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  • Welcome to the third installment of our miniseries on the history of life on Earth!

  • 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.

  • Thanks for watching this episode of SciShow, which was brought to you by our President

  • of Space, Morgan, and The Big Try Hard.

  • Morgan just completed a bicycle trip across the U.S. raising money for YouTube channels

  • he loves.

  • Thank you, Morgan!

  • You can catch up on his journey at thebigtryhard.com and if you want to help support this show,

  • just go to patreon.com/scishow.

  • And don't forget to go to youtube.com/scishow and subscribe!

Welcome to the third installment of our miniseries on the history of life on Earth!

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