When Birds Had Teeth

In 1861, a man working in a Bavarian quarry discovered a fossil that would change the way we think about

birds

. The fossil was a single, beautiful feather, exquisitely preserved in Jurassic limestone. But the feather turned out to belong to an animal that looked like no other bird humans had ever seen. The remains of Archeopteryx, a creature that by all accounts resembled a bird, were found nearby. But it had some very strange features, such as a long, bony tail and claws on its fingers. It also did not have the large sternum of modern

birds

. And it had...

teeth

.

Experts are still arguing over whether Archeopteryx was a true bird, a paravian dinosaur, or some other type of dinosaur. But regardless of which side you're on, how does this fascinating bird-like animal relate to today's birds, like ducks, crows, emus, and that sort of thing? Well, the path from Archeopteryx to modern birds was not a straight line. Those

teeth

were a clue that this story goes back to what we now call non-avian dinosaurs. Scientists began to realize that birds are actually dinosaurs shortly after Archeopteryx was discovered in the 1860s and 1870s. This is because, around the same time as that discovery, another discovery was made in those same Bavarian quarries.

It was the skeleton of a tiny carnivorous dinosaur called Compsognathus. And it was the first more or less complete skeleton of any dinosaur ever found. Compsognathus had many bird-like features: it had a curved neck, stood on two legs, and had three main toes. And now we think it might even have had hair-like feathers. But it is indisputable that it was not a bird. It had very short arms instead of wings. And it had a pelvic bone, called the pubis, that pointed forward instead of backward, as it does in birds. So Compsognathus was a non-avian dinosaur that looked like a bird.

Then, in the 1870s, American paleontologist O. C. Marsh described two more species that illuminated the transition between dinosaurs and birds, found in late Cretaceous rocks of Kansas. One was Ichthyornis, which looked very much like a seagull. And the other was Hesperornis, which looked like a huge flightless loon. Ichthyornis was clearly a bird: it had clawless wings, a short tail, and a large sternum, like most modern birds. But both it and Hesperornis also had teeth, like most non-avian theropods. So Marsh and other experts began hypothesizing that birds evolved from dinosaurs. In these specimens, Marsh saw how the birds gradually acquired their curved necks, their bipedal posture and their three toes.

He watched as the modified arms turned into feathered wings and the loss of the claws and long tail. Hesperornis and Ichthyornis were soon classified as ornithurins, a group of Mesozoic birds whose name means "bird's tail." These ancient birds had short tails, rather than the longer bony tail of Archeopteryx. Most of them also had teeth. And they used to live on the ground. We know this because they were generally found in near-coastal environments, rather than inland forests, and did not have the adaptations associated with perching, such as curved claws on their feet or claws on their wings.

So, at the end of the 19th century, many scientists believed that it was this group, the ornithurines, that gave rise to today's birds. They didn't know what happened to their teeth, but they knew the connection was there. But still, not everyone was convinced that birds derived from dinosaurs. In the early 20th century, some researchers, led by Danish paleoartist Gerhard Heilmann, argued that the similarities between Compsognathus and birds were convergent, and that birds evolved from some other mysterious reptile. It was not until 1970 that, by chance, paleontologist John Ostrom found the key that would unlock the origin of birds.

He was studying pterosaur fossils from the same German limestones that produced Archeopteryx and Compsognathus. But one of these fossils was not of a pterosaur, as many had thought. It turned out to be the hand of an animal very similar to Archeopteryx. In fact, for a long time, Ostrom and others thought it was an Archeopteryx. But in 2017, it was redescribed and assigned to a new genus, named after Ostrom himself. Now, this animal's hand resembled the hand of a non-avian dinosaur that Ostrom had discovered just a few years earlier in Montana, called Deinonychus. Both animals had short thumbs, long second fingers, and medium-length third fingers.

And in both specimens, most of the wrist bones were fused into a crescent-shaped block that allowed movement from side to side, but not up and down. Living birds have this same crescent shape on their wrists, which they use to fold their wings. So Ostrom's discovery revived the theory that birds evolved from small theropod dinosaurs. And yet, we had no idea how incomplete our understanding was until the 1980s and 1990s,

when

dozens of new fossil discoveries showed us how truly diverse birds were in the Mesozoic. Most of the fossils found around this time belonged to a completely new lineage of extinct birds that we didn't know existed.

They came to be called enantiornithines or "opposite birds." Which is a terrible name, because they were totally birds. They were only called opposite birds because one of their shoulder joints was the opposite of what is seen in modern birds. But unlike ornithurins, which lived mainly on the ground, these opposite birds lived mainly in trees. Their fossils were commonly found in rocks that formed in ancient forests, and most of them had curved talons like those found in perching birds today. And these turned out to be the most common and diverse group of birds of the entire Mesozoic era, performing many of the same functions as modern birds.

For example, Bohaiornis may have hunted small vertebrates 120 million years ago in ancient China. And it lived alongside Longipteryx, a perching bird with a long snout that may have dived for fish from tree branches, as kingfishers do today. Most of these opposite birds had teeth and clawed fingers. And instead of having a fan of tail feathers, they had blunt butts that sometimes sported just two long ribbon-shaped feathers. So it became clear to scientists that these "opposite birds" belonged to a different lineage than Hesperornis, Ichthyornis and other ornithurins. Meanwhile, experts were also making important discoveries about ornithurins. Especially what happened to his teeth.

