Why did some birds survive when dinosaurs went extinct? Molting feathers could hold a clue

• Fossilized feathers of an extinct bird show it molted differently than modern birds.
• The prehistoric bird lost its feathers all at once, while many modern birds molt in waves.

During the Cretaceous period, 145 to 66 million years ago, the flying species called enantiornithines thrived on our planet. They were the most diverse group of birds of their time, but they didn't survive the era's mass extinction event.

Now, scientists have a better clue as to why thanks to one enantiornithine in particular that didn't make it. Trapped in a tiny, 2-gram piece of amber, the 99-million-year-old feathers are remarkably preserved.

The type and growth pattern of feathers in the fossil suggests that these ancient avians molted differently compared to modern birds, according to a new study.

The discovery startled Jingmai O'Connor, associate curator of fossil reptiles at The Field Museum and lead author of the study who said she thinks it could help explain why these species went extinct.

Why do birds molt?

In 2018, researchers proposed that enantiornithines went extinct when an asteroid hit the earth 66 million years ago and destroyed their forest niches. This would've led to food scarcity, O'Connor said, which would've been especially detrimental for some molting birds.

There doesn't seem to be a singular reason why modern birds' ancestors survived and the others went extinct, O'Connor said in a statement. "I think it's a combination of characteristics," she said. "But I think it's becoming clear that molt may have been a significant factor in which dinosaurs were able to survive."

Molting is an energy-intensive process because birds need to synthesize keratin. Usually, birds avoid molting in cold weather or during migration, reproduction, and other rigorous activities.

Living birds are divided into two groups that affect the way they molt. Precocial chicks, like ducks, molt in waves, O'Connor said, losing feathers from different parts of their bodies at different times. But altricial chicks, like pigeons, lose all their feathers at once during a "simultaneous molt."

The fossil of the young enantiornithine, however, suggests an unusual combination of altricial and precocial attributes.

O'Connor said that like altricial chicks, enantiornithine was born without any protective down to keep it warm. But it was also born with a full set of flying feathers, a trait found in certain types of modern precocial birds. They probably would have looked something like this as young chicks:

"It's just something totally bizarre, a combination that you wouldn't see in any living bird," O'Connor said. "And we think that this really odd combination of characteristics is one of the reasons that they went extinct."

A strange mixing of altricial and precocial features that led to extinction?

The body feathers of the fossilized enantiornithine are all at a similar stage of development, suggesting they were lost all at once, according to the "Cretaceous Research" study.

"It's hard to understand why they would do this, but that's what the evidence shows us," O'Connor said.

If the juvenile enantiornithine was similar to modern precocial birds, it would have had less interaction with its parents than an altricial chick and couldn't have relied on adults for thermoregulation. The chicks might have survived in warm climates with abundant food, but after the asteroid hit, temperatures plunged.

Dust and debris partially blocked the sun, and plants couldn't photosynthesize, "which caused this chain-reaction collapse of ecosystems globally," O'Connor said. Coupled with the increased energy demands of colder weather, the impact winter may have doomed the enantiornithine. "

It just seems that together it would've been – I'm not going to say the reason that this group went extinct – but a major factor," she said.

O'Connor thinks developing a better understanding of molting and how it evolved could have important implications for modern birds.

"Right now we're experiencing mass extinction, and this kind of information can help guide conservation efforts and to understand how organisms today are going to be affected by the current environmental changes," she said.