Fossils reveal that ancient birds molted, which could explain why modern birds survived while other dinosaurs died

Fossils reveal that ancient birds molted, which could explain why modern birds survived while other dinosaurs died

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Feathers from a baby bird that lived 99 million years ago, preserved in amber. Credit: Shundong Bi.

Every bird you’ve ever seen—every bird, every pigeon, every peacock in the zoo—is a living dinosaur. Birds were the only group of dinosaurs to survive the asteroid-triggered extinction event 66 million years ago. But not all birds were alive at that time. Why the ancestors of modern birds lived while their relatives died is a mystery that paleontologists have been trying to solve for decades. Two new studies point to one possible factor: differences between modern birds and their ancient relatives molt feathers.

Feathers are one of the most important characteristics that all birds have. They are made from a protein called keratin, the same material as our fingernails and hair, and birds rely on them to fly, swim, protect, attract mates, stay warm, and protect against sunlight.

But feathers are complex structures that cannot be repaired, so as a way to keep them in good shape, birds shed their feathers and grow replacements in a process called molting. Baby birds molt in order to lose their feathers and grow into adults; Mature birds continue to molt about once a year.

“Molt is something that I don’t think a lot of people think about, but it’s basically an important process for birds, because feathers are involved in many different functions,” said Jingmai O’Connor, Fossil’s reptile curator. at the Field Museum of Chicago. “We want to know, how does this process evolve? How does it differ across groups of birds? And how does the evolution of birds shape, shape the survival of all these different clades?” Two of O’Connor’s recent papers examine the molting process in prehistoric birds.

Journal paper Cretaceous research Details of the discovery of feathers preserved in amber from baby birds Lived 99 million years ago.






Illustrations of what a newly released Enantiornithine bird might look like. Credit: Yu Chen and Shundong Bi.

Today, baby birds are on the spectrum of how they develop when they are born and how much help they need from their parents. Altricial birds hatch naked and helpless; Their lack of fur means that their parents can efficiently transfer body heat directly to their babies’ skin. On the other hand, Precocial species are born hairy and self-sufficient.

All baby birds go through a series of molts – the time when they lose their existing feathers and grow a new set of feathers, before reaching full plumage. Molting takes a lot of energy, and the loss of many feathers at once can make it difficult for birds to keep themselves warm. As a result, precocious chicks tend to molt slowly, so they maintain a constant supply of feathers, while older chicks who can rely on their parents for food and warmth undergo a “simultaneous molt,” losing all their feathers around the same time. .

The feathers preserved from the amber in this study are the definitive fossil evidence of juvenile molting, and they reveal a young bird whose life history does not match that of any bird alive today.

“This sample shows an unusual combination of precocial and social characteristics,” said O’Connor, who was the first author of the paper along with senior author Shundong Bi of Indiana University of Pennsylvania. “All the hairs on the body are basically at the same stage of development, so this means that all the hairs start growing at the same time, or close to the same time.”

However, this bird is almost certainly part of a now-extinct group called Enantiornithines, which O’Connor’s previous work has shown to be highly precocial.






Enantiornithine is an older child’s specimen. Credit: Yu Chen and Shundong Bi.

O’Connor hypothesized that the stress of being a precocious bird to keep itself warm, while molting quickly, may have been a factor in the Enantiornithines’ eventual demise.

“Enantiornithines were the most diverse group of birds in the Cretaceous, but they went extinct along with other non-avian dinosaurs,” O’Connor said. “When asteroids fall, global temperatures will drop and resources will be scarce, so not only do these birds have higher energy needs to stay warm, but they don’t have the resources to meet them.”

At the same time, more studies were published on July 3 in Communication Biology By O’Connor and Field Museum postdoctoral researcher Yosef Kiat examines molting patterns in modern birds to better understand how the process first evolved.

In modern adult birds, molting usually occurs once a year in a sequential process, in which they change their feathers only a few times over a period of a few weeks. That way, they can still fly throughout the molting process. Simultaneous molts in adult birds, in which all the flight feathers fall off at the same time and are recovered within a few weeks, are rare and often appear in waterfowl such as ducks that do not really need to fly to find food. and avoid predators.

It’s very rare to find evidence of molting in avian and other feathered dinosaurs, and O’Connor and Kiat wanted to know why. “We have this hypothesis that birds that have simultaneous molts, which happen in a short period of time, will be less represented in the fossil record,” O’Connor said – less time spent molting means less chance of dying during your molt and becoming. Fossils showing signs of molting. To test their hypothesis, the researchers delved into the modern bird collection of the Field Museum.






An illustration showing a young Enantiornithine bird. Credit: Yu Chen and Shundong Bi.

“We tested more than 600 skins of modern birds stored in the ornithology collection of the Field Museum to find evidence of active molting,” said Kiat, the first author of the study. “Among birds that molt sequentially, we found dozens of examples in active molt, but among simultaneous molters, we found almost none.”

While these are modern birds, not fossils, they provide a useful proxy. “In paleontology, we have to be creative, because we don’t have a complete set of data. Here, we used a statistical analysis of random samples to guess what something doesn’t really tell us,” O’Connor said.

In this case, the lack of molting in fossil birds, despite the fact that molting is very active in modern bird samples, indicates that fossil birds simply did not molt as often as most modern birds. They may molt simultaneously, or they may not molt on an annual basis the way most birds do today.

Both examples of amber and studies of molting in modern birds indicate a common theme: prehistoric birds and feathered dinosaurs, especially groups that did not survive extinction, molted differently than birds today.

“All the differences that you can find between the crested and the crested birds, basically, become hypotheses about why one group survives and the rest don’t,” O’Connor said. “I don’t think there’s any particular reason why birds, the group that includes modern birds, survived. I think it’s a combination of traits. But I think it’s becoming clear that molt may have been an important factor in how dinosaurs were able to survive.”

Additional information:
Yosef Kiat et al, Rarity of molt evidence in early pennaraptoran dinosaurs suggests annual molt developed later among Neornithes, Communication Biology (2023). DOI: 10.1038/s42003-023-05048-x

Jingmai O’Connor et al, Feathers preserved in Burmite provide evidence of rapid molting in enantiornithines, Cretaceous research (2023). DOI: 10.1016/j.cretres.2023.105572

Journal information:
Communication Biology

Cretaceous research

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