All language subtitles for NOVA - S49E07 - Dinosaur Apocalypse - The Last Day_track4_[eng]

DAVID ATTENBOROUGH:
Dinosaurs...

Perhaps some of the most dramatic
animals ever to have walked the Earth.

[BELLOWS]

They dominated the world
for over 150 million years,

until a huge asteroid struck the planet.

But how exactly did they die?

Palaeontologists have been
searching for the answer for decades.

[BELLOWS]

And now new evidence is coming to light.

STEVE BRUSATTE: We're out
looking for clues...

[BELLOWS]

...and each fossil is a clue,

and that tells us something about
what the world was like at that time.

ATTENBOROUGH: Since 2012,
a team of palaeontologists

has been investigating a remarkable site,

deep in the Badlands of North Dakota.

The team's leader, Robert
DePalma, hopes it holds evidence

of what happened on the
very last day of the dinosaurs.

[THUNDERCLAP]

Could it even contain
the remains of an animal

that bore witness to that terrible event?

We've got all these bones
in the ground right now,

but the one thing that we would
just dream of finding

is that one dinosaur that died
on the day of the impact.

BRUSATTE: The idea that
there is a dinosaur fossil,

potentially, that's a direct
victim of that, that's very exciting.

DEPALMA: Whoa,
that's skin right there.

That's actually scaly skin.

ATTENBOROUGH: Can they find a dinosaur
that died on the day the asteroid hit?

ATTENBOROUGH: For 10 years,
Palaeontologist Robert DePalma and his team

have been digging in a small
corner of the Hell Creek Formation.

An area famous for more than a
century of major dinosaur discoveries.

They've already found a wealth
of fossilised creatures

in a patch of land
they call "Tanis."

Oh, wow!

ATTENBOROUGH:
What appears to be

a piece of fossilised skin
from a Triceratops...

The unhatched egg, and what
looks like a pterosaur embryo...

Jawbones of a mammal called a Pediomyid.

And teeth and footprints of
carnivorous dinosaurs like T. rex.

There is no other dinosaur
that has teeth like this.

ATTENBOROUGH: Many of these fossils were
found in a thick layer of crumbly rock.

The rock here is really not quite rocky,

and it just falls apart in your hands.

ATTENBOROUGH: Right above
the crumbly rock is the K-Pg boundary,

a layer of iridium-rich
debris from the asteroid impact

that hit the Earth 66 million years ago.

It marks the end of the age of dinosaurs.

CATHY PLESKO: If you look
below this layer,

you see fossils of dinosaurs.

If you look above this layer,
no dinosaurs.

ATTENBOROUGH: The four-foot thick layer of
rock at Tanis is full of ejecta spherules...

DEPALMA: And beautiful,
look at that.

ATTENBOROUGH:
...tiny glass droplets

created in a major asteroid impact.

Robert thinks that
this is compelling evidence

that everything in the layer was
buried while the spherules fell.

If he is right,

and the spherules he's found

can be matched to the asteroid impact,

this dig site could provide

a snapshot of what happened on
the very last day of the dinosaurs.

Stories like this are eminently plausible.

Proving them is more challenging.

EMILY BAMFORTH:
It opens up that whole debate

about how do we link

catastrophic events
to fossil and geologic deposits?

SONIA TIKOO: If we can both
match spherules to the impact site,

geochemically, and in terms
of radiometric ages,

that's pretty accurate.
That's a smoking gun.

After 10 years of digging,
there is now enough evidence

to piece together
much of the story of Tanis

and the creatures which lived here.

But how exactly did they die?

The asteroid that struck the
Earth 66 million years ago,

created what is today
known as the Chicxulub crater.

To find out if the ejecta spherules
they've found in North Dakota

can be linked to Chicxulub,

Robert has come to the Diamond
Light Source synchrotron in the UK.

Joining him is Phil Manning
of the University of Manchester.

They've already run
initial tests in America

on over a dozen spherules found in
different areas of the crumbly layer.

What have you found out so far?

