A fossil feather preserved inside dinosaur poop could help explain why birds survived the mass extinction
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. And 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.
“It’s such a beautiful, well-preserved feather, from such an unexpected source, and it’s exciting that it could help us answer this huge question in paleontology,” says Jingmai O’Connor, the lead author of the new paper and the associate curator of fossil reptiles at the Field Museum in Chicago.
Birds are a specialized kind of dinosaur, and they've been around for a long time. The earliest known bird is Archaeopteryx, from 150 million years ago, and birds continued to evolve and co-exist alongside their fellow dinosaurs for nearly 100 million years. In the aftermath of the Earth’s collision with an asteroid 66 million years ago, nearly all the dinosaurs, including nearly all the birds, went extinct. But one group of birds, Neornithes, survived, and every living bird today is one of their descendants.
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.
“As far as I know, no one has ever thought to look for feathers or to study feathers in coprolites, so this project was really exciting,” says O’Connor.
The fossil itself came to light in 2016, when David DeMar, Jr., a research scientist and the Hell Creek Project collections manager at the University of Washington Burke Museum and co-author of the paper, was conducting fieldwork in northeastern Montana.
"I was crawling up a rocky outcrop collecting fish fossils when I came across a dark, reddish-brown nodule about half the size of a golf ball. I picked it up and scanned its surface through my hand lens, and that's when I couldn't believe what I was seeing, a tiny fossil feather. I was cautiously optimistic about its discovery, because feathers had not yet been found in the Hell Creek Formation, even after more than 150 years of prospecting," says DeMar.
Back in the lab, the researchers examined the specimen’s mineral composition and took CT scans of it—essentially, thousands of X-rays that are digitally stacked to reveal the contents of an object.
“Although the preservation was better than many of the feathers I’d been studying in Burmese amber for my thesis, this was in a rock—and a fairly unremarkable one at that. I suspected it might be a coprolite, but it wasn’t until we got it into the micro-CT scanner at USC’s medical campus that the full feathery fabric of this fossil feces became apparent,” says Nate Carroll, a co-author of the paper and paleontologist at the Carter County Museum in Ekalaka, Montana. “Every hour processing the data revealed another feather, another scale, another bone—in stunning 3D. As someone who had been relying on far-flung amber mines as my main source of 3D feather data, realizing that fossil poop from my home state could yield such exceptional specimens was a game changer.”
The coprolite contained multiple feathers, tiny fish scales from a gar, and leg bones from a hesperonithiform bird. Since the bones and feathers, 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 were 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 selectivity 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 water-proof, 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 or enantiornithines, the discovery of the feathers in the coprolite also help 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.
Greg Wilson Mantilla, a professor at the University of Washington, curator of vertebrate paleontology at the Burke Museum, and co-author of the study, says that the study’s findings are important because, “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. On top of that, these bird feathers found within a large fossilized dinosaur dung give us an incredible window into predator-prey interactions 66 million years ago."
For O’Connor, the study also highlighted the way that paleontology is like solving a mystery. “I usually work with fossils that are preserved in big stone slabs, and the entire skeleton and even the soft tissue is preserved—they make it easy for me. But with this project, we just had this coprolite—and its contents—to go off of, and it made me feel like a detective, piecing together all these little clues,” says O’Connor. “And since no one has studied feathers in coprolites before, this opens up a whole new avenue for investigation. We only knew to look at this one because of how it happened to be split open, with the feather exposed—it was literally a lucky break. I hope more scientists start CT scanning coprolites and taking a closer look at them to see what might be inside.”
This study was contributed to by Jingmai O’Connor (Field Museum), David DeMar Jr. (Burke Museum of Natural History and Culture/University of Washington), Nathan Carroll (Carter County Museum), Karen Chin (University of Colorado, Boulder), Michael Holland (Burke Museum of Natural History and Culture/University of Washington), Alex Clark (Field Museum and University of Chicago), Christian Cooper (Field Museum), Pei-Chen Kuo (Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences), Thomas Tobin (University of Alabama), Aaron Celestian (Natural History Museum of Los Angeles County), David Bottjer (University of Southern California), Luis Chiappe (Natural History Museum of Los Angeles County), and Gregory Wilson Mantilla (Burke Museum of Natural History and Culture, University of Washington).