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   PID: hpt/lnx 1.9.0-cur 2019-01-08   
   TID: hpt/lnx 1.9.0-cur 2019-01-08   
    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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   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."   
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   Source: Materials provided by Field_Museum. Note: Content may be edited   
   for style and length.   
      
      
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   Related Multimedia:   
       *   
       Feathers_from_a_baby_bird_that_lived_99_million_years_ago,_preserved_in   
         amber.   
      
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   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   
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   Link to news story:   
   https://www.sciencedaily.com/releases/2023/07/230705154016.htm   
      
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