These little glass spherules, these globs
of molten material from the impact site

have a chemical signal that
ties it with where they came from.

'Cause when an asteroid hits,

it melts the ground that it hits,

but also that glass has a little bit of
contamination from the asteroid itself.

And that gives you a
unique geochemical fingerprint.

MANNING: We can see once
we've scanned it,

and looking at other sites
from around the world,

Haiti, Mexico and North Dakota,

we can get a baseline for what
the ejecta should look like

when it's related to the Chicxulub crater.

You can see each element here
and the ratios of those elements.

And when we look at Tanis, it's a match.

I mean, it perfectly overlays.

So I think this is powerful evidence

supporting that Tanis
and Chicxulub are linked.

Wow!

And what do these findings mean
for the rest of the fossils

that you're finding in Tanis?

This data is key for the entire site,

because once you have that link and
you know what impact affected Tanis,

then you essentially know
that every object in that site,

all the animals, and the plants, and
everything buried in those sediments

are linked to the last day
of the Cretaceous.

ANUSUYA CHINSAMY-TURAN:
This is very important,

because it immediately gives a time stamp

for the locality itself.

BAMFORTH: The Tanis site is

like a window into a snapshot of time.

ATTENBOROUGH: With ejecta
spherules found everywhere

throughout the four-foot thick deposit,

Robert and his team seem to be
able to link their site to a single day.

And the synchrotron here in the UK,

reveals something even more remarkable.

DEPALMA: So this is showing
a beautiful synchrotron scan

of the half of one spherule.

The glass is
a good geochemical fingerprint,

but when we look at the entire thing,
we see something quite unexpected.

That's your entire spherule.

What's this?

DEPALMA: In this, we've got
a little bit of a nugget.

There was a little particle right there.

So we scanned it, and
that's a lot of iron in there.

Over here, we've got chromium,
a big peak in chromium.

Over here, we've got a big peak in nickel.

And the abundances of iron,
nickel and chromium all together,

that matches what you'd expect
to see in a meteoric body.

That does not match what you
would normally have down here.

So this is extra-terrestrial material?

If you were to sort of grind up

and stuff into a spherule,
a piece of meteorite,

that's what it's gonna look like.

This could be a piece
of the Chicxulub asteroid.

The piece of the bullet
that killed the dinosaurs

No.

ATTENBOROUGH: Robert's team
may have found

a fragment of the asteroid
itself in North Dakota.

Physical evidence linking this
site to the Chicxulub impact.

But Tanis is almost 2,000 miles
away from where the asteroid hit.

So, exactly how did the asteroid
cause the death of the animals here?

To answer that question,

Robert is searching in something
he calls the mass death assemblage.

DEPALMA: Right here, we've got
this intertangled mass of fish.

There's one fish here,
another sturgeon goes this way,

underneath the body of a paddlefish.

There's another sturgeon
that goes this way,

underneath this log, and
continues out the other side.

And his head hit that log

and has deflected downward
at a 90-degree angle.

ATTENBOROUGH: Robert uncovered
a tangled mass

of fossilised creatures and logs
surrounded by spherules,

and crushed together
in what's known as a logjam.

He has a theory that the
creatures were swept to their death

in some kind of turbulent surge of water,

and quickly entombed in sediment,

which is why they are so well-preserved.

But what could have caused the wave?

One hypothesis is a tsunami.

The asteroid hit at sea.

We're talking about a tsunami
of completely different scale,

much higher, much larger

than we've ever seen before
in modern tsunamis.

SEAN GULICK: So
if you had 6,000 feet of water,

at least half of that

would have left as the rim wave.

So at least 3,000 feet high, at a minimum.

ATTENBOROUGH: The tsunami
raced towards land.

TITOV: When they
reach the coastlines,

they were still very high
waves of up to 300 feet high,

at least probably as high
as 1,000 feet high wave.

That's very impressive wave.

Imagine the waves that's the size of
a building approaching the coastline.

ATTENBOROUGH:
In the Late Cretaceous,

North America was divided by a narrow sea

that has been called
the Western Interior Seaway.

