As every schoolchild now knows, birds are dinosaurs, linked to their extinct relatives by feathers and anatomy. But birds' beaks—splendidly versatile adaptations that allow their owners to grasp, pry, preen, and tear—are nothing like stiff dinosaurian snouts, and how they evolved has been a mystery. Now, 3D scans of new fossils of an iconic ancient bird capture the beak just as it took form.
"This region of the [bird family] tree is populated almost exclusively by flattened specimens," in which delicate features of the skull are lost, says Amy Balanoff, a paleontologist at Johns Hopkins University in Baltimore, Maryland, who was not involved in the research. By bringing details from multiple specimens together, the new scans offer an early glimpse of key features of bird skulls, including a big brain and the movable upper jaw that helps make beaks so nimble.
Ichthyornis, an ancient seabird from about 90 million years ago, has long been famous for having a body like a modern bird, with a snout lined with teeth like a dinosaur. Paleontologists studying the first Ichthyornis fossil, discovered in the 1870s in Kansas, initially thought the body came from a small bird and the jaw from a marine reptile. Further excavation convinced them that the pieces belonged to the same animal. In 1880, Charles Darwin wrote that Ichthyornis was among "the best support for the theory of evolution" since On the Origin of Species was published 2 decades earlier.
But in the original Ichthyornis fossil, the upper jaw is missing, and the toothed lower jaw resembles that of other dinosaurs. So paleontologists assumed that early birds made do with a fixed upper jaw, like most other vertebrates.
In 2014, paleontologists in Kansas found a new specimen of Ichthyornis. They shared the fossil with Bhart-Anjan Bhullar at Yale University and his colleagues.
Instead of extracting the fossil from the limestone in which it is embedded, the researchers used computerized tomography to scan the entire block of rock. Then they scanned three previously unrecognized specimens that they found in museum collections, and combined all the scans into a complete model of Ichthyornis's skull. They also re-examined the original fossil from the 1870s, housed at Yale's Peabody Museum of Natural History. Among unidentified pieces stored with the fossil, they found a small fragment that, when scanned, turned out to contain two key bones from the upper snout—bones that were missing in the new specimens.
The resulting 3D model captures Ichthyornis's transitional position between modern birds and other dinosaurs, Bhullar and colleagues report this week in Nature. Despite its dinosaurlike teeth, Ichthyornis had a hooked beak, likely covered by a hard layer of keratin, on the tip of its snout. It also could move both top and bottom jaws independently like modern birds.
That means beaks appeared earlier than thought, perhaps around the same time as wings, Bhullar says. The agile jaw probably allowed the bird to preen its feathers and gave it a pincerlike grasp. At the same time, Ichthyornis retained powerful jaw muscles. "more similar to what you'd see in velociraptor than what you'd see in living birds," says Daniel Field, a paleontologist at the University of Bath in the United Kingdom who helped lead the work.
This mosaic of dinosaurian and avian characteristics shows birds in the act of evolutionary transformation, says Patrick O'Connor, a paleontologist at Ohio University in Athens—and offers a reminder that evolution rarely takes a straight path.