A quarter of a billion years ago, an epic story was starting to unfold all over the
world.
The heroes of this saga were the survivors of a near apocalypse.
They emerged from it as humble creatures, but in time, they came to dominate the Earth.
These were the reptiles, which grew to become some of the largest forms of life ever to
stomp, swim, and soar across the planet.
There were, however, some other noteworthy players in this story -- the animals and plants
that diversified in the shadows of the reptiles, many of which would go on to play key roles
in our planet’s future.
Over millions of years, this whole troupe of characters adapted to a rapidly
changing world.
But, even though they grew to immense sizes and dominated all environments, many of the
reptiles couldn’t adapt to the changes that would bring their reign to an end.
This Age of Reptiles was a spectacular prehistoric epic, and it all took place in a single era:
the Mesozoic.
The Mesozoic Era began 252 million years ago in the aftermath of the most destructive mass
extinction of all time, The Great Dying.
It brought to a close the previous era, the Paleozoic, and wiped out most marine and land
species, leaving a world ripe for the taking, at least for … anything that survived.
In the Early Mesozoic, Earth’s landmasses had almost finished merging into a single
supercontinent called Pangea.
And that meant life could traverse the globe, free of ocean barriers.
The stage was set for the first act of the Mesozoic, the Triassic Period.
At first, the planet was populated only by the survivors of the Paleozoic.
On land, there were the amphibian-like temnospondyls and early relatives of mammals called therapsids.
Meanwhile, the seas were home to many groups of ancient fish but also lots of reptiles
that were adapted to life in the water.
These first marine reptiles were mostly amphibious, but they quickly developed fully aquatic traits,
including the ichthyosaurs, which came to resemble fish and later marine mammals, even
though they were neither.
And in the background, a special group of reptiles was beginning to take advantage of
this newly open world.
These were the archosaurs, a clade that had its origins in the Paleozoic but truly came
into its own during the Triassic.
Archosaurs include many of the animals that you think of, when you think of ancient life
-- like dinosaurs, pterosaurs, and the croc-like phytosaurs.
And these creatures became key players in the Mesozoic, because they were very well
adapted to its environments.
The skulls of archosaurs were lighter than their reptile ancestors.
Most archosaurs also had teeth that were set in deep, protective sockets.
Plus they pioneered a unique unidirectional respiratory system which was a network of air sacs
that let them breathe more efficiently in the low-oxygen atmosphere after the Great
Dying.
With the help of adaptations like these, archosaurs spread and diversified.
And by the middle of the Triassic, around 243 million years ago, the real rising stars
of the archosaurs appeared: the first dinosaurs.
This is when the earliest known proto-dinosaur appears in the fossil record: Nyasasaurus,
in what’s now Tanzania.
Soon after, we find evidence of the tiny omnivore Eoraptor and the predator Herrerrasaurus,
both in South America.
And all of the earliest known dinosaurs were members of the same group, known today as
saurischians.
They had the same basic things in common, like long necks and tails and a generally
reptilian body plan.
But the thing I wanna point out here is their … pubis.
That’s one of the three bones that makes up the pelvis, including in you.
You have a pubis.
And in the case of saurischian dinosaurs, the pubis bone always pointed down, and forward.
I know it’ll be hard for ya, but try to remember that word, pubis, because I’m gonna
come back to it.
Now, as dinosaurs started playing larger roles in their ecosystems, they also became larger
and more specialized.
And around 230 million years ago, they diverged into two of their most iconic groups: the
long-necked sauropods and the two-legged, mostly-meat-eating theropods.
But while dinosaurs were becoming more diverse and widespread, another lineage of archosaurs
was adapting to another environment: the sky.
By the Late Triassic, pterosaurs became the first vertebrates in the history of the world
to take flight, with Eudimorphodon and others like it appearing in the fossil record throughout
Europe some 210 million years ago.
Meanwhile, other, non-archosaurian reptiles were dominating the seas.
Ichthyosaurs had been swimming the world’s oceans since near the start of the Triassic.
And by 200 million years ago, another group of semi-aquatic reptiles had given rise to
plesiosaurs, which adopted a totally different body plan for life in the water.