The tsunami could have, theoretically,

travelled up this toward Hell Creek.

Tsunamis generally travel at
about the speed of a jet plane.

It's not something you could,
say, run away from.

It had plenty of energy
to get over the coastline.

It could easily still have been
tens of metres high

by the time it reached
well into the seaway.

ATTENBOROUGH: Could the rapid deposition
at Tanis have been caused by a tsunami?

To test the idea, the team
needs to look at the timing.

-DEPALMA: Which fish is that? It's a new--
-MAN: New one.

-DEPALMA: ...it's a new contact.
-MAN: Yeah.

ATTENBOROUGH: If a tsunami
buried the fish,

it would have to have hit while
the ejecta spherules were falling,

because spherules were found everywhere,

including in the fishes' gills.

So much depends on determining when
these spherules were falling at the site.

RILEY BLACK: Modelling the
ejecta always has error bars on it,

in that we're not there to measure it,

and we've had no equivalent
impact like this

on Earth since then.

PLESKO: But we can look
at the computational models

that we do and say, "All right, this
material is coming from this point.

"It's now moving away this fast
with about this much mass."

And then we can tell
with the sorts of equations

that we might use to calculate the trajectory
of a cannonball, where it would go.

And we can observe from these
simulations how long it takes

these ejecta to reach
their final destinations,

down to the order of a few minutes.

ATTENBOROUGH: What the
calculation shows is surprising.

Robert and his team have
found that these ejecta spherules

landed at Tanis between 13
minutes and 2 hours after the impact.

So, if a wave buried the fish,

it must also have reached
the site within two hours.

Data from recent tsunamis

show even a powerful wave
would take much longer than this

to travel almost 2,000 miles from
the impact site to North Dakota.

So, if it wasn't a tsunami,

what could have caused
the surge of water at Tanis?

ATTENBOROUGH: Stein Bondevik
is an expert in tsunamis.

BONDEVIK: The fjords
in Norway are very special.

We have tall mountains
surrounding bodies of water.

So the water is usually very calm.

ATTENBOROUGH: In 2011,
something very strange happened.

The water in the fjord
began to move violently.

The height of the water increased

by one-and-a-half meter,

like a maelstrom with the turbulent water.

Someone said that the fjord was boiling.

ATTENBOROUGH: News started
to roll in,

there'd been an earthquake
5,000 miles away in Japan.

BONDEVIK: A journalist from
the local newspaper called me

and he said that people were
observing waves here in the fjords.

I got a video clip of the waves.

I saw immediately that they
looked like a tsunami wave.

Here you can see that
the fjord is perfectly calm,

but at the beach here, you could see
that the water is sloshing back and forth.

And no one had ever seen anything like it.

Some people got very upset and afraid.

ATTENBOROUGH: A magnitude nine earthquake
had devastated the northeast of Japan.

But how did that affect
a fjord so far away?

BONDEVIK: So no one in Norway
could feel the earthquake,

but I could see that the times matched

the arrival of the waves
here in the fjord.

ATTENBOROUGH: Eventually,
Stein and his team realised

that this might have something
to do with seismic waves,

shockwaves that pass quickly
through the Earth during an earthquake.

BONDEVIK: So, it took only
12 minutes before the first signal

of the earthquake in Japan reached
all the way here to western Norway.

ATTENBOROUGH: So,
it was the seismic waves

that caused the normally
calm water in the fjord

to slosh turbulently back and forth.

Just thinking of that...
scientifically, it's fantastic.

ATTENBOROUGH: Could something
similar have happened in Tanis?

MAN: Large weather fronts
coming through large...

ATTENBOROUGH:
Geophysicist Mark Richards

has been studying
the site for several years.

The events in Norway support a hypothesis

that he's been working on
with Robert's team,

about what could have caused
the surge of water here.

RICHARDS: A tsunami can't get
here in less than minimum 12 hours,

but seismic waves travelling
from the Yucatan impact site

to North Dakota
can arrive here fairly quickly.

