found in Denmark and dated to the Early Eocene.

Yet, alongside these modern features, other feathers on the same bird were small, fuzzy, and primitive, resembling the simple down found on earlier dinosaurs.

How do these ancient feathers help explain extinction?

This strange mix of ancient and modern plumage could explain why these birds did not survive the mass extinction event that followed the asteroid impact. Paleontologists have long wondered why some bird lineages made it through the disaster while others died out. One popular theory suggested that water-dwelling birds were insulated from the worst of the global fallout. But hesperornithiforms were aquatic, and they still vanished.

Dr Jingmai O’Connor, associate curator of fossil reptiles at the Field Museum and one of the study's authors, suggests the answer lies in their plumage. The combination of waterproof outer feathers and primitive, poorly insulating inner feathers, or perhaps the way these birds shed and regrew them, may have left them vulnerable to the sudden global cooling that followed the asteroid strike.

"We think the types of feathers that these birds had, and/or the way they molted those feathers, may have been one of the underlying causes of the selectivity of the end-Cretaceous mass extinction," Dr O’Connor said.

Why is this discovery so rare?

For paleontologists, finding any bird feathers from this era is a major breakthrough. Bird bones are hollow and fragile, making them incredibly rare in the fossil record. Finding their delicate feathers preserved inside the digestive tract of a Cretaceous giant is rarer still.

"We rarely find fossils of birds and even more rarely their feathers, giving us such important insight into the evolution of this key aspect of their biology," said Professor Gregory Wilson Mantilla, curator of vertebrate paleontology at the Burke Museum of Natural History and Culture.

He added that finding the plumage preserved inside a predator’s dung provides an "incredible window into predator-prey interactions" 66 million years ago.

The fossil was originally found in 2016 by Burke Museum paleontologist David DeMar Jr., but it has taken years of careful analysis to extract the secrets locked inside the prehistoric waste.