Feathers are a primitive trait among pennaraptoran dinosaurs, which today are represented by living birds, the only clade of dinosaurs to survive the end-Cretaceous mass extinction. Feathers are central to many important functions and therefore, maintaining plumage function is of great importance for survival. Thus, molt — by which new feathers are formed to replace old ones — is an essential process. The limited knowledge regarding molt in early pennaraptoran evolution is based largely on a single specimen of Microraptor, a small feathered dinosaur that lived 120 million years ago. In a new analysis of 92 feathered non-avian dinosaur and ancient bird fossils, paleontologists did not find additional molting evidence. They suggest that the evolution of the flight feather molt evolved with the development of powered flight, among ancient birds or a more derived subset of pennaraptoran dinosaurs, and as a response to the high dependence of this group on the flight feathers and the aerodynamic ability they impart.
Primary sequential molt in Microraptor; the arrow indicates the location of the molt-related gap within the primary feathers. Scale bar – 10 cm. Image credit: Y. Kiat & J.K. O’Connor, doi: 10.1038/s42003-023-05048-x.
“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,” said Dr. Jingmai O’Connor, a researcher at the Field Museum of Natural History.
“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?”
In their study, Dr. O’Connor and Field Museum of Natural History postdoctoral researcher Yosef Kiat examined 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 he authors 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 — less time spent molting means fewer opportunities to die during your molt and become a fossil showing signs of molting,” Dr. O’Connor said.
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,” Dr. Kiat said.
“Among the sequentially molting birds, we found dozens of specimens in an active molt, but among the simultaneous molters, we found hardly any.”
“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.”
The findings were published in the journal Communications Biology.
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Y. Kiat & J.K. O’Connor. 2023. Rarity of molt evidence in early pennaraptoran dinosaurs suggests annual molt evolved later among Neornithes. Commun Biol 6, 687; doi: 10.1038/s42003-023-05048-x