ATTENBOROUGH:
In the Late Cretaceous,

the Western Interior Seaway
that bisected North America

could have been connected to Tanis

through the extensive river
system that once flowed here.

RICHARDS: If you have
a very large body of water,

like the Western Interior Seaway,

and you can shake it back and forth,

you can generate a large water
wave coming up this river at Tanis.

GULICK: So this is bigger than
any tectonic-generated earthquake.

You would have shaking
literally everywhere on the planet.

ATTENBOROUGH: So,
their hypothesis suggests

seismic waves from the impact

could have caused surges of water

in the Hell Creek river system.

RICHARDS: Seismic waves
get here quickly enough

to cause this wall of water,

coming up the Tanis river,
inundating this area,

arriving at the same time these spherules
are still falling out of the air.

ATTENBOROUGH:
If they're right,

seismic waves travelling through the Earth

could have caused a powerful
surge of water at Tanis,

at the same time as spherules fell.

And ultimately, dumping it
on the Tanis sandbank,

burying everything in the churned up mud.

Debris and fine iridium dust
from the asteroid

would have gradually covered the deposit,

forming the K-Pg boundary.

Over millions of years,

the surge of mud would become the
four-foot-deep layer of crumbly rock.

RICHARDS: And that's
the beauty of Tanis.

What you're seeing is the deposit

that is literally recording the last,

say, forty-five minutes to an
hour-and-a-half of the Cretaceous.

ATTENBOROUGH: If the extinction
of the dinosaurs was a crime,

the detective solving it
would have plenty of evidence.

They would see that the asteroid
was in the right place at the right time.

They would see that no dinosaurs
survived after the hit.

They would have a piece
of the murder weapon,

a fragment of the asteroid,

but they would be missing
one very important thing,

a body.

A lot of the bones that exist
from those last Cretaceous days

were basically destroyed.

BRUSATTE: As far as we know,

we've never actually
found a fossil of a dinosaur...

individual, a single skeleton let's say,

that we can unequivocally say
was there on the day the asteroid hit.

ATTENBOROUGH: But before the site
was timestamped to the Chicxulub impact,

Robert's team did find
part of a Triceratops

in the crumbly layer at Tanis.

So could that be the body?

A dinosaur that died on that day.

Something that would help them
would be establishing the cause of death,

which can be difficult when you only
have a piece of skin and horn to go on.

This is the horn
after they've cleaned it up.

The team is particularly
interested in these lines here.

And they found that the
fractures go right through the horn.

So rather than dying
as a result of the impact,

they wondered whether
it had been killed in a fight.

[GROWLING]

ATTENBOROUGH: But when they
looked at the fractures in more detail,

they found signs of new bone growth here.

An indication that the bone
had started to heal.

So it looked as though the Triceratops
survived the event that broke its horn.

[SNORTING]

[GRUNTS]

Could this Triceratops have survived

until the day of the impact?

This drooping in the skin and the
disarticulation of some of the bones

suggested to the team
that there was decay underneath.

That means its body had started to rot

before it was entombed
and preserved by the surge.

So, it seems that this dinosaur didn't die

as a result of the asteroid impact.

[SNORTING]

ATTENBOROUGH: Given the signs
of partial decay,

it's likely this Triceratops wouldn't have
lived to see the last day of the dinosaurs.

[THUNDERCLAP]

However, the Triceratops fossil does show

that dinosaurs were alive
shortly before the asteroid hit.

Perhaps even within weeks of the impact.

This is an extraordinary discovery,

and one that has never been found before.

But if it's true that dinosaurs were here

until the final weeks before the impact,

there could be even more
still to find in this deposit.

DEPALMA: Looking down
on to the sight of a dinosaur

that died weeks to months
before the impact,

that is such a cool thing.

We've got all these bones
in the ground right now,

but the one thing that we would
just dream of finding

is that one dinosaur that died
on the day of the impact.

ATTENBOROUGH: And the weather
isn't helping his search.

God...

