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 of a single, beautiful feather,
exquisitely preserved in limestone from the Jurassic. But the feather turned out to belong to an
animal that was unlike any other bird humans had ever seen. Nearby, the remains were found of Archaeopteryx–
a creature that by all accounts was bird-like. But it had some very weird features, like
a long bony tail and claws on its fingers. It also didn’t have the large breastbone
of modern birds. And it had … teeth. Experts are still arguing over whether Archaeopteryx
was a true bird, or a paravian dinosaur, or some other kind of dino. But regardless of
what side you’re on, how did this fascinating, bird-like animal relate to today’s birds,
like ducks, ravens, emus, that sort of thing? Well, the path from Archaeopteryx to modern
birds wasn’t a straight line. Those teeth were a clue that this story goes
all the way back to what we now call the non-avian dinosaurs. Scientists started to realize that birds actually
are dinosaurs not long after Archaeopteryx was discovered in the 1860s and 70s. That’s because, around the same time as
that find, another discovery was made in those same Bavarian quarries. It was the skeleton of a tiny meat-eating
dinosaur called Compsognathus And it was the first more-or-less complete
skeleton of any dinosaur ever found. Compsognathus had a lot of traits that seemed
bird-like — it had a curved neck, it stood on two legs, and it had three main toes. And
we now think it might’ve even had hair-like feathers. But it was indisputably 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 kind of 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 from Kansas. One was Ichthyornis, which looked pretty much
like a seagull. And the other was Hesperornis, which resembled a huge, flightless loon. Ichthyornis was clearly avian — it had wings
without claws, a short tail, and a large breastbone, like most modern birds do. But both it and
Hesperornis also had teeth, like most non-avian theropods did. So, Marsh and other experts began to hypothesize
that birds evolved from dinosaurs. In these specimens, Marsh saw how birds gradually
acquired their curved necks, their two-legged stance, and their three toes. He saw modified arms transition to feathered
wings, and the loss of claws and the long tail. Hesperornis and Ichthyornis were soon classified
as ornithurines, a group of Mesozoic birds whose names means “bird tail.” These ancient birds had short tails, instead
of the longer bony tail of Archaeopteryx. Most of them also had teeth. And they tended
to live on the ground. We know this because they were usually found
in environments near shorelines, instead of inland forests, and they didn’t have the
adaptations that are associated with perching, like curved foot claws, or claws on their
wings. So by the late 1800s, 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 at all. In the early 20th century, some researchers
— led by Danish paleo-artist Gerhard Heilmann — argued that the similarities between Compsognathus
and birds were convergent, and that birds evolved from some other, mystery reptile. It wasn’t until 1970 that, quite by accident,
paleontologist John Ostrom came across the key that would unlock the origin of birds. He was studying fossils of pterosaurs from
the same German limestones that produced Archaeopteryx and Compsognathus. But one of these fossils was not from a pterosaur,
as many had thought. It turned out to be the hand of an animal that was very similar to
Archaeopteryx. Actually, for a long time, Ostrom and others
thought it was an Archaeopteryx. But in 2017, it was redescribed and assigned to a new genus,
named after Ostrom himself. Now, the hand of this animal looked just like
the hand of a non-avian dinosaur that Ostrom had uncovered just a few years earlier in
Montana, called Deinonychus. Both animals had short thumbs, long second
fingers, and third fingers of medium length. And in both specimens, most bones in the wrist
were fused together into a half-moon shaped block that allowed for movement from side
to side, but not up and down. Living birds have this same half-moon feature
in 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 90s, 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 whole new lineage of extinct birds that we didn’t know existed. They
came to be called the enantiornithines or “opposite birds.” Which is a terrible name, because they totally
were birds. They were only called opposite birds because one of their shoulder joints
was the reverse of what’s seen in living birds. But, unlike the ornithurines, which mostly
lived on the ground, these opposite birds mostly lived in trees. Their fossils were
commonly found in rocks that formed in ancient forests, and most of them had curved claws
like perching birds do today. And these turned out to be the most common
and diverse group of birds in the whole Mesozoic era, filling many of the same roles 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 dove for fish from tree branches, like kingfishers
do today. Most of these opposite birds had teeth and
clawed fingers. And instead of having a fan of feathers in their tails, they had blunt
rear-ends that sometimes sported just two long, ribbon-like feathers. So it was clear to scientists that these “opposite
birds” belonged to a different lineage than the likes of Hesperornis, Ichthyornis, and
the other ornithurines. Meanwhile, experts were also making some important
discoveries about the ornithurines, too. Especially, what happened to their teeth. For example, in the early 2000s, Chinese researchers
reported finding the oldest fossil evidence of a toothless bird. They named it Confuciusornis,
and it lived 130 million years ago — way back in the early Cretaceous. And the strangest thing was: Confuciusornis
