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Though it was a contentious idea for a long time, we know today that birds are the last living dinosaurs. But there seems to still be a lot of confusion about what that means.

You might think that birds are the descendants of dinosaurs, and that is true— but they didn't just appear after the other dinos died out. They also lived alongside them. Think ancient egrets riding on Triceratops.

Ok, we don't know that that was a thing, but it's not out of the question. Because if you went back in time, you would see some pretty familiar-looking feathered animals filling many of the same ecological roles that modern birds do today. Archaeopteryx — that half-bird half-lizard thing — gave us our first clue that birds evolved from dinosaurs all the way back in the 1860s.

But for more than a century, our actual knowledge of what birds looked like in the Mesozoic. Era was pretty sparse. In the past forty years, though, scientists have unearthed all sorts of new fossils, confirming that birds aren't just an afterthought of the dinosaur lineage.

And if you know anything about the paleontology community, you will not be surprised to hear this has lead to some pretty heated debates— but more on that later. Let's start with a very old bird: Archaeornithura. Found in China, this bird lived over 130 million years ago, making it the oldest known ornithuromorph—a grouping that includes modern birds.

Despite being only about 20 million years younger than Archaeopteryx, nobody would confuse this animal with a lizard. While Archaeopteryx had a long reptilian tail, Archaeornithura had a short little rump with fan-shaped tail feathers, like a modern bird. And while it still had a two tiny claws, it also had a special set of feathers on its wings known as alula, which are really important for controlling flight.

Archeopteryx could maybe fly, albeit awkwardly. But with its new-and-improved tail and the alula,. Archaeornithura probably had some real maneuverability and may have flown alongside pterosaurs.

But when it came to mealtime, it was probably a wading bird. Most likely, it darted around shorelines looking for small invertebrates to snack on, kind of like sandpipers do today. Actually, a lot of the early Mesozoic birds that paleontologists have found probably lived and ate near water— though, not all.

Take, for example, a little bird called Eogranivora. It lived in the same area of China as Archaeornithura, though probably a few million years later. It was also a good flier, though it likely ran along the ground a lot— maybe to escape from tiny Tyrannosaurus relatives.

What makes Eogranivora interesting, though, isn't it's feathers or legs, but rather, its diet. When scientists examined the fossil, they found what looked like seeds in its digestive tract—the remains of the bird's last meal. There was evidence it had a special organ called a gizzard where it stored swallowed pebbles to help it grind these up.

And that, combined with a toothless beak, has led scientists to declare Eogranivora the oldest known seed-eating bird. Fossils like these have taught paleontologists that many signature “bird” traits are truly ancient and have occurred in lots of divergent lineages. Because though Eogranivora and it's kin looked a lot like modern birds, and they belong to the same larger group of birds, technically, they weren't modern birds.

They were more like old cousins. Modern birds all fall under the Neornithes—a subgroup of ornithuromorphs. And debates about when those evolved —and whether they lived alongside dinosaurs—get really heated.

You see, you get two very different answers depending on how you go about trying to determine when modern birds arose. Molecular clocks suggest the first modern birds appeared fairly far back— up to 100 million years ago in the Cretaceous Period, the last period of the Mesozoic Era. But the fossil record for them in the Mesozoic remained nonexistent for decades, which suggested they only really evolved after the dinosaurs died off.

This is often known as the “rocks versus clocks” problem. They named it, it's that big of a deal. Then, in 2005, scientists announced they'd found something amazing: a Mesozoic duck.

Or duck-relative, at least. Named Vegavis iaai, it was actually unearthed in Antarctica in 1992. And at first, paleontologists weren't sure what it was, but now, they place it in Anseriformes, the order of birds that contains ducks, geese, and swans.

It even has the oldest known syrinx, a voice-box-like organ in birds, which would have let it make honking or quacking noises. What's really special, though, is that it was found in a layer of rock that's definitely, 100% pre-mass extinction. This means when Vegavis was alive, duck-billed dinosaurs and raptor-like dromaeosaurs would have walked Antarctica's beaches.

And this little duck may have had to dive past sea monsters like plesiosaurs and mosasaurs to get its food. The fossil was the first solid evidence that modern birds were already beginning to diversify into the many different families we know today before other dinos died out. And now, many paleontologists think that there were early ostrich relatives running alongside tyrannosaurs, ancestral chickens hiding from velociraptors, and maybe even penguins snacking on fish!

Of course, the rocks vs. clocks problem hasn't been completely solved. As you may see if there's any paleontologists in the comments. There are still a lot of questions — and arguments — about things like how many families of birds there were in the Cretaceous and how the mass extinction affected them.

But the more we learn, the more it's becoming clear that birds aren't just the descendants of dinosaurs. Millions of years before the other dinosaurs went extinct, birds had already become diverse, complex, and recognizable, and they were filling some of the ecological niches their kin do today. So, yes, if you went back in time, you would see some distinctly avian-looking creatures amongst T. rex, Triceratops, and the other famous dinosaurs.

And maybe even riding on them. Who knows?

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The first 500 SciShow viewers to sign up using the link in the description can join them with a 2 month free trial. Yeah, free -- so you can give it a spin and see if it's right for you. [ outro ].