Oh!

DEPALMA: That therapod print
is toasted.

DAVID BURNHAM: Yeah,
it was in a low corner.

DEPALMA: Look, it's full
of mud and water.

The problem is it's wet, look.

See? If we're not careful,
we're gonna lose the print.

And that's the biggest
theropod print we've got.

I see some areas that
could use glue right now too.

ATTENBOROUGH: The team is
racing to excavate dozens of fossils,

before the rains wash them away.

DEPALMA: We're up
against the clock here.

This stuff that could be exposed right
now is gonna get ruined by the rain.

ATTENBOROUGH: But then,
the team comes across something

that looks very unusual.

DEPALMA: Look at that.

BURNHAM: What is going on
right there?

LOREN GURCHE: Are we sure
this isn't crocodilian?

-That's not crocodilian.
-BURNHAM: No.

GURCHE: Let me try this piece
right here.

I'll go in from the top and then twist up,

and it separates right on that line.

DEPALMA: Oh,
that's skin right there.

That's actually scaly skin.

No, no, no, no, no.
Look, look, look.

Look at that pattern right there.

Have you ever seen elongated
scales like that before, Dave?

BURNHAM: Scutellates in birds.

DEPALMA: Just, careful.

-Oh, my God.
-BURNHAM: It's changing again.

-DEPALMA: It's changing again.
-Oh, my God.

DEPALMA: We're seeing it for
the first time in 66 million years.

I think we got ourselves a dinosaur.

ATTENBOROUGH:
A dinosaur fossil.

In the same mass death assemblage

as the fish with the spherules
in their gills.

DEPALMA: This is the most incredible
thing that we could possibly imagine here,

the best-case scenario.

We're excavating
this mass death layer of fish

from the surge sent up by the impact,

and we've got dinosaur remains.

The one thing

that we would always want
to find at this site,

and here we've got it.

This is unreal, I, I, I cannot
process this in my brain.

No, I am absolutely blown away by this.

Just, my heart is literally
pumping outta my chest,

wondering what is behind there, just a
couple of centimetres back in the outcrop.

What is waiting for us back there?

GURCHE: This is amazing.

ATTENBOROUGH: The team
keeps digging.

BURNHAM: This could be a ribcage. It could
be laying against ribs that are curved.

DEPALMA: There's something here.
That's hard.

That's bone right next to the skin.

That's an articular surface right there, so
this is either a hip or a shoulder element.

ATTENBOROUGH: After hours
of painstaking work...

DEPALMA: And we can go
from the thigh of the animal...

There's the knee.

And then you've got the

little calf muscles of the
dinosaur over there bulging out,

and you go down to the anklebones,

and these are the toes of the feet.

We have got nails at the tips of the toes.

It's a beautifully preserved leg,
all articulated, covered with skin.

ATTENBOROUGH: The complete
leg of a dinosaur.

DEPALMA: In my wildest dreams,

I never expected to find
a dinosaur leg in this deposit.

Yeah. I mean, and then
it's got skin and tissue.

It does look just like a drumstick.

DEPALMA: It looks like a Thanksgiving
turkey, just laid out in the ground.

ATTENBOROUGH: Robert
and his team

think they have found the body
missing from the crime scene.

A dinosaur that might itself have
witnessed the cataclysmic impact.

DEPALMA: Dinosaur fossils are not known
from the last years of the Cretaceous.

And it was unclear whether
they were already extinct,

or in decline, or what was going on.

So they were just sort of absent.

And this answers that question,

were dinosaurs still there then?

Well, yes, they were there
weeks to months before the impact.

This one likely died in that surge.

ATTENBOROUGH: But such
big claims need verification.

Robert is in the process

of sharing the team's finds with
the wider scientific community.

As part of this process,

he has brought the dinosaur
leg to London, England,

to get a second opinion from
palaeontologist Paul Barrett,

an expert in ornithischian dinosaurs,

from the Natural History Museum.

So what do you think this might be?

When we look at the leg, it has claws.