For example, in the early 2000s, Chinese researchers reported finding the oldest fossil evidence of a toothless bird. They called it Confuciusornis and it lived 130 million years ago, back in the early Cretaceous. And the strangest thing was: Confuciusornis was neither ornithurin nor enantiornithine. It turned out that missing teeth was a convergent trait that appeared in many different groups at different times. Some opposite birds did not have teeth and some of the early ornithurins did not have teeth either. So where did his teeth go? Well, for a long time scientists thought that birds lost their teeth to lighten their bodies.

But now we know that some birds with teeth could fly, so it wasn't as if having teeth kept them on the ground. As more fossils of Mesozoic birds were found, more clues began to present themselves. For example, experts began to notice that although most ancient birds had teeth, they tended to have fewer teeth than their dinosaur ancestors. And the fossils also revealed that birds began to have fewer teeth after they developed gizzards. The gizzards are a muscular part of the stomach where, with the help of swallowed pebbles, birds can grind their food. Then, with the appearance of gizzards at least 120 million years ago, the need for teeth began to disappear.

And as the birds' teeth changed, so did their beaks. Most opposite birds retained their teeth at the front of their mouths. But in ornithurins this same area was covered by keratinous tissue from the beak, while they maintained the teeth that were further back in their jaws. Now, some experts think that replacing teeth with beak tissue may have had something to do with making certain foods, such as seeds, easier to eat. But another theory, proposed in 2018, suggests that the spikes may have been more advantageous for the little birds. Teeth take a long time to grow in developing embryos, but if that step is shortened or removed completely, the eggs can hatch sooner.

And the shorter a bird's incubation time, the less vulnerable its young will be to predators. So birds may have lost their teeth for many reasons. But one thing we do know for sure is that, in the Cretaceous, teeth disappeared in the group of ornithurins that would give rise to today's birds. These are the neornithines. Their fossils remain very rare, but in the last two decades, paleontologists have found bones of the first members of this group. One of the best known is a 68-million-year-old relative of the Antarctic duck called Vegavis. And its remains show that, already at the end of the Cretaceous, it was already well adapted to swimming and diving.

So today we finally have enough fossils of different types of birds to know that, about 66 million years ago, there were both toothed and toothless birds from many different groups. And in general, everyone was doing well. Until the Cretaceous Period ended in extinction. There were many causes behind the extinction event that wiped out non-avian dinosaurs. But

when

it comes to birds - and almost everything else - the worst was probably the asteroid impact in the Gulf of Mexico. The impact sparked huge fires that destroyed entire forests and spewed dust into the atmosphere, likely blocking much of sunlight for years.

For birds, their chances of survival could have come down to a number of factors, including their size, where they lived, and what they ate. For example, if you lived in trees, as most birds did, you were in the worst possible place after impact. Not only did its habitat suddenly disappear, but if it ate plants, it would be weeks or months before new growth became available. And if you ate meat, there was almost nothing to hunt. But if you were smaller and lived on the forest floor, there's a chance you would have found shelter. And it might have been much easier for you to find food, like seeds in the ground.

Likewise, if it were aquatic, its habitat and food supply might have been less disturbed by fires. So, of what may have been thousands of dinosaur species that lived at the end of the Cretaceous, only three groups of neornithes had that perfect combination of survivors: being small, terrestrial or aquatic, and eating seeds. One of these lineages was that of the paleognaths, which today include ratites such as ostriches and emus, as well as strange South American birds called tinamous. Galloanserans also survived, like Vegavis and its relatives, possibly thanks to their aquatic lifestyle. Today they include chickens, ducks and other types of birds.

And the third lineage is the Neoavians, which includes essentially all other living birds. Free from competition from other tree-dwelling birds, many neoavians quickly began living in trees at the dawn of the Cenozoic. All of these birds have diverse diets and habitats today. But by analyzing the earliest known members of each group, scientists have discovered that the last common ancestor of all living birds was in fact a ground-dwelling seed-eater. Each of these three lineages may have had only a handful of species at the time of extinction, but they are the ancestors of the approximately 10,000 species of birds that are alive today.

There are many things we still have to learn about the history of birds. But our understanding has increased enormously since that beautiful feather fossil was first discovered in Germany more than 150 years ago. And, in just my own lifetime, a wealth of bird fossils have been discovered that are only beginning to reveal the wonderfully strange and diverse world of the Mesozoic, when birds had teeth. I hope you enjoyed this delightful episode and all the difficult words I had to say. Here's something new for you! “ReInventors” is a new show from PBS Digital Studios and KCTS9 in Seattle that will introduce you to scientists and inventors at the forefront of green technology.

They'll eat edible plastic, so you don't have to, and they'll take you to unexpected places, like a garage that has its own nuclear reactor. Check out ReInventors and subscribe to them at the link below. And of course, don't forget to tell me what you want to learn and, if you haven't already,go to youtube.com/eons and subscribe. And we want to thank our two eontologists, David Reed Rasmussen and Steve. . Thank you very much for your support! If you want to join them, head over to patreon.com/eons and pledge for some nifty, nerdy rewards.