This three-pronged takeover of the land, sea, and sky allowed the reptiles to rule Pangea.
But it was not until the very end of this period that the last remnants of Paleozoic
life would truly be swept away.
201 million years ago, Pangea began to break apart, as North America drifted away from
the rest of the continent.
This caused a spike in volcanic activity that sent greenhouse gases into the atmosphere.
The resulting rise in global temperatures triggered an event known as the End-Triassic
Mass Extinction.
The casualties included most of the therapsids and other holdovers from the Paleozoic.
And this left many niches open for the dinosaurs, pterosaurs, and other reptiles that would
come to define the Mesozoic.
Which brings us to the second act of this story: the Jurassic Period, when the Age of
Reptiles reached its peak.
Hence the name of the park
By this time, dinosaurs had already acquired a variety of body plans and adaptations.
And one of the most important was the spread of the Ornithischians
Now, remember that pelvic bone I was talking about?
The pubis?
You remembered! Very good
Well that’s where a major change occurred in ornithischian dinosaurs.
Instead of pointing down and forward, in ornithischians, the pubis was reversed.
It pointed backward.
This allowed ornithischians to have a larger gut cavity that could hold expanded digestive
organs, and this helped them eat some of the toughest plants of the Mesozoic.
Because of this adaptation, and with the help of new, chisel-like teeth, ornithischians
became the eating machines of the Mesozoic!
Ornithischians probably first appeared back in the Triassic, but this was when they started
to spread and diversify.
The group became so successful that eventually it would grow to encompass many of the most
prolific kinds of dinosaurs -- all the hadrosaurs, ceratopsians, armored dinosaurs and pachycephalosaurs,
to name just a few.
So, in the Early Jurassic, reptiles took center stage, moving into most of the large animal
niches.
But one group of fuzzy therapsids managed to survive into the Jurassic, and for a long
time its members played only minor roles.
These were the very first true mammals, like Megazostrodon -- a tiny, nocturnal insectivore
that scurried around the feet of the dinosaurs.
While there’s some debate over whether mammals actually appeared at the end of the Triassic,
they certainly diversified in the Jurassic.
By the middle of the period, 164 million years ago, mammals had diversified beyond little
shrew-like things to include species that could swim like beavers and glide like flying
squirrels.
And alongside these new types of mammals was a group of theropods that would also take
to the air, acquiring the first complex wing feathers.
By 150 million years ago, the first paravian dinosaurs -- or stem birds -- were taking
to the wing, although these animals, like Anchiornis, probably weren’t very good fliers.
As for the non-avian dinosaurs, the Late Jurassic was when their really famous forms appeared.
Legendary characters like the spiky ornithischian Stegosaurus, and the carnivorous Allosaurus
patrolled the plains of North America.
Meanwhile, giant sauropods like Giraffatitan roamed Africa.
For these reptiles, the Late Jurassic was a golden age, where they were the most obvious
forms of life all over the planet.
But as the period came to a close, Jurassic Earth was changing.
The breakup of Pangea was still underway.
Sea levels began to rise, creating shallow seas in North America and Europe
And as these landmasses moved, more events unfolded that led to a complex series of extinctions
about 145 million years ago,
Rather than being a single clear incident, these losses were the result of a constantly cycling
climate of cooling and warming, and a jolt of volcanic activity again in the Pacific Ocean.
These events ushered in the third act of the Mesozoic: The Cretaceous Period.
I know that traditionally the third act is supposed to be the shortest
part of any drama but in this case the Cretaceous is
actually the longest period of the Mesozoic. And the Cretaceous saw some of the most extreme changes
ever recorded in both flora and fauna.
One of the first breakthroughs of the Cretaceous was the appearance of flowers, which appear
in the fossil record about 130 million years ago.
Before these early bloomers, conifers, ferns and cycads were the dominant plants.
Now they had competitors, although it would be a while before flowering plants became
major players in the landscape.
Meanwhile, dinosaurs were going through their own revolution.
Feathered theropods, called coelurosaurs, rarely got larger than dogs during the Jurassic,
but they reached new heights in the Cretaceous.
By 125 million years ago, big, predatory coleurosaurs like Utahraptor were roaming North America,
while Yutyrannus was hunting in China.
