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This episode is brought to you by the Music for Scientists album, now available on all streaming services. So you think you know what a bird is.

Beaks, feathers, wings, screams for peanuts way too early in the morning. But birds have been around for a good 150 million years, and in that time, they’ve come in all sorts of forms, not all of which are familiar to us today. Because when evolution gets the chance to riff on the basic bird plan… it takes it.

So here are some extinct examples whose strange features might make you rethink what you know about evolution -- and maybe how you define birds altogether. Let’s start at the beginning, with the most famous weird bird of all time: Archaeopteryx. Archaeopteryx was discovered in the 1860s in 150-million-year-old rocks from the Late.

Jurassic Period. It’s been nicknamed the “first bird,” since it’s among the earliest known fossil animals with obvious bird features. It’s got wings, feathers, and an overall bird-shaped body.

But this wasn’t a bird-like we know them today. For one, it had a toothy snout instead of a beak. It also had a small breastbone that didn’t support large flight muscles.

And it had grasping hands in its wings and a long bony tail. Now that’s a weird bird. But it wasn’t the only one.

It shared these features with plenty of Late Jurassic cousins, like Xiaotingia and Anchiornis. These animals are so strange that there’s debate among paleontologists over whether they’re really birds at all. Here’s the deal: all birds alive today evolved from ancestors that lived much later, during the Cretaceous Period.

This group is often called Neornithes, or crown group birds, because they make up the crown of birds’ evolutionary tree. Think of them as “birds as we know them.” But the earliest birds-as-we-know-them lived in a world that was already full of birds, including lots of offshoots from the trunk of that tree, below the crown. Those ones are all extinct now.

This whole collection of modern birds, their ancestors, and their extinct cousins are often called Avialae -- sort of the expanded universe of birds. See, birds aren’t just defined by their features, but also by their ancestry. Birds include all species, living and extinct, who share more recent ancestors with modern birds than with non-bird dinosaurs.

So Archaeopteryx’s toothy grin, long tail, and weak flight muscles don’t disqualify it from being a bird as long as it falls on the bird side of that ancestral divide. But when we look so far back in the story of bird evolution, it gets tricky to tell which of those feathery critters are early birds and which fall just outside the bird lineage. The very earliest birds were so, well, dinosaur-y that even if you saw them in person, they would have been hard to tell apart from their closest dinosaur cousins -- like the famous.

Velociraptor or the four-winged Microraptor. And that’s just the beginning. Birds didn’t just start weird in the days of Archaeopteryx and then quickly become familiar -- they stayed weird for a while, even as they took over the skies.

Take for instance Jeholornis, a bird from the Early Cretaceous of northern China, over 120 million years ago. These were definitely flyers; the anatomy of their chest and shoulders is very similar to modern birds. But they also held onto lots of early features, including teeth in their jaws and a long bony tail.

In fact, their tail was even longer than the tail of Archaeopteryx. Birds as we know them don’t have bony tails. Instead, they have a small series of fused tailbones called a pygostyle, which supports a fan of feathers that help in flight.

You might think a long tail would get in the way while flying, but some paleontologists have suggested the tail of Jeholornis might have acted as a stabilizer in the air. Another strange feature of these birds was their growth rate. Modern birds grow super-fast, but studies of Jeholornis bone tissue show much slower growth, more like reptiles.

So while birds as we know them typically spend very little time as juveniles, Jeholornis was younger for longer, and might even have taken a longer time to start flying. These weird flyers represent early branches in bird evolution. The suite of modern features we see today didn’t pop up all at once -- instead, evolution came up with a bunch of combinations of ancient and newer characteristics over millions of years.

Around that same time, there was a much more familiar-looking bird named Protopteryx. It had long wings, grasping feet for perching on branches, and a complex breastbone to support flight muscles. And it was a trendsetter: Protopteryx is one of the earliest members of a group called.

Enantiornithes. These so-called “opposite birds” were the most diverse group of Cretaceous birds. From a distance, you’d have a hard time telling an opposite bird from a modern one, but a closer look would reveal the oddities.

Like many early birds, most of these had clawed hands and toothy snouts, although some evolved beaks independently of birds-as-we-know-them. That actually seems to happen a lot. Also, their shoulder and wing bones were structured differently from modern birds.

Which seems to be where the “opposite birds” moniker comes from -- their wing anatomy is downright backwards. But despite their weird wings, studies have found that these birds were not only capable of flight -- they exhibited a wide variety of flight styles, just like birds today. Another strange feature of Enantiornithes is their intrepid babies.

Baby birds today tend to be helpless little things that can’t do much on their own, much like human babies. But some birds are independent at a very young age. Fossil remains of hatchling opposite birds show advanced development in their skeletons, including their wings, which suggests they were built to face the world and even fly shortly after hatching.

That’s not totally unheard of in birds today. But it might have been the norm for Enantiornithes. Just another way opposite birds were, well, opposite.

But as strange as they were, they were also pretty common birds in the Cretaceous. They may be opposite birds to us, but at the time, they were just… birds. But ancient birds weren’t just strange in the sky; they were also weird in the water.

