How Did Birds Lose Their Teeth And Get Their Beaks? Study Offers Clues

May 2, 2018
Originally published on May 2, 2018 10:06 pm

Scientists are one step closer to understanding how modern birds evolved to have beaks, and the answer starts millions of years ago with some of the sexiest dinosaurs.

Modern gulls, with their large eyes, long beaks and distinctly ancient-looking and bony faces, descended from animals such as the velociraptor and T. Rex. (Next time you get a hungry look from a seagull, remember that.) For more than a century, paleontologists have used fossils from all over the world to piece together how large, toothy, land-bound lizards evolved into flying, toothless, feathered animals.

The key is fossils of so-called stem birds, which are ancient birdlike dinosaurs that offer clues about the bones and brains of modern birds. One such creature, Ichthyornis dispar, is the subject of a newly published study that fills in some of the missing links in the evolution of birds.

"Ichthyornis had the aspect of a seabird, like gull or a tern," says Bhart-Anjan Bhullar, an assistant professor of geology and geophysics at Yale University. It had a long beak and large eyes, and lived in Kansas back when Kansas was an inland sea, between 100 million and 66 million years ago. But it had two things modern gulls don't: teeth and a muscular jaw to use those teeth.

"It was probably flying about, picking out morsels of fish and shellfish, grabbing them with its little pincer beak and then throwing them back into its strong, dinosaurian toothed jaws — crunching them a few times and then swallowing them," Bhullar says.

That combination of beak, teeth, flight and jaws make it a crucial link in bird evolution, but its skull has been difficult to study because the only fossils available were "some crushed brain cases and some lower jaws and a few other fragmentary bits," Bhullar explains.

Then, in 2014, scientists working in Kansas found a complete skull fossil.

Bhullar's team got a high-resolution CT scan of the fossil, which was still completely enveloped in chalky rock. In essence, the researchers did digitally what their predecessors had been forced to do manually: They extracted the bones from the surrounding stone to get a complete, three-dimensional picture of the skull.

When they analyzed the images and compared them with previously discovered fossils, the researchers discovered two things. First, Ichthyornis could move its beak in a very modern way, lifting its upper beak without moving the rest of its skull, the way all birds today do. It gives the beak more dexterity, so the bird can use it to pinch very precisely.

The fact that Ichthyornis could do this reinforces a theory about why birds have beaks at all: Perhaps the beak is basically a surrogate hand.

However, the second finding challenges a different theory, about bird brains. "Bird-brain" insults notwithstanding, modern birds actually have relatively large brains compared with their velociraptor ancestors.

"Bird brains are larger relative to their body size than is the case for reptiles, and the relative size of bird brains is comparable to that of placental mammals," paleontologist Kevin Padian of the University of California, Berkeley, says in response to the study published today in the journal Nature. "As birds evolved from their dinosaur ancestors, the bones that protect the brain enlarged to keep pace with the changes in brain size.

Bhullar and others had hypothesized that as the bird skull expanded to hold a larger brain — maybe to provide more processing power for flight — the muscular jaws around the head would shrink. But Ichthyornis has both a large brain and strong jaws.

"Ichthyornis fills in an important gap, but of course everything that fills a gap makes two more gaps on either side," Bhullar says. "Now we need to figure out how the remainder of the transformations toward the bird's skull occurred."

Padian points out other questions the new study raises. It's still unclear what range of things ancient birds like this one used their beaks for. In addition to eating, modern birds use their beaks for everything from preening their feathers to building nests to moving their eggs. And he notes that the new information about how Ichthyornis moved its beak could offer clues to what it ate and how it gathered its food.

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AUDIE CORNISH, HOST:

Scientists are one step closer to understanding how modern birds evolved to have beaks. NPR's Rebecca Hersher reports on a new study that goes back millions of years to a birdlike dinosaur from Kansas.

REBECCA HERSHER, BYLINE: Birds are direct descendants of dinosaurs, but researchers still don't know exactly how we got from a velociraptor to a seagull. Bhart-Anjan Bhullar is a paleontologist at Yale who's focused on one crucial part of that evolution - the head.

BHART-ANJAN BHULLAR: The first thing I think one would notice is that birds have a beak. The other thing they have is an enormously enlarged brain.

HERSHER: Dinosaurs don't have any of that.

BHULLAR: It's a big step between a tooth, snout and a beak. The heads of dinosaurs like velociraptor - the jaws were filled with large teeth, and they had large jaw muscles and a powerful bite.

HERSHER: To figure out how snouts turned into beaks and the little dino brains grew into big bird brains, Bhullar studies the ancient creatures that were evolutionarily halfway between velociraptors and birds. One of them is a creature that lived in Kansas about a hundred million years ago called Ichthyornis. Ichthyornis looks like a really aggressive seagull, like a perfect mash-up between a bird and a dinosaur.

BHULLAR: It sits at a really pivotal kind of position in the evolution of birds.

HERSHER: It had a beak with teeth in it. But the overall shape of Ichthyornis' skull was less clear because the fossils available were from the 1870s and weren't that complete, which is why Bhullar was really excited in 2014 when he got his hands on a newly discovered Ichthyornis fossil embedded in a slab of rock. He did a CT scan of the whole rock and ended up with a high-resolution 3D image. Two things jumped out about what he saw in the new fossil. First, the bone configuration would have allowed the animal to pinch with its beak the way parrots and other birds grasp nets very carefully. It's the earliest example of that pinching ability, and it appears to support a very cool idea.

BHULLAR: Perhaps the bird beak is basically a surrogate hand.

HERSHER: Think of it this way. As the claws of velociraptors evolved into the wings of birds, their faces compensated by gaining dexterity. The team also found Ichthyornis's brain was large like a modern bird. But surprisingly, the jaw was muscular like a dinosaur. Until now, scientists had assumed that no animal would have both of those things.

BHULLAR: Ichthyornis fills in an important gap. But of course everything that fills in a gap makes two more gaps on either side.

HERSHER: The search is on, he says, for more dino-bird fossils. Rebecca Hersher, NPR News. Transcript provided by NPR, Copyright NPR.