was neither an ornithurine nor an enantiornithine. It turned out that toothlessness was a convergent
trait that showed up in lots of different groups at different times. Some opposite birds didn’t have teeth, and
some early ornithurines didn’t either. So, where did their teeth go? Well, for a long time, scientists thought
that birds lost their teeth to make their bodies lighter. But now we know that some
toothed birds were able to fly, so it wasn’t like having teeth was keeping them on the
ground. As more fossils of Mesozoic birds were found,
more clues started to present themselves. For instance, experts began to notice that,
even though most ancient birds had teeth, they tended to have fewer teeth than their
dinosaur ancestors. And fossils also revealed that birds started
to have fewer teeth after they developed gizzards. Gizzards are a muscular part of the stomach
where, with the help of swallowed pebbles, birds can grind up their food. So with the
advent of gizzards at least 120 million years ago, the need for teeth started to fade. And as birds’ teeth changed, so did their
beaks. Most opposite birds retained teeth at the
very front of their mouths. But in ornithurines this same area became covered in keratinous
beak tissue, while they kept the teeth that were farther back in their jaws. Now, some experts think that replacing teeth
with beak tissue may have had something to do with making it easier to eat certain foods,
like seeds. But another theory, proposed in 2018, suggests
that beaks may have been more advantageous for baby birds. Teeth take a long time to grow in developing
embryos, but if you shorten that step or cut it out altogether, then eggs can hatch earlier.
And the shorter a bird’s incubation time is, the less vulnerable its young will be
to predators. So birds may have lost their teeth for many
reasons. But one thing we know for sure is that, in the Cretaceous, teeth disappeared
in the group of ornithurines that would give rise to today’s birds. These are the neornithines. Their fossils are still very rare, but in
the past couple of decades, paleontologists have found bones of the very earliest members
of this group. One of the best known is a 68 million year
old duck relative from Antarctica called Vegavis. And its remains show that, even by the Late
Cretaceous, it was already well adapted to swimming and diving. So today, we finally have enough fossils of
different kinds of birds to know that, about 66 million years ago, there were both toothed
birds and toothless birds from many different groups. And generally, they were all doing
just fine. Until, the Cretaceous Period ended in extinction. There were many causes behind the extinction
event that killed off the non-avian dinosaurs. But when it came to birds – and most everything
else – , the worst was probably the asteroid impact in the Gulf of Mexico. The impact set off huge fires that destroyed
entire forests and vaulted dust into the atmosphere, probably blocking much of the sunlight for
years. For birds, their odds of survival might have
come down to a number of factors, including how big they were, where they lived, and what
they ate. For example, if you lived in trees, like most
opposite birds did, you were in the worst possible place in the aftermath of the impact. Not only was your habitat suddenly gone, but
if you ate plants, it would be weeks or months before any new growth would be available.
And if you ate meat, there was almost nothing around to hunt. But if you were smaller and lived on the forest
floor, there’s a chance you could have found shelter. And it might’ve been a lot easier
for you to find food, like seeds on the ground. Likewise, if you were aquatic, your habitat
and your food supply, might have been less disturbed by the fires. So, out of what may have been thousands of
species of dinosaurs living at the end of the Cretaceous, only three groups of neornithes
had that perfect survivor’s combination of being small, ground-dwelling or aquatic,
and seed-eating. One of these lineages was the paleognathes,
which today include ratites like ostriches and emus, as well as weird South American
birds called tinamous. The galloanserans also survived, like Vegavis
and its kin, possibly thanks to their aquatic lifestyle. Today they include chickens, ducks
and other kinds of fowl. And the third lineage is the neoavians, which
includes essentially every other living bird. Free of competition from the tree-dwelling
opposite birds, a lot of neoavians quickly took to life in trees at the dawn of the Cenozoic. These birds all have diverse diets and habitats
today. But by analyzing the earliest known members of each group, scientists have found
that the last common ancestor of all living birds was indeed a ground-dwelling seed-eater. Each of these three lineages might have had
only a handful of species at the time of the extinction, but they are the ancestors of
the roughly 10,000 species of birds that are alive today. There are many things we have yet to learn
about the story of birds. But our understanding has grown tremendously since that lovely fossil
of a feather was first discovered in Germany, more than 150 years ago. And, just within my own lifetime, a wealth
of bird fossils has been discovered that are only beginning to reveal the wonderfully weird
and diverse world of the Mesozoic, when birds had teeth. I hope you enjoyed this toothsome episode and all the difficult words I had to say. Now,
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 on the cutting
edge of green technology. They’ll eat edible plastic, so you don’t have to, and 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 let me know
what you want to learn about, 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 so much for your support! If you’d like to join them, head over to patreon.com/eons and pledge for some neat n nerdy rewards.

