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,
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