Like the claws we see
in small, agile, bipedal,

running dinosaurs that are plant eaters.

We can also rule out things
like Triceratops,

partly just because
it's not big and stocky.

And the proportions
of those legs are also different

from some of the other
plant eaters we see,

in that they have this
rather long ankle and shin,

compared with its thigh bone.

So as we narrow those possibilities down,

what we're left with probably is
an animal called a Thescelosaur.

[GRUNTS]

ATTENBOROUGH: Thescelosaurus
are thought to have lived next to rivers.

They had leaf-shaped teeth,
common amongst herbivores.

And claws on their short front limbs,

which they may have used for digging.

At the front of their mouth, they
had specialised, pointed teeth

that could help them to
pull roots out of the ground.

So it's possible they dug for food.

But how did the Thescelosaur
that Robert's team found die?

Could it have been killed
by another dinosaur?

BARRETT: It's a possibility.

This is a relatively agile animal.

And that turn of speed
would've been its primary defence

against the large predators
living alongside it.

DAVID MARTILL: Whenever
we're excavating a dinosaur,

one of the things that
we're always keen to know

is how did the animal die?

It's not always easy to do that.

So maybe we can find evidence for things
like broken bones that didn't heal back up.

Sometimes, we can even see things

like bone tumours and gout.

CHINSAMY-TURAN: There are some wonderful
fossils where you can find bite marks on them,

you can even find a predator
tooth buried within the bones.

BAMFORTH: In science, we don't prove
things. We just disprove some things.

Generally speaking, unless

a cause of death leaves a signature
on the skeleton, it's hard to tell.

ATTENBOROUGH: CT scans Robert and
the team have taken of the dinosaur leg

allow a closer look at what the animal
might have gone through before it died.

It doesn't seem to me like there is any
evidence that this animal was predated.

None of the obvious tooth marks

or left-over bits of carnivore
teeth to suggest it's been eaten.

So we can see that the bones look okay.

So this was an animal that was
probably living and healthy

at the time that this happened to it.

ATTENBOROUGH: Could this be a
dinosaur that was swept up in the surge?

BRUSATTE: The idea that
there is a dinosaur fossil,

potentially,
that's a direct victim of that,

that's very exciting.

I think, ultimately, it comes
down to a couple of things.

You know, are there injuries on that
fossil that show that this dinosaur was

bobbing about, heaving about in the water?

So are there things
like breaks on the bone,

or other things that have not healed?

BARRETT: This is actually
a shoulder bone,

and this bone in the living animal
would actually be way over here.

And similarly, this little bone here

would have been from about,
maybe, a third of the way

along the tail, maybe halfway down.

So somehow these two bones
have been telescoped together.

So maybe this animal
has been tumbled around.

Could this be a victim
of the meteor strike?

I think it's entirely possible.

We've ruled out a lot of other
possible causes of death for this animal,

so it could well be that this
was an animal that was there,

being tumbled around in
its death throes in that river,

as a result of the asteroid impact.

Palaeontologists do depend
a lot on tragedy.

Every little disaster

is the material we need
to actually develop our subject.

Tragic for the individual concerned,

but we're just really happy
that it happened.

ATTENBOROUGH: After years
of work at this site,

Robert and his team have
uncovered unprecedented detail

about the animals living there.

And he thinks that many of them
were alive on that fateful day

when the asteroid devastated our planet.

But how exactly did they die?

The team's finds give us new
clues to answer that question.

One of the most important days
in Earth's history

probably started much like
any other late spring morning.

We think it was late spring,
because palaeobotanists

have found key evidence about
the season from fossilised flowers.

The Tanis finds are consistent with this,

including the fossils of young fish

that died at the size they
reach at that time of year.

Perhaps this day, that would
end with so much death,

began with something different.

A new life.

[SQUAWKING]

ATTENBOROUGH: No one can be
certain of the exact timings of the day

when the asteroid
collided with our planet.

But it's estimated that within
just 40 minutes of the impact,

the consequences
for the creatures of Tanis

would have been profound.