In the middle Cretaceous, a new group of sauropods, the titanosaurs, were outgrowing all of their
Jurassic relatives.
Some, like Argentinosaurus, are thought to have grown over 30 metres long and weighed
nearly 70 metric tons!
In the skies, pterosaurs also got much bigger, and by the Late Cretaceous, they became the
largest animals ever to fly.
These were the giant azhdarchids, which were as tall as giraffes, had wingspans the size
of small airplanes, and were more than capable of feeding on small dinosaurs.
Beneath the wings of these animals, the Cretaceous continents continued to drift apart, and dinosaur
groups became more and more isolated, and also more distinct.
For instance, titanosaurs became much more common on the southern landmasses, while in
the north, a group of feathered coelurosaurs was reaching the rank of apex predator.
These were the tyrannosaurids, the largest of the tyrannosaurs.
They first appear in the fossil record in the middle Jurassic, but by the late Cretaceous,
they had developed powerful crunching jaws and swift legs to deal with a whole new cast
of ornithischians on the northern continents.
Some of their prey, like the ceratopsians, grew wild head-gear, while ankylosaurids acquired
armour to attract mates and fend off predators.
But the duckbilled hadrosaurs were the most prolific herbivores in the north, thanks to
their powerful, beaky mouths and complex teeth that allowed them to eat just about any kind
of plant.
By the Late Cretaceous, many of these dinosaurs were the largest and most bizarre that these
groups had ever produced, showing just how successful the reptiles had become
Yet, they were about to see their Age come to an end.
For reasons that experts aren’t quite sure about, some dinosaur groups, like hadrosaurs
and ceratopsians, were becoming less diverse toward the end of the Cretaceous.
The low species density means that some dinosaur groups were already vulnerable to extinction.
And as it happened, doom was on the horizon.
Rocks dated to 66 million years ago from a region of India called the Deccan Traps show
signs of massive volcanic eruptions.
These were among the largest eruptions in Earth’s history, lasting for tens of thousands of
years, and the volcanic gases likely had powerful effects on the air and oceans.
And in the midst of these eruptions, another disaster came: A giant asteroid struck the
Gulf of Mexico, spewing ash into the atmosphere, creating an impact winter that starved plants
and phytoplankton.
These twin disasters threatened all life on Earth, but the largest animals -- the ones
that needed the most food -- were most affected.
The giant titanosaurs, and ceratopsians, and other herbivores wouldn’t have been able
to find enough plants to sustain their bulk.
And their decline meant that the large carnivores, like the tyrannosaurs, were doomed as well.
The seas saw similar losses in reptile groups, including the extinction of the plesiosaurs
and mosasaurs.
Other marine reptiles, like the ichthyosaurs and pliosaurs were already long gone by this
time.
Nearly 200 million years after they pulled themselves from the ashes of the Great Dying,
all of the giant reptiles were wiped out.
It’s now known as the K-Pg Extinction, after the German abbreviation for Cretaceous and
Paleogene, the two periods whose boundary is marked by this event.
And it brought about the disappearance of 75% of the world’s species.
But, thanks in part to their small size and their more varied diets, many Mesozoic animals
survived into modern times.
The three modern mammal groups -- the placentals, marsupials and monotremes -- all made it.
The flowers that first bloomed in the Cretaceous are now more numerous than ever.
And even some of the mighty archosaurs have persisted into our day, as crocodilians and
.. birds, the last surviving saurischian theropod dinosaurs.
But for the giant reptiles of the Mesozoic, their dominance turned out to be their downfall.
When disaster struck, the niches that demanded large size and specialization were the first
to go.
It turned out playing smaller parts in the story of our planet was a key to survival.
And so as the next era dawned, the Cenozoic, it would be those once-minor characters that
would inherit the Earth.
Thanks for sticking around for this long story about the Mesozoic, I appreciate it
But please, tell me what you want to learn about.
Because you have a lot of good ideas!
So leave me a comment, and don’t forget to go to youtube.com/eons and subscribe.
By the way, have you checked out Physics Girl?
She’s been nominated for a Webby, and if you check out her channel, you’ll understand
why.
Your brain will thank you!