Fossils reveal how ancient birds molted their feathers -- which could
help explain why ancestors of modern birds survived when all the other dinosaurs died

Date:
July 5, 2023
Source:
Field Museum
Summary:
Birds are the only group of dinosaurs that survived the
asteroid-induced mass extinction 66 million years ago. But not all
the birds alive at the time made it. Why the ancestors of modern
birds lived while so many of their relatives died has been a mystery
that paleontologists have been trying to solve for decades. Two
new studies point to one possible factor: the differences between
how modern birds and their ancient cousins molt their feathers.


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FULL STORY ========================================================================== Every bird you've ever seen -- every robin, every pigeon, every penguin at
the zoo -- is a living dinosaur. Birds are the only group of dinosaurs
that survived the asteroid-induced mass extinction 66 million years
ago. But not all the birds alive at the time made it. Why the ancestors
of modern birds lived while so many of their relatives died has been a
mystery that paleontologists have been trying to solve for decades. Two
new studies point to one possible factor: the differences between how
modern birds and their ancient cousins molt their feathers.

Feathers are one of the key traits that all birds share. They're made of
a protein called keratin, the same material as our fingernails and hair,
and birds rely on them to fly, swim, camouflage, attract mates, stay warm,
and protect against the sun's rays. But feathers are complex structures
that can't be repaired, so as a means of keeping them in good shape,
birds shed their feathers and grow replacements in a process called
molting. Baby birds molt in order to lose their baby feathers and grow
adult ones; 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 is fundamentally such an important process to birds, because feathers
are involved in so many different functions," says Jingmai O'Connor,
associate curator of fossil reptiles at Chicago's Field Museum. "We want
to know, how did this process evolve? How did it differ across groups of
birds? And how has that shaped bird evolution, shaped the survivability
of all these different clades?" Two of O'Connor's recent papers examine
the molting process in prehistoric birds.

A paper in the journalCretaceous Research published in May 2023 detailed
the discovery of a cluster of feathers preserved in amber from a baby
bird that lived 99 million years ago.

Today, baby birds are on a spectrum in terms of how developed
they are when they're born and how much help they need from their
parents. Altricial birds hatch naked and helpless; their lack of feathers
means that their parents can more efficiently transmit body heat directly
to the babies' skin. Precocial species, on the other hand, are born with feathers and are fairly self- sufficient.

All baby birds go through successive molts -- periods when they lose the feathers they have and grow in a new set of feathers, before eventually reaching their adult plumage. Molting takes a lot of energy, and losing
a lot of feathers at once can make it hard for a bird to keep itself
warm. As a result, precocial chicks tend to molt slowly, so that they
keep a steady supply of feathers, while altricial chicks that can rely
on their parents for food and warmth undergo a "simultaneous molt,"
losing all their feathers at roughly the same time.

The amber-preserved feathers in this study are the first definitive
fossil evidence of juvenile molting, and they reveal a baby bird whose
life history doesn't match any birds alive today. "This specimen shows a totally bizarre combination of precocial and altricial characteristics,"
says O'Connor, who was the first author of the paper alongside senior
author Shundong Bi of the Indiana University of Pennsylvania. "All the
body feathers are basically at the exact same stage in development,
so this means that all the feathers started growing simultaneously,
or near simultaneously." However, this bird was almost certainly part
of a now-extinct group called the Enantiornithines, which O'Connor's
previous work has shown were highly precocial.

O'Connor hypothesizes that the pressures of being a precocial
baby bird that had to keep itself warm, while undergoing a
rapid molt, might have been a factor in the ultimate doom of the Enantiornithines. "Enantiornithines were the most diverse group of
birds in the Cretaceous, but they went extinct along with all the
other non-avian dinosaurs," says O'Connor. "When the asteroid hit,
global temperatures would have plummeted and resources would have
become scarce, so not only would these birds have even higher energy
demands to stay warm, but they didn't have the resources to meet them." Meanwhile, an additional study published July 3 in Communications 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 happens once a year in a sequential process, in which they replace just a few of their feathers at a time over
the course of a few weeks. That way, they're still able to fly throughout
the molting process. Simultaneous molts in adult birds, in which all the
flight feathers fall out at the same time and regrow within a couple
weeks, are rarer and tend to show up in aquatic birds like ducks that
don't absolutely need to fly in order to find food and avoid predators.

It's very rare to find evidence of molting in fossil birds and other
feathered dinosaurs, and O'Connor and Kiat wanted to know why. "We had
this hypothesis that birds with simultaneous molts, which occur in a
shorter duration of time, will be less represented in the fossil record,"
says O'Connor -- less time spent molting means fewer opportunities to
die during your molt and become a fossil showing signs of molting. To
test their hypothesis, the researchers delved into the Field Museum's collection of modern birds.

"We tested more than 600 skins of modern birds stored in the ornithology collection of the Field Museum to look for evidence of active molting,"
says Kiat, the first author of the study. "Among the sequentially molting birds, we found dozens of specimens in an active molt, but among the simultaneous molters, we found hardly any." While these are modern birds,
not fossils, they provide a useful proxy. "In paleontology, we have to
get creative, since we don't have complete data sets.

Here, we used statistical analysis of a random sample to infer what
the absence of something is actually telling us," says O'Connor. In
this case, the absence of molting fossil birds, despite active molting
being so prevalent in the sample of modern bird specimens, suggests that
fossil birds simply weren't molting as often as most modern birds. They
may have undergone a simultaneous molt, or they may not have molted on
a yearly basis the way most birds today do.

Both the amber specimen and the study of molting in modern birds
point to a common theme: prehistoric birds and feathered dinosaurs,
especially ones from groups that didn't survive the mass extinction,
molted differently from today's birds.

"All the differences that you can find between crown birds and stem birds, essentially, become hypotheses about why one group survived and the rest didn't," said O'Connor. "I don't think there's any one particular reason
why the crown birds, the group that includes modern birds, survived. I
think it's a combination of characteristics. But I think it's becoming
clear that molt may have been a significant factor in which dinosaurs
were able to survive."
* RELATED_TOPICS
o Plants_&_Animals
# Birds # Zoology # Bird_Flu_Research # Nature
o Fossils_&_Ruins
# Early_Birds # Fossils # Dinosaurs # Paleontology
* RELATED_TERMS
o Dinosaur o Homo_(genus) o Bird o Turkey_(bird) o
Bird_intelligence o Feathered_dinosaurs o Dodo_bird o
Dodo_and_related_birds

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========================================================================== Related Multimedia:
*
Feathers_from_a_baby_bird_that_lived_99_million_years_ago,_preserved_in
amber.

========================================================================== Journal Reference:
1. Yosef Kiat, Jingmai Kathleen O'Connor. Rarity of molt evidence
in early
pennaraptoran dinosaurs suggests annual molt evolved later
among Neornithes. Communications Biology, 2023; 6 (1) DOI:
10.1038/s42003-023- 05048-x ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2023/07/230705154016.htm

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