About 66 million years ago, a dinosaur—maybe a T. rex or a Nanotyrannus—ate a bird. The record of that meal, in the form of fossilized poop, has survived to today, preserving the best example of a feather ever found from the age of dinosaurs. By studying this feather, scientists reported in a new study in the journal Current Biology, researchers have found clues that help explain why birds are the only group of dinosaurs that survived the mass extinction 66 million years ago.

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One of the biggest mysteries in paleontology is why this one small group of birds survived when all the other birds and all the other dinosaurs died out. O'Connor has spent years studying fossil birds to try to answer this question. But this is the first time she's studied feathers that were preserved inside fossilized droppings, or coprolites.

The coprolite contained multiple feathers, tiny fish scales from a gar and leg bones from a hesperornithiform bird. Since the bones and feathers were found together, it stands to reason that the feathers came from that bird.

"Hesperornithiforms were aquatic birds, ecologically similar to loons," says O'Connor. "Most couldn't fly, and instead, they used their specialized feet to dive down into the water to hunt for things like fish. The feathers showed adaptations for being underwater that we see in living aquatic birds."

The hesperornithiforms are close cousins of the Neornithes birds that survived the mass extinction and still live today, but they were not part of Neornithes themselves.

"The most common birds alive in the Cretaceous were part of a group called the enantiornithines. The modern Neornithes branch of birds was separate from that group, and so were the hesperornithiforms," says O'Connor.

Some scientists have hypothesized that the Neornithes branch of the bird family survived because they lived near water, and something about this habitat helped buffer them from the effects of the mass extinction. However, the hesperornithiforms also lived by water, and they went extinct. So, there must be another reason to explain why Neornithes are the only birds that survived. O'Connor thinks one of the main reasons has to do with differences in their feathers.

"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 end-Cretaceous mass extinction—essentially, why some birds died out and why others survived," she says.

The feathers found in the coprolite are the first hesperornithiform feathers ever found, and their features seem to represent a middle ground between the feathers of enantiornithines and modern birds.

"Some of these diving birds' feathers seem to have been modern-looking and waterproof, but they also had some smaller, fuzzy, primitive body feathers that we associate with dinosaurs and enantiornithines," says O'Connor.

The feathers on a bird's body help insulate it from the cold. If the hesperornithiforms' and enantiornithines' feathers weren't as good at keeping their bodies warm as Neornithes' feathers, that could have been a major factor in these groups' abilities to survive the impact winter following the asteroid strike 66 million years ago.

And since hesperornithiforms are neither neornithines nor enantiornithines, the discovery of the feathers in the coprolite also helps explain why these diving birds died out.

"The hesperornithiforms retain primitive feather types that may not have been as efficient for insulation as modern plumaceous feathers, and that could explain why they went extinct along with the enantiornithines," says O'Connor.