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Birds are known for having beaks, however at what point between being a humongous therapod and tiny sparrow did they get them, and why?

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Sources:

https://www.nature.com/articles/s41586-018-0053-y
https://www.ncbi.nlm.nih.gov/pubmed/25964090
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https://www.pnas.org/content/114/41/10930
Thanks to Brilliant for supporting this episode of SciShow.

Go to Brilliant.org/SciShow to learn more. Birds have beaks.

It's one of their things. I mean, they're not the only ones. Turtles have them and so did a bunch of extinct dinosaur groups.

But birds are by far the more diverse group of beaked animals with more than 10,000 living species and an amazing variety of beak shapes and functions. This has left many a scientist wondering exactly how birds got their beaks and, also, why. And many fossils later, they are starting to put together a pretty fascinating explanation.

Today, birds may not look much like reptiles, but their ancestors were very reptilian. They had scaly bodies, four clawed limbs, and snouts full of teeth. But as birds evolved besides gaining wings and a bunch of other adaptations, they turned those snouts into beaks.

This change involved restructuring the bones of their jaw. Many reptiles have a pair of small bones at the tip of their upper jaw called premaxillae. But in birds these bones are fused into one much larger piece and the jaws are covered in a keratin sheath.

This is all a big part of what makes a beak a beak. And a 2015 study even identified some of the genetics behind how this happens. By studying developing chicken embryos, researches identified a sequence of genome that's active during facial development and when they interrupted that genetic activity the beaks of those embryos didn't fully develop.

Instead the premaxillae remained separate and rounded much more like the snouts of modern reptiles and birds' reptilian ancestors. But while genetic data are teaching us how birds developed beaks, fossils are cluing us into when, and maybe even why, those changes happened.

In 2018, scientists described an exquisite fossil, a nearly-complete skull of a late Cretaceous bird from Kansas named Ichthyornis dispar. And this skull helped them get a better-than-ever view of the bird's face, revealing a mixture of bird-like and reptile-like features. Its premaxillae were fused together, lacked teeth, and were hooked at the end like we see in birds. And the bone had passages for blood and nerves to flow to a keratin coat, also, like modern birds.

But on the other hand, the premaxillae were small, and the rest of the mouth was full of teeth, classic reptile traits. So Icthyornis seems to sit partway through the transition from snouts to beaks, which is an amazing thing to find. And here's the really big thing, the scientists found that Icthyornis already had a flexible skull like modern birds.

In birds today the upper and lower beaks can flex to act like pincers that allows them to make precise movements that are great for catching food, cleaning feathers, and building nests. And since Ichthyornis already had this, this feature must have evolved fairly early on, and might partly explain why beaks became such a big success in birds. After all, a nimble snout would be good for any animal, but for birds who gave up their fingers for wings, a beak might work like a surrogate hand.

I say might, because evolution is complicated and there could have been other factors at play too, but still Ichthyornis is helping us figure out. Now, if we're going to talk about how birds got their beaks, there is one final thing we should mention, how they lost their teeth, because that could have been a problem. So far as we can tell, as the beaks grew larger through evolution, they seem to have gradually replaced teeth.

In fact, some extinct species started life with teeth, but developed into toothless, beaked adults. At some point in their lives, teeth just stopped developing. Overall, scientists think that as birds evolved, this signal to stop tooth development moved earlier and earlier until birds were born toothless.

But teeth serve an important role, after all, most reptiles and mammals use their teeth not just for grabbing food, but also for grinding it up to start digestion. And bird beaks just aren't great for grinding. Thankfully, scientists think they were able to get away with this because of a special organ called a gizzard, this is basically a muscular pouch full of swallowed grit and stones that acts like a food grinder at the start of the bird's digestive tract.

And luckily for birds, there's fossil evidence for gizzards in early birds as well as other dinosaurs. So, this seems to be something that evolved fairly early on. So, with their gizzards doing the chewing, birds could lose their teeth while still eating varied diet in varied habitats.

This may explain in part why birds have become so much more varied and widespread than other groups of beaked animals. And these days modern birds have used their toothless, flexible beaks to live in just about every habitat on the planet. If figuring out how things work and why is your jam, you might enjoy one of Brilliant's courses, like their one on algebra fundamentals.

Algebra can help you understand a surprising amount about the world, and Brilliant's course is so interactive and clearly explained that it's really easy to get a handle thing which is saying a lot for a math course. If you want to check it out, you can head over to Brilliant.org/SciShow and if you're interested right now the first 200 people to sign up at Brilliant.org/SciShow will get 20% off their annual premium subscription.