A hesperornithiform feather preserved inside dinosaur dung has given researchers their clearest clue yet about why only one group of birds survived the mass extinction 66 million years ago. Scientists at the Field Museum in Chicago say a dinosaur consumed a bird around that time, and its dung preserved what they describe as the best-preserved feather ever recovered from the dinosaur age.
The fossil was found in 2016 by study co-author David DeMar Jr. during fieldwork in Montana. DeMar Jr., a research scientist at the University of Washington Burke Museum, described the moment of discovery: ‘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.’ The find was reported by Chicago City Wire.
What the dung contained
CT scans and mineral analysis of the specimen revealed more than one feather inside. The nodule also held tiny fish scales from a gar and leg bones from a hesperornithiform, an aquatic bird ecologically similar to modern loons. ‘Hesperornithiforms were aquatic birds, ecologically similar to loons,’ said study lead author Professor Jingmai 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.’
Hesperornithiforms are the earliest known avian lineage to give up flight in favour of a foot-propelled diving lifestyle. They were widespread during the Cretaceous period but did not survive beyond it. The feathers found in the droppings are the first hesperornithiform feathers ever recovered, and their structure appears to sit midway between the feather types of the dinosaur age and those of birds living today.
The hesperornithiform feather extinction link
O’Connor has spent years studying fossilised birds to understand why only one group (the Neornithes) made it through the catastrophic impact winter that followed the asteroid strike. Every living bird today descends from that single surviving lineage, making the question one of the more consequential open problems in palaeontology. ‘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,’ she said.
The hesperornithiform feather extinction question centres on insulation. O’Connor believes differences in feather quality between bird groups may have determined which ones could endure the cold. ‘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,’ she said. If those feathers were less efficient at retaining body heat than the plumaceous feathers of the Neornithes, the consequences in a prolonged impact winter could have been fatal. ‘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,’ O’Connor said.
The enantiornithines were another group of birds that also perished at the end of the Cretaceous, leaving no descendants. The pattern, aquatic specialists and tree-dwelling birds alike disappearing while the ancestors of today’s birds survived, has long resisted a clean explanation. Feather insulation now looks like one credible part of the answer, though O’Connor frames it as a contributing factor rather than the sole cause.
‘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 study co-author Professor Greg Wilson Mantilla of the University of Washington.
CT scanning coprolite collections
The study, published in Current Biology, calls for researchers to run CT scans on existing fossilised dung samples held in museum collections. The authors argue that more feathers (and potentially other soft tissue evidence) could be sitting undetected inside specimens that have never been closely examined. The 2016 Montana nodule spent years in storage before its contents became clear. More may be waiting.
