Even paleontology has ugly duckling stories.
In 2000, a fossil collector found a chunk of rock about as big as a deck of cards in a limestone quarry in Belgium. Sticking out of it were femur and shin bones the size of twigs that looked like the remains of a miniature chicken dinner.
Eighteen years later, Daniel Field, a paleontologist at the University of Cambridge in England, and Juan Benito, a doctoral student, took a CT scan of the chunk and found a prize within: an almost perfectly preserved ancient bird skull.
“It just totally blew our minds,” Dr. Field said.
The fossil, described by Dr. Field and colleagues today in Nature, is between 66.7 and 66.8 million years old, making it “the oldest known fossil that’s clearly part of the modern bird family tree,” Dr. Field said. It’s also the first fossil that resembles a modern bird without any ambiguity found in the northern hemisphere, he said, although at least one expert takes issue with this characterization.
Bhart-Anjan S. Bhullar, a paleontologist at Yale who was not involved in the study, said that the fossil is “one of those great discoveries that come up a few times in a lucky lifetime.”
The researchers have given the bird the scientific name Asteriornis maastrichtensis along with a pet name: the Wonderchicken.
Sometime during the Jurassic period, around 165 million years ago, the earliest birds diverged from theropod dinosaurs. Over the next 100 million years, the bird family tree continued to branch out, leading to everything from broad-winged Archaeopteryx to toothy Enantiornithes.
When the asteroid hit about 66 million years ago, many of these branches were sheared off. Those that survived became the birds we know today.
Dr. Field noted that modern bird fossils from before the asteroid impact are extremely rare. Examining them can help experts learn about the features that might have allowed modern birds to survive that mass extinction event while other animals, including the dinosaurs, died out.
Based on its bones, the Wonderchicken was likely a small, ground-dwelling bird — about the size of a male green-winged teal.
Dr. Field’s previous research suggested that only small creatures survived the asteroid impact and its fallout, and that ground dwellers did better than birds that lived in trees, perhaps because of the wildfires that followed. That the Wonderchicken’s relatives survived to become today’s birds supports these hypotheses, Dr. Field said.
The fossil also helps to shed light on the path evolution took after this worldwide shock. The Wonderchicken’s skull “looks like if you took the skull of a chicken, added the skull of a duck, and divided by two,” Dr. Field said.
The back of the Wonderchicken’s skull was duck-like, and it had a chicken-like beak. This suggests relatedness with today’s waterfowl and land fowl.
Trevor Worthy, a paleontologist at Flinders University who was not involved in the study, said that “until now, we have had no fossil birds that can be considered the ancestor” of the duck and chicken group, or of the group that contains ducks, chickens, and most other birds. The Wonderchicken, Dr. Worthy said, “appears to fill that gap.”
It also supports an image that many paleontologists love: “that modern chickens and other fowl are living fossils, hardly modified since the days of the dinosaurs,” Dr. Bhullar said.
Thomas Stidham, a paleontologist at the Chinese Academy of Sciences who was not involved in the study, said that this is an “exciting discovery.” But other, less complete modern bird fossils from a similar time period have been found in the Northern Hemisphere. The Wonderchicken “helps to support those other, more fragmentary records,” he said.
Dr. Field hopes that the Wonderchicken’s surprise appearance will spur fossil hunters to search for more complete fossils in Europe and North America that could fill gaps in our knowledge of bird evolution.
He also hopes there is another lesson: “Look more carefully at specimens that, at first glance, might look kind of uninspiring,” he said.