100 thoughts on “When Birds Had Teeth

  1. Can we have one about how communication developed? Visual too but if possible verbal etc i kinda think it might also be interessting to know not just how the past looked like but how it sounded like ^^

  2. So did Pterosaurs who ate fish went extinct because they suffocated, because in this video you said that birds who ate fish had enough food to eat but pterosaurs ate fish too. was it because there size was to big to get enough food or was it because most Pterosaurs lived in higher elevations and any ash that was still in the air during the meteor impact or from a volcanic eruption suffocated them Im just curious.

  3. What is up with all the pebbles from the bird stomach being quartz variants (agate/chalcedony)? Less biologically reactive? Just because they are shiny?

  4. Small nitpick here. You said that a theory was proposed, but theories aren't proposed. That's hypotheses. Theories are conclusions arrived at and agreed upon by many scientists independently.

  5. Mammals: 5400 known species
    Birds: 9990 known species

    Conclusion: We're still on the world of the dinosaurs!

    (well, at least we're more on theirs than on ours)

  6. ichthyornis is an incredibly annoying bird if you play Ark: survival evolved…diving down and stealing your stuff…

  7. I think this channel is the best thing I have ever found on YouTube. I would love to learn more about cephlapod history, really early ones like orthocerida are kind of surreal seeming to me.

  8. I find it interesting that for the longest time Christians claimed that humans and dinosaurs lived at the same time, and the scientific community said that we didn't. But now Christians have gotten with the idea that we didn't, but the scientific community is now basically saying that birds are dinosaurs, so humans and(at least one variety of) dinosaurs did and do live at the same time.

    Not hating on Christians, I am one. I just thought it was interesting.

  9. I feel like if we call Birds Dinosaurs or Dinosaurs birds, then we should be calling Dimetrodon a mammal. Dinosaurs and species like Dimetrodon were similar in that they were a transition from Reptile to their respective families. Dinosaurs shared a lot in common with birds, but also a lot with their reptilian ancestors, and the same goes for Dimetrodon with Mammals. I dunno, taxonomy can get convoluted very fast.

  10. I personally don't think that anything outside of Aves is a true bird, just a very birdlike dinosaur, since they aren't descended from the common ancestor of modern birds. It's really just pedantic, since "bird" isnt really a scientific classification, but it's my interpretation.

  11. It’s interesting to see that the future of our avian dinosaurs would result in their developing gizzards, loosing teeth and evolving keratin beaks. It’s those little changes you can believe in.

  12. How did birds survive the meteor? I mean mammals were under earth, crocodiles probobly too or in deep swamps, but birds are very fragile

  13. This why I can't stand people like Ken Ham or Frank Turk. They know nothing about evolution, but yet talk about it and fail to debunk it. They think that evolution is animal popping out of nowhere. That's creation not evolution. Evolution is the why and not the how.

  14. What is the narrators name and educational background? Is she in the same league as Hank and John Green from Crash Course or Dr. Joe Hanson from it’s OK to be Smart? Just wondering because I love your content.

  15. Evolving a gizzard that needs to be filled with rocks doesn't seem a very logical thing to do while evolving to become lighter for better flight ?

  16. 10:15 wait… Galloanserans which were semi-aquatic include ducks and chickens!? are you telling me ancient chickens used to be aquatic?! please PBS Eons explain this!

  17. I dont know why this is still "controversial" embryonic modern birds have teeth, and a tail, they loose these features later on the embryonic stage and this is due to genes that influence their embryonic development.

  18. I know this video isn't about hair, but I've got to say, I love your braid!
    This video is really fascinating! The variety of birds and pre-birds is really amazing!

  19. Ah, so dino nuggets aren't too inaccurate after all! In fact, I guess technically ALL chicken nuggets are dino nuggets.

  20. Some birds like Turkeys, chickens and waterfowl still have a single small claw on the ends of their wings.

  21. It makes more sense to have beaks you can cut me in a smaller slivers allowing it to digest better allowing you to have more calories to spend more energy

  22. "Scientists have found that the last common ancestors of all living birbs was indeed a grounddwelling seedeater".
    I do love me some toothless grounddwelling seedeating birbs.

  23. I think about the extinction of dinosaurs, and it makes me sad until I consider the fact that birds are nothing more than the best survivors dinosaurs had to offer. Your lineage still lives and affects the world 65 million years later.

  24. Saw this video in the recommended of my teacher's YouTube account while she was showing us a video.
    No regrets.
    (Am now scared that she will show us this video as well and see my comment)

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