Based on the team's finds,

and the latest evidence
from other scientists,

this is how the catastrophe
might have unfolded.

The asteroid is around seven miles
across, bigger than Mount Everest.

And traveling at close
to 45,000 miles an hour.

The impact causes an explosion

with over a billion times the power...

-[EXPLOSION]
-... of the first atomic bomb.

GULICK: It comes in so fast

that it wouldn't even have been visible

passing through the atmosphere.

Right, it would have just come
and hit in a moment.

ATTENBOROUGH: At Tanis,
almost 2,000 miles away,

there might have been
an initial flash of light,

yet it is completely silent.

But at the impact site...

the asteroid vaporises.

More than three-trillion tons
of rock are ejected into space

in a blast of superheated violence.

[WIND GUSTING]

Winds higher than 600 miles an hour.

A colossal earthquake,

followed by a ring of massive tsunamis.

All the while, the creatures at Tanis

go about their business.

Just like any other day.

[CHITTERS]

[SNORTS]

The evidence suggests that baby pterosaurs

may have emerged from eggs
ready to fend for themselves.

And that includes...

flying.

Well, almost.

Elsewhere, as the
reverberations of the impact

race out across North America...

dinosaurs and creatures of all shapes
and sizes are obliterated by the blast.

Incinerated in a firestorm
unlike anything seen since.

If I were a dinosaur standing
on the coast of North America,

I would just see a flash and a fireball
coming at me, and then I would be fried.

All you feel is an awfully
sharp, stabbing pain in your ears,

then you explode.

ATTENBOROUGH: At Tanis,
for a few more precious minutes,

life continues.

But the clock is ticking.

[GROWLS]

The blast from the impact
never reaches Tanis,

but seismic shock waves do.

[RUMBLING]

They are far more powerful

than any earthquake ever recorded.

PLESKO: If you were standing

on the Gulf Coast of Texas,

that magnitude 12 earthquake
would have been strong enough

to actually jam your femurs
up into your body cavity.

ATTENBOROUGH:
While the earthquake

that reached Tanis was likely
less destructive,

the effects would have been felt

by all that lived there.

Seismic waves are now slowly
shaking the whole region,

causing water to slosh and churn.

At Tanis, these strange
currents in the river

may be some of the first signs
of what is coming.

Next, it begins to rain.

Ejecta spherules are falling
back to Earth.

As the spherules plummet,

friction heats them
until they are red hot.

They soon transfer their heat
to the atmosphere...

which grows hotter by the second.

As the searing heat builds, the creatures
of Tanis are fighting for their lives.

And then, as seismic
waves rock the whole region...

a violent surge wave

30 feet high rushes up the Tanis River.

Surviving the turbulence of the surge is
a challenge even for the best swimmers.

Then, the slow but powerful rocking of
the river system draws the water back.

A large, robust animal
like a T. rex

might have survived the surge.

As might a hard-shelled reptile.

But there is much more to come...

As billions of tons of superheated
spherules continue to fall,

the atmosphere gets even hotter...

igniting dead leaves
and sparking wildfires...

earthquakes,

fires,

devastation.

Little would survive for long,

on land...

[BELLOWING]

...or in the air

[YELPS]

Those that live underground
may have had a better chance.

As the slow rocking
of the river system continues

to move the water to and fro,

another powerful surge hits the riverbank.

[RUMBLING]

For most, there is no escaping
the destruction.

For many of the creatures here,

their stories end underwater.

In less than two hours,

the world has changed forever.

The mud the two waves
leave behind will gradually turn

into the thick layer of crumbly rock,

entombing the creatures which died here.

Until 66 million years later,

when they are finally unearthed.

We have a general idea of what
horrors were unleashed

on the landscape by the asteroid impact.

But I think these sites may give us

the ability to actually
put them in sequence

and understand exactly what
these organisms went through.

Even though there is a lot of debate,

and there is a lot of controversy,

every new thing that we find, every
new hypothesis that's put forward,

whether it's accepted or rejected,

gets us a little bit closer
to doing that mental time travel

and imagining ourselves back
in that Cretaceous world.

