When we left off, life had been brought to its knees by a mass extinction at the end

of the Paleozoic era.

The Mesozoic era, from 251 to 65 million years ago, followed the great extinction and would

produce some of the weirdest and most fascinating animals of all time.

Including the dinosaurs!

It also led to most of the major land animal groups we know today.

Like other eras, the Mesozoic is divided into periods -- in this case, three of them.

The first period, the Triassic, lasted from 251 to 199 million years ago.

It was a time of transition, when the dominant vertebrates of the late Paleozoic, the therapsids,

pretty much disappeared.

A new group of reptiles, the dinosaurs, then rose to become Earth's new dominant land vertebrates.

Throughout the Mesozoic, Earth was warmer than it is now, and had no polar ice caps.

At the beginning of the Triassic, Earth's landmasses were lumped together into the dry

supercontinent Pangea.

Slowly, life started to repopulate the place.

Repopulating meant diversifying.

The therapsids were declining, but a new group of vertebrates was starting to take over -- the

archosaurs.

Archosaurs descended from one of the earliest major groups of land vertebrates, the diapsids,

at the end of the Paleozoic.

These major animal groups -- the archosaurs and the diapsids -- are defined by the holes

in their skulls, which attach to muscles and mean that the big, heavy bones weigh a little less.

It might seem like kind of a strange way to tell animals apart, but it's actually a

very clear marker:

Diapsids have two openings behind their eyes.

Archosaurs have two extra openings, one in front of the eye and one in the lower jaw.

Dinosaurs are archosaurs, but so is another group that almost took over instead of the

dinosaurs: the pseudosuchians, which evolved very similar body plans to the dinosaurs that

came later -- including standing on two legs.

They came in a lot of different shapes and sizes, but if you want know what a pseudosuchian

looked like, picture something a bit like a crocodile, but about twelve times more terrifying,

because it's able to stand up straight on long legs and run really fast over land.

Pseudosuchians were nearly wiped out in another mass extinction at the end of the Triassic.

Only one lineage survived, one that took to living in swamps and gave rise to modern crocodiles

and alligators.

Although thankfully, the modern versions don't run around on two legs.

The only other archosaurs around today are birds.

So the archosaurs took over from the therapsids, which had mostly died out at the end of the Paleozoic.

But some therapsids hung on through the Triassic, and some didn't die out at all -- luckily for us.

The therapsids are the descendants of another major branch of land vertebrates, the synapsids,

who are also classified by their skull holes.

Synapsids have one skull opening, not four, and they're the ancestors of mammals.

The earliest mammals appeared in the middle of the Triassic -- at practically the same

time as dinosaurs.

Yes -- we mammals started out as dinosaur buddies, and we still hang out with dinosaurs

now, just in bird form.

Speaking of dinosaurs: there probably weren't many Triassic dinos in the coloring books

you had when you were six, because they hadn't developed much yet.

But they had some advantages that would eventually make them the ruling reptiles.

Like I mentioned earlier, pseudosuchians and dinosaurs had very similar body plans.

But dinosaurs had a slight physiological edge: their breathing was more efficient.

And while both groups evolved legs that were positioned straight under them instead of

sprawling to the sides, dinosaurs were the stronger movers.

The earliest dinosaurs that we can be confident about go back 230 million years.

But there are some very dino-like animals from 10 million years before that.

An animal called Nyasasaurus may or may not be a true dinosaur, depending on who you ask,

but it's definitely close.

And it comes from 243 million years ago in present-day Tanzania.

As is often the case with evolution, it's hard to draw the line between true dinosaurs

and their immediate ancestors.

But somewhere between Nyasasaurus and later dinosaurs like Eoraptor, they had officially

evolved, ready to take over the world.

A couple of other animal groups turned up during the Triassic.

One was the ichthyosaurs, the first reptile group to become fully aquatic again after

evolving a land-based lifestyle.

They're also one of only two groups to evolve a fish-shaped body from a four-footed animal body.

The other group being the whales.

Finally, toward the end of Triassic, a group of archosaurs that were closely related to

the dinosaurs -- but weren't dinosaurs themselves -- evolved the power of flight: the pterosaurs.

