New look at ancient jaw fossil rewrites bird evolution – Science

A tiny broken bone, misidentified for decades, has upended scientists’ view of bird evolution. For nearly 200 years, zoologists have divided birds into two categories: those with mobile joints in their upper jaw that allow their upper beak to move, and a much smaller group, including ostriches and emus, with a fused upper palate that gives them a less agile upper beak. This fused palate is also found in dinosaurs, including the feathered ones that were ancestors to today’s birds, so zoologists thought ostriches and their kin were the evolutionary older group of birds, with mobile upper beaks arriving later in the history of birds.
Now, paleontologists have identified a key skull bone in an ancient bird that lived nearly 67 million years ago—just before the devastating asteroid impact that killed off the dinosaurs. The bone, a piece of the upper jaw, closely resembles its mobile counterpart in today’s chickens or ducks, leading the researchers to conclude the ancient bird also had a jointed upper beak. They suspect the jointed beak was present in even older birds, because the rest of the specimen indicates it was a relative of Ichthyornis, another ancient bird that lived about 20 million years earlier. Overall, the new analysis suggests a jointed beak was already present in the ancestor of modern birds, and a fused palate re-evolved later in ostriches and their kin.
“It’s changing how we’ve been looking at the evolution of birds since the time of Linneaus,” says Christopher Torres, a paleontologist at Ohio University, Athens, who was not part of the new work. “We thought we had this worked out centuries ago, and now we’re finally finding fossils that are showing that we didn’t. We got it mixed up.”
The fossil, discovered more than 2 decades ago in a Belgian quarry near the Dutch border, was partially described for the first time in 2002, but many of its pieces remained inside a block of sediment. Juan Benito and Daniel Field, paleontologists at the University of Cambridge who study bird evolution, borrowed the fossil in 2018 from the Natural History Museum of Maastricht so they could use computerized tomography to image these remaining bones.
They hoped to find more of the animal’s skull, but initial scans only turned up vertebrae and ribs. Disappointed, they put the project aside for more than a year. When Benito returned to the fossil, he was puzzled by a bone the earlier analysis had identified as part of a shoulder but that seemed too small. He realized the piece was a fragment of a bone that had been broken in two.
After identifying the companion piece and putting the two together, Benito, Field, and colleagues concluded the full structure was a particularly delicate part of the upper palate, a bone called the pterygoid that is key to the jointed upper beak. The researchers, who describe the more complete fossil today in Nature, argue the bird is a previously unknown species and name it Janavis finalidens, for Janus, the Roman god of beginnings, endings, and transitions. It was a coastal flyer, plying the shallow seas that at the time covered what is now Belgium and the Netherlands, and weighed an estimated 1.5 kilograms—
about the size of a gray heron.
University of Texas, Austin, paleontologist Julia Clarke, who studies bird evolution, calls the Janavis fossil “an important snapshot that shows the skull in a new way and adds to the evidence for what collection of traits was present in the ancestors of modern birds.”
Several skulls of its older Ichthyornis relative have been described in recent years with bones that suggested the bird’s upper palate might have been jointed, but the evidence was still fuzzy. Now, in the Janavis fossil, “the specific skull bone that materialized was the particular one we needed” to show the upper beak was flexible, Field says. Torres agrees. “It’s like a puzzle where that one piece was missing, and now we have it,” he says
Gretchen Vogel is a contributing correspondent for Science magazine based in Berlin, Germany.
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