Birds are inextricably linked to feathers, which allow them to fly, keep warm and put on dramatic displays. Feathers, however, predate birds—having first belonged to extinct dinosaurs. Finding out exactly when feathers evolved, and which animals had them, could offer important new insights into the distant past.
In the 1990s, the first fossilized feathers were found in extinct dinosaurs. But, almost 30 years later, many questions remain about these distinctive features.
Did all dinosaurs inherit feathers from a common ancestor, or did feathers evolve multiple times in the group? Are they exclusive to birds and their closest relatives, or are they more widespread across the reptile family tree? At the moment, the jury’s still out.
With such important details still shrouded in mystery, a new review has sought to bring together the latest knowledge about the evolution of feathers to help find answers. Professor Paul Barrett, one of our dinosaur researchers, co-authored the paper alongside Professor Xu Xing.
“While dozens of feathered dinosaurs have been identified since Sinosauropteryx was first unveiled in the 1990s, they’re mainly meat-eaters that are close relatives of birds,” explains Paul. “Everyone agrees that these dinosaurs had feathers, but there’s little consensus beyond that.”
“To settle these disagreements, we need more fossils and to define what a feather is. If we can sort this out, it could reveal where feathers first came from and whether they predate the dinosaurs themselves.”
The study was published in the journal Biology Letters.
When did feathers evolve?
At the moment, paleontologists are split into a few different groups depending on their thoughts about when feathers evolved. The first group supports a concept known as the avemetatarsalian origin, which refers to the group containing birds, dinosaurs and pterosaurs.
This theory suggests that feathers first evolved in a common ancestor of the dinosaurs (including birds) and pterosaurs, meaning both groups would have inherited their feathery coatings. Some would have later lost their ability to grow feathers, explaining why some species don’t have feathers but are scaly instead.
Other researchers believe that feathers evolved at a different time. Some think that feathers might have evolved among the ancestors of dinosaurs, but that the fuzzy features found in pterosaurs are something else entirely.
The tetanuran theropod origin goes even further. It suggests that only the two-legged, meat-eating dinosaurs known as the theropods developed true feathers, and that feather-like features seen in other fossils are similarly adapted scales.
One of the main reasons behind these different theories is that most feathered dinosaur fossils come from the Cretaceous Period, long after feathers would have first evolved. Finding earlier fossils will be crucial to change this.
“We need to find new sites with fossilized feathers, particularly from the Jurassic and the Triassic,” Paul says. “The Triassic is especially important, as finding feathered dinosaurs at this early time in dinosaur evolution would suggest that all dinosaurs had an ancestor with feathers.”
“If feathered dinosaurs aren’t found among early dinosaurs, then it makes it more likely feathers were an innovation among later groups.”
Ongoing questions
As well as hunting for more fossils, researchers will need to agree on the precise definition of a dinosaur feather. While feathers belonging to today’s bird species are helpful to refer to, millions of years separate them from those of the dinosaurs—meaning new features could have evolved in the interim.
“To identify a true feather from a living bird, we’d look to see what it’s made out of, and its structure,” Paul explains. “One of the characteristics we have to identify is that the feather grows out of a follicle in the skin.”
“It’s possible, however, that follicles and other modern characteristics might not have been present in all dinosaur feathers. Detailed anatomical work at a microscopic level to compare preserved skin and feathers with examples from living birds could help to resolve this.”
One promising source of new details is preserved skin. Comparisons between fossilized skin in some dinosaurs and early birds from China suggest that they had similar tissues, supporting the idea that it was present in a common ancestor.
Other dinosaurs, however, had more reptile-like skin, or a mix of feathers and scales, which instead make it more possible that feathers evolved independently.
Ultimately, there’s still a lot of work to do until the origin of feathers can be pinned down. Until then, paleontologists will continue to search the world for the fossils that can finally settle this decades-old debate.
More information: Xing Xu et al, The origin and early evolution of feathers: implications, uncertainties and future prospects, Biology Letters (2025). DOI: 10.1098/rsbl.2024.0517
Journal information: Biology Letters
Provided by Natural History Museum
This story is republished courtesy of Natural History Museum. Read the original story here.