Birds are so different than other animals. They uniquely have feathers. They're
built to fly. And they just behave differently than other animals do.
But while birds have carved out their own niches in the world, they have their origin
with a different group of animals, the reptiles. And specifically, dinosaurs.
To understand why, we have to go back a long time.
The group of dinosaurs called Theropods that first appeared in the middle of the
Triassic period. Theropods walked on two legs and most ate other animals,
like the famous Theropods called Velociraptor and Tyrannosaurus rex.
Theropods became a very diverse group including species that were omnivores and plant eaters.
Theropods can be recognized by their hollow bones, three-toed feet, and claws on their
hands and feet. Many Theropods had feathers as well as scales, and even other dinosaurs
have been found with feathers. A smaller group of Theropods called Avialae developed powered
flight. This diversity of Theropods lived around the world in all kinds of different environments.
In the Cretaceous-Paleogene extinction around 66 million years ago, an asteroid hit the Earth and
killed every land species weighing more than about 50 pounds. 75% of all existing species
went extinct. But three groups of avian dinosaurs were able to survive through this catastrophe. The
ancestors of ducks, the ancestors of chickens, and the ancestors of ratites like ostriches and emus.
These three groups were probably able to survive because they
could eat seeds buried in the ground until plant life started to recover.
We call these three surviving groups the "avian dinosaurs", because they then evolved
into birds - avians. These avian dinosaurs were left with a world completely open. They spent
tens of millions of years diversifying into the thousands of modern birds.
Djinn506 made this custom model of a turkey skeleton so we can see a modern bird skeleton.
Let's look at some of the similarities birds have with reptiles. The more unusual
similarities we find, the more likely it is that birds and reptiles are related.
Birds and reptiles both have a lower jawbone made of five fused bones instead of one solid bone.
All birds and most reptiles lay eggs outside their bodies. There are reptiles that hatch
eggs internally and give birth to live young. But every reptile, and every bird lays eggs.
Probably connected to their egg-laying, both reptiles and birds produce uric acid as their
nitrogen waste. Uric acid takes more energy to produce, but is less toxic at high concentrations,
so it's useful if you spend part of your life sealed inside an egg.
Birds and reptiles both have scales. The feet of most birds have obvious scales. See?
Many different Theropods had both scales and feathers. And some of the same genes
are used to make feathers and scales. One other similarity is that scales
of reptiles and birds, and feathers, are made of an unusual form of keratin, called beta keratin.
A Theropod trait that is in almost every bird. Some diving birds like penguins and
loons have lost this trait. But for flying birds,
having hollow bones helps their bones to be more rigid without burdening them with more weight.
Another Theropod trait that has been brought into modern birds is feet with three forward-facing
big toes, and one smaller toe facing backwards. The vast majority of birds have this foot plan.
A Theropod named Archaeopteryx shares traits with both reptiles and birds.
One example is that it has a fused clavicle, also known as the wishbone
in modern birds. This trait is shared by most birds today, and was also in some dinosaurs.
Birds, dinosaurs, and crocodilians all have one attachment point between their
neck vertebrae and skull. This is part of what allows owls to rotate their heads so
far. Mammals and amphibians have two attachment points, so their heads can't rotate as much.
No people were around to document the time when dinosaurs became the birds that we see
today. But if we assemble enough clues, it tells us that dinosaurs never went completely extinct.
There are still thousands of species of Theropod dinosaur alive today. So if you are celebrating
with a Thanksgiving turkey this year, you can accurately say, "Please pass the dinosaur."
Thanks to Djinn506 for making this custom skeletal model just for this video. He makes these great 3D
artworks that you can see on Instagram if you follow him like I do. Link is in description.
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