[FIRE CRACKLING]

ATTENBOROUGH: Robert's finds
have helped us understand

in remarkable detail
what happened at Tanis

in the minutes after the asteroid impact.

But what about the rest of the world?

Fires rage, destroying
many of the world's forests.

As the horrific day draws to a close,

many of the world's dinosaurs
are likely already dead.

Research shows that the angle
at which the asteroid hit,

and the sulphur-rich rocks
at the impact site,

amplified the devastation.

Without sunlight, most plants
died, and food became scarce.

As the weeks and months passed,

any dinosaur left alive
would have died of hunger.

In the oceans, it was the same.

Nearly all of the world's plankton died,

leading to the starvation
of most marine creatures.

It's thought that the
impact winter that followed

caused a global temperature drop

of at least 48 degrees Fahrenheit.

After this huge change in climate,

the fossil record tells us that
three-quarters of all species,

including the dinosaurs, were wiped out.

PLESKO: The location
of the Chicxulub impact

really was a worst-case scenario.

If the asteroid had actually
come in, you know,

30 seconds earlier, 30 seconds later,

it would have actually hit the
Atlantic Ocean or the Pacific Ocean,

and not the sediment-rich, sulphur-rich
target of the Yucatan Peninsula.

Forests collapsed.

The plant-eaters didn't have
any food to eat. They died.

The meat-eaters didn't have
any plant-eaters to eat.

They died. Ecosystems collapsed
like houses of cards.

BLACK:
This unintentional accident

that just was set in motion

long before dinosaurs even existed.

And it just happened to be
the one case of bad luck,

the one worst day
in the history of the planet.

ATTENBOROUGH: Studies suggest
that the planet was in semi-darkness

for around a decade, as dust
and soot slowly fell to Earth.

But then came something wonderful,

a new beginning.

Once the dust cleared from the atmosphere

and the sunlight returned,

plant life was gradually restored,

led by ferns,

the spores of which had lain
dormant deep underground.

And the world began
to turn green once more.

But what about the animals?

One of the reasons some
mammals survived the great extinction

were burrows.

During the impact winter,

a burrow would have provided warmth,

protection and a place to store food.

Mammals which were able
to thrive in the aftermath

were resourceful omnivores.

And the insects which survived

could have been one source of food.

Their size would have been
another advantage.

When catastrophe strikes
and food is scarce,

the largest tend to die out,

whilst the smallest often survive.

And they weren't alone...

The turtle found at the dig
site may have been unlucky,

but many others survived.

As did crocodiles, snakes,

and many fish species.

Life has found a way,
and life is now thriving again,

and it is those ecosystems

formed in the recovery from the asteroid

that are the foundations
of our ecosystems today.

It's kind of amazing that we're able to put
our finger on this one line in the rock.

And as much as we miss the dinosaurs,

say, if this hadn't happened,
we wouldn't be here.

ATTENBOROUGH:
And as for the dinosaurs...

Did the impact really kill them all?

Well, this beautiful fossilised
feather isn't from a bird,

but from a predatory dinosaur.

So, we have to be careful when
we say that dinosaurs are extinct,

because what we call birds,

originally evolved from the
smallest feathered dinosaurs.

So, to be correct, we should say
all non-avian dinosaurs are extinct.

The finds from Robert and his team
have given us a more detailed picture

about what might have happened
on the day that destroyed

the largest beasts ever to walk the Earth.

Dinosaurs were perhaps some of
nature's most extraordinary creatures,

dominating the planet
for over 150 million years

before they became extinct.

But extinction comes in different forms,

and many of the amazing creatures and
plants alive today are also threatened.

It's possible that humanity is
having as big an impact on the world

as the asteroid that ended
the age of the dinosaurs.

As human beings, we are unique in our
ability to learn from the distant past.

The question is, will we use
that ability wisely,

and do our very best to
protect the millions of species

for whom, alongside us,
this planet is home?