A beautifully preserved fossil Archaeopteryx, a famed flightless bird from the age of the dinosaurs, adds to the evidence that feathers evolved well ahead of the ability to fly.
Long seen as one of the first birds, Archaeopteryx has been joined by a flock of fellow feathered dinosaurs discovered in recent decades. (Related: "Archaeopteryx's Evolutionary Humiliation Continues.")
About 150 million years old, the new fossil disclosed Wednesday in the journal Nature reveals the iconic birdlike dinosaur sported long feathers from head to foot. Earlier fossils showed such feathers on only its wings and tail.
These so-called pennaceous feathers are the long-shafted ones birds use to fly today, distinct from the downy ones that act as insulation. The new Archaeopteryx fossil, says the study's senior author, Oliver Rauhut of Germany's Bavarian State Collections of Palaeontology and Geology in Munich, gives scientists a glimpse into the evolution of flight.
"The only thing we can say with some certainty is that [feathers] did not primarily evolve for flight, as it was long believed," Rauhut says.
About the size of a raven, Archaeopteryx (ARK-ee-OP-tur-ix) was first discovered in fossil form in 1861 at a limestone quarry in Germany. Only 11 fossils are known, including the latest one which came from the same quarry.
"The discovery of a new specimen of Archaeopteryx with such a beautiful preservation of feathers is truly exciting," paleontologist Zhou Zhonghe of China's Institute of Vertebrate Paleontology and Paleoanthropology in Beijing writes in an email.
Many dinosaurs belonging to the two-legged theropod family (the same lineage as Tyrannosaurus rex) had feathers, scholars now know, and debate has turned to how birds developed flight. Knowing pennaceous feathers covered the flightless Archaeopteryx adds to the evidence that birds started out close to the ground. (Related: "New Candidate for World's First Bird.")
The new fossil retains traces of the feathers adorning Archaeopteryx in imprints in the fine-grained limestone surrounding the fossil's bones. (The specimen's skull was crushed, but much of the rest of its skeleton is intact.) Feathers even covered the creature's legs like trousers.
The feathers on the fossil's wings were shorter than expected based on comparison with earlier fossils. That adds to the certainty that it wasn't much of a flier.
"It probably didn't fly like a bird at all—maybe like a turkey if it really tried," says Mark Norell, a paleontologist at the American Museum of Natural History in New York who has studied feathered dinosaur fossils.
A reasonable argument could be made that the long feathers evolved from the insulating ones seen on other feathered dinosaurs, Norell says. Thereafter they could have served many uses before birds ever took to the air, from lining nests to camouflage. "It's very hard to say feathers evolved for any one reason," he says.
The study's authors suggest that pennaceous feathers primarily served as a display in early bird species, as the tail of the peacock does today to attract mates. The feathers evolved in ancestors of Archaeopteryx and other early flightless birds, they suggest. They also evolved rapidly across the family of dinosaurs to which Archaeopteryx belonged.
Once pennaceous feathers had evolved, early feathered dinosaurs could have relied on them to eventually fly, Rauhut says, "including the possibility that flight evolved more than once in advanced predatory dinosaurs."
Essentially, the study team is arguing for a "ground up" evolution of feathered flight, with flapping offering an advantage to early flightless birds (perhaps in escaping predators as wild turkeys do today) that led later species to take to the air. The team is arguing against a "trees down" argument, where gliding by early feathered birds drove the evolution of flight.
While he agrees that pennaceous feathers served as displays in early birds, Jakob Vinther of the United Kingdom's University of Bristol sees at least some early birds as gliders, with feathered feet that would have helped them steer in air, for example. "I doubt their evolutionary scheme," Vinther says.
Zhou is similarly skeptical. "It is indeed very difficult to separate out a single fitness advantage to explain the origin or proliferation of feathers," he writes by email. "In fact, in most cases, it is a mixture of more than one selection force that drives the evolution of feathers and results in the diversity of feather distributions during the transition from dinosaurs to birds."
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