Like the Late Cretaceous bird Hesperornis, as recently as 70 million years ago. Hesperornis was a big deal when it was first discovered in the 1870s. It was one of the first birds known to have teeth -- but that’s far from its most unusual feature.

Hesperornis is clearly built for the water. It has a streamlined body with powerful back legs for foot-propelled swimming, similar to modern-day diving birds like ducks and cormorants. Nowadays, we know there was a whole group of these birds: the Hesperornithiformes.

But while many modern diving birds can still fly, most Hesperornithiformes definitely couldn’t. Many species had shrunken front arms that couldn’t hope to get the birds airborne. Some species of Hesperornis did away with their arms entirely.

With huge, splaying back legs and no arms, Hesperornis would be pretty useless on land. They might have had to push themselves along the ground like seals, or just avoid land altogether. These are another peculiar branch early in bird evolution, but one that wasn’t totally bananas -- many water-loving birds today follow in the bizarre footsteps of these strange swimmers.

I’m kind of glad ducks still have wings, though. Around 66 million years ago, the Mesozoic Era ended with a mass extinction that brought an end to most dinosaurs, including all the groups of ancient birds we’ve been talking about. The only dinosaurs that made it through to our present Cenozoic Era were early members of crown group birds -- birds as we know them.

Those survivors were equipped with familiar bird features like toothless beaks and pygostyles, and they passed those traits onto their descendants, which are all the birds that have existed since. But they didn’t stop getting weird. For example, in Europe during the Paleocene Epoch, over 56 million years ago, the largest animals on land were birds called Gastornis.

These birds stood up to two meters tall and weighed over 150 kilograms. As you might imagine, they didn’t fly, but walked on strong back legs like an ostrich. Gastornis has such an unusual skull that for a long time, paleontologists weren’t sure what they ate.

Some thought they might even have been giant apex predators. But more recent evidence, including detailed study of the structure of their jaws, suggests they were huge herbivores, filling a similar role to modern-day mammals like deer and wildebeest. A bird that can’t fly may seem unusual, but it’s not limited to ostriches and penguins.

Flightlessness is estimated to have evolved in birds more than 150 times. From South America’s terror birds to Madagascar’s elephant birds, it’s actually kind of a thing. It seems odd now, but flightless mega-birds have been a winning strategy for tens of millions of years.

The last entry on our list are the pelagornithidae, the “bony-toothed birds.” That’s right -- teeth are back! Well, sort of. Like I said earlier, birds as we know them don’t have teeth.

So what do you do when evolution has taken away your ability to develop teeth, but you really need teeth to catch your food? Well, pelagornithids grew bony spikes right out of their beaks. They didn’t have true dental tissue, but the spikes were probably still useful for grabbing food like fish and squid.

And this wasn’t an isolated group. They lived all over the world from the Paleocene to the Pliocene, a legacy of over 50 million years! Strangely enough, there are still birds with false teeth today.

Some waterfowl, like geese, have similar, but smaller, spikes in their beaks. But the bony-toothed birds didn’t just have huge fake teeth. They had huge bodies!

The largest species are estimated to have been 20 to 40 kilograms, with wingspans over six meters across, making them some of the biggest flying birds of all time. For comparison, among today’s flying birds, albatrosses are the largest with wingspans of 3.5 meters, and Great Bustards are the heaviest at 19 kilograms. Now, that’s impressive, but it does kind of leave us wondering why they didn’t get even bigger.

After all, the largest pterosaurs had ten-meter wingspans and weighed 250 kilograms, so there must be a reason birds can’t match them. And it might be… because they were birds. See, as birds evolved, they inherited their dinosaur ancestors’ beefy back legs, useful for running, grabbing, and of course, for kicking off the ground to take flight.

But a heavier bird needs bigger leg muscles to get off the ground, and bigger leg muscles are heavy, which means the bird needs stronger wings, so it needs even bigger legs to get off the ground, and, well, you get the idea. Those legs help birds get into the air, but they also weigh them down. Big pterosaurs, on the other hand, are thought to have used their wings to take off -- sort of a jumping push-up -- so they’re using their wing muscles for take-off and for flight.

So, birds’ legs, which have been so important over 150 million years of running, perching, swimming, and grabbing prey, might also limit how big they can get while still flying. And that might be one of the weirdest things we’ve learned about birds: what makes a bird isn’t just what they have, but also their limitations. All birds started off with the same set of features, and since then they’ve had to work with the hand -- or wing -- they were dealt.

But evolution has gone wild pushing that bird body to the limits, producing strange, unexpected, and sometimes surprisingly successful forms. The evolutionary story of birds is one of untold diversity, and it includes a lot more than what first comes to mind when you think of birds. If you enjoy not just the beauty of what evolution comes up with, but also the elegant work it takes to discover it all, you might like Music for Scientists.

Music for Scientists is an album of appreciation for everyone who spends their time doing science -- and communicating about it, too. And it was inspired by all that beauty and elegance in the world around us, from the tiniest microbes to the vastness of space. If you think you’d enjoy Music for Scientists, you can find it at the link in the description. {♫Outro♫}.