Hey there! Squeaks and I are here with our friend Dino.

And our library just got a new book that has pictures of all the different kinds of dinosaurs that used to live on Earth.

And it tells a little about them, too...

...like what they ate!

This page says that some dinosaurs like Stegosaurus and Triceratops ate plants.

So I was thinking that they might have been friends who hit up the salad bar together.

Uh, Dino...

...but they definitely weren’t friends with the meat eaters, like Allosaurus and T. Rex!

I bet they had amazing dinosaur races as they ran to catch the best food.

Dino...

Yes, Jessi. What do you think?

I think… that it’s fun to imagine what these animals might have done if they’d met.

And I also think we need to remember that Stegosaurus and Triceratops weren’t around at the same time.

Wait. Wait, what...

I’m sorry, Dino… but the same is true for Allosaurus and T. rex.

These dinosaurs lived in different times from one another!

I don’t get it! They all lived back in the time of the dinosaurs, right?

Yep, you’re right about that! But the time of the dinosaurs was a very long time.

It lasted millions of years! So all dinosaurs didn’t live at the same time!

Oooh, good point, Squeaks.

We built a pretend time machine to imagine what it would be like to travel back in time and see living things from the past.

But to see all of these dinosaurs, we would have to make more than one trip!

So… if we took your time machine back to the time of the dinosaurs, we’d actually have to go to lots of different times.

Exactly! Scientists divide “dinosaur time” into three parts… called periods.

There’s the Triassic period… which started over 200 million years ago.

Some small dinosaurs like Eoraptor lived then.

Next came the Jurassic period.

Allosaurus and Stegosaurus were alive during this period.

The Jurassic period lasted over 50 million years.

It was after that that dinosaurs like Triceratops and T. rex

came along in a time called the Cretaceous period!

Oh, you’re right, Squeaks, it can be pretty hard to think about long periods of time like that.

But I have an idea we can use to show how long ago dinosaurs lived.

We can use our bodies as a model!

We use models to helps us understand or explain something in the world around us.

By using our bodies as a model of "Dinosaur Time," we can get a better picture of just how long ago those different periods were!

It won’t be perfect… but it will be a way for us to think about things that happened a long time ago!

Now… let’s say that the bottom of your shoes is the bottom of our model, when the very first dinosaurs appeared.

That’s the beginning of the Triassic period, about 245 million years ago.

Okay!

Stegosaurus and Allosaurus didn’t show up until the Jurassic period, about 50 million years later.

That means that these dinosaurs would have lived in the time that starts somewhere below your knee and ends somewhere around your hips.

That distance on your body represents about 56 million years.

Then... it was T. rex time!

The Cretaceous period was about 74 million years long.

That means if it starts at your hips, it should end at about your chest.

And the time of the dinosaurs ended about 65 million years ago.

So everything from your chest to the top of your head represents the last 65 million years!

Oooh, good question, Squeaks.

Humans and their ancestors have only been on Earth for about 6 million years…

so in our model, that would be only the last couple centimeters of your body.

Maybe from the top of your forehead up!

Wow, so people haven’t been around for very long at all!

No! Life on Earth was around a long time, even before there were dinosaurs...

and I mean a long time.

And over that time, the way earth looked and the plants, animals, and other kinds of life that lived here changed a lot!

Some of these changes happened very fast, like volcanic eruptions!

But others happened very slowly.

So slow that we wouldn't have noticed they were happening, even if people had been around.

Oooh, great question!

Squeaks wants to know how we know plants and animals lived during different times if no one was there to see them!

I think I know this one!

Is it because of fossils

Good job, Dino!

We’ve found fossils bones, footprints, and other things left behind by ancient animals, plants, and other kinds of life.

We use these fossils as clues to try and figure out how long ago they might have lived.

How

Well… one way we can tell which fossils are older and which are newer is by how deeply they’re buried.

Think about paper in a recycling bin.

The paper that you put into the empty bin last week is on the very bottom of the can.

And it gets buried by the paper you throw in later.

The very top layer of paper in the recycling bin is the paper you put in there today,

this morning, or even a few minutes ago.

Oh… I see!

So the fossils that are deep in the ground, like Stegosaurus, are older than the ones closer to the top of the ground, like the T. rex.

That’s right! T. rex is pretty young.

… in dinosaur years, anyway!

Yes! Now let’s see what else we can learn about in this new book.

Thanks for joining me here on SciShow Kids.

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be sure to hit the subscribe button, and we'll see you next time here at The Fort!

Bye!