After the Triassic came a period that you might have heard of: the Jurassic period,

which lasted from 199 to 146 million years ago.

Although I do feel like I need to point out that some of the dinosaurs in the movie franchise

are partly or totally made up, and others aren't from the Jurassic at all.

During the Jurassic, Pangea was beginning to separate into two continents, Laurasia

and Gondwana.

Shallow seas covered parts of the land.

This is when dinosaurs diversified into their more familiar forms.

The Jurassic was a great time to be a sauropod, for example: a huge, long-necked plant-eater

that walked on four legs.

Diplodocus, Brachiosaurus, and Apatosaurus all lived during the Jurassic.

Then there were the theropods, the meat-eaters that walked upright.

Allosaurus was one major predator, but even bigger and meaner theropods were yet to come.

The Stegosaurus also evolved during the Jurassic.

It was a big plant-eater with plates all along its back and a spiked tail weapon called a

thagomizer, because if you're going to pick a name for a giant spiky tail-weapon you might

as well make it awesome.

Meanwhile, the Plesiosaurs, a group of reptiles not closely related to dinosaurs, joined ichthyosaurs

in the oceans.

There was one other major group of dinosaurs that appeared in the Jurassic: birds.

We know that birds are descended from dinosaurs because of the similarities of their skeletons,

and the fact that many dinosaurs had feathers.

And because people who study evolution like to include all of a group's descendents

in that group, birds technically are dinosaurs.

So, if you've ever fed a chicken nugget that's shaped like a dinosaur to a child:

that's a weird experience.

That's a whole, strange thing.

Archaeopteryx, which is usually considered the earliest bird, dates back to the Jurassic,

and so do lots of other early birds.

They, along with the pterosaurs, were the two kinds of flying archosaurs during the

Jurassic -- and during the next period, the Cretaceous.

The Cretaceous, which means chalk-bearing, lasted from 146 to 65 million years ago and

was even warmer than the earlier Mesozoic.

The continents continued to drift apart, heading for where they are now.

As the seafloor spread, it released carbon trapped in the Earth's crust and caused

some serious global warming.

Ichthyosaurs disappeared sometime during the Cretaceous.

But a new type of marine reptile appeared: the mosasaurs, aquatic lizards related to

the monitor lizards we have today -- but not closely related to dinosaurs.

Another new arrival?

Flowering plants, which were excellent at getting animals to spread their pollen.

That's why, at the same time as flowers, we see pollinators like bees appearing in

the fossil record.

Whether flowers or pollinators came first is a kind of evolutionary chicken-and-egg question.

Probably neither one of them came first, exactly.

The flowers and pollinators influenced each other's evolution and became more interdependent

as time went on.

Mammals -- which, you'll remember, had been around since the Triassic -- evolved into

the major lineages alive today: placental mammals like us, marsupials like the opossum,

and monotremes like the platypus.

The Cretaceous also meant even more dinosaurs!

Like the frilled ceratopsians, the duck-billed hadrosaurs…

...and, of course, Tyrannosaurus rex.

I don't know about you, but I think T. rex is pretty cool.

I'm also very glad predators that size aren't around today to snack on us.

Why aren't they around any more?

Like the Paleozoic, the Mesozoic ended in a mass extinction.

But there were a few differences between the two die-outs.

For one thing, the extinction at the end of the Mesozoic wasn't as bad.

Only about 50% of Earth's species went extinct, which is a lot, but not nearly as many as

during the extinction at the end of the Paleozoic, when almost all life died out.

And while we don't know exactly what caused the earlier Paleozoic extinction, we have

a major clue about the event at the end of the Mesozoic.

It's a crater in the Yucatan region of Mexico.

Most scientists agree that a meteor impact at this site must have been what wiped out

all of the dinosaurs except for birds.

There might have been other factors at play, but the meteor didn't help.

So most of the diversity dinosaurs had to offer is gone for good.

Sure, birds are cool, but they only represent one lineage of dinosaurs.

Those big four-footed plant eaters and the walking around armored vehicles?

They're not around anymore.

But once they were gone, mammals had a chance to take over -- which is what happened during

the Cenozoic, the era that we are still in, which we will talk about next time.

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