♪ ♪ (creatures croaking) ♪ ♪ DAVID ATTENBOROUGH: 66 million years ago, planet Earth was very different from today.

♪ ♪ Some of our ancestors at the time might have looked like this furry creature.

♪ ♪ (animal chittering) (dinosaur grunting) The rulers of the land were giant reptiles.

♪ ♪ (animal chittering, dinosaur grunting) (footsteps thudding) ♪ ♪ Dinosaurs.

That's one of the most infamous, a carnivorous T. rex.

And just behind are the bison of their time, a common plant eater, Edmontosaurus.

But what happened to them all?

66 million years ago, an asteroid hit the Earth, and scientists think that it was this collision that wiped out the dinosaurs.

But no one has ever found the fossil of a dinosaur that they know for certain died as a result of the impact.

(dinosaur roaring) ♪ ♪ However, a truly extraordinary dig site might change that.

Hell Creek Formation, North Dakota.

♪ ♪ These sedimentary rocks are rich in dinosaur remains.

(roaring) From triceratops... (growling) ...to T. rex.

Now, in a patch of land no bigger than two football fields, a long-buried secret is coming to light.

♪ ♪ Because this place might hold evidence of one of the most dramatic events in all the four- and-a-half-billion-year history of our planet.

♪ ♪ RILEY BLACK: Everything was fine on Tuesday in the Cretaceous, and the next second, the world just wasn't the same.

STEVE BRUSATTE: Any time that an asteroid the size of Mount Everest smashes into the Earth, that's not going to be a good day.

EMILY BAMFORTH: It's actually pretty remarkable that anything survived.

♪ ♪ ROBERT DEPALMA: Let me get down here between you.

ATTENBOROUGH: For almost ten years, a team of scientists has been trying to find out exactly what happened here.

♪ ♪ DEPALMA: You're at the edge of your seat every moment trying to dig this stuff up.

ATTENBOROUGH: They call the site Tanis, after an ancient Egyptian city, and believe it could be a mass graveyard of creatures which were killed in the asteroid strike 66 million years ago.

♪ ♪ This site might reveal the remarkable story not just of how the dinosaurs lived, but how they died.

♪ ♪ The impact really was a worst-case scenario.

DAVID UNWIN: It's almost beyond what we can imagine.

ATTENBOROUGH: If the dig team is right, Tanis could be a place where the remains of a long-lost world are frozen in time.

♪ ♪ A place that gives us, for the first time, an unprecedented window... (dinosaur screeching) into the lives of the very last dinosaurs.

♪ ♪ And a minute-by-minute picture of what happened on the day the asteroid hit.

♪ ♪ "Dinosaur Apocalypse: The New Evidence"-- right now on "NOVA."

♪ ♪ ♪ ♪ ATTENBOROUGH: This landscape is full of fossils dating from the Late Cretaceous, the period which began around 100 million years ago and ended 66 million years ago, when the dinosaurs vanished.

Paleontologist Robert DePalma wants to find out more.

♪ ♪ I think anybody who has ever liked dinosaurs in the past, or still does, has thought at one point or another, well, what happened to them?

Why are they not here anymore?

♪ ♪ ♪ ♪ ATTENBOROUGH: At the end of the Late Cretaceous, fossil evidence tells us Hell Creek might have looked like this.

(creature croaking) There were low-lying marshy flood plains, intercut by river channels, and covered with horsetails, ferns, and trees.

Back then it was warm and wet here all year round.

(creature bellowing in distance) ♪ ♪ STEVE BRUSATTE: If we go back to about 66 million years ago, the Earth in some ways was very similar to today, and in other ways it was an alien world.

The climate was very different, the temperature was different.

There were no ice caps at the poles.

ATTENBOROUGH: Hell Creek is one of the most famous and well-studied areas for digging up dinosaurs.

BRUSATTE: Hell Creek is really the only place in the world, at least right now, where we have a really good record of the last surviving dinosaurs.

BAMFORTH: Hell Creek records the very, very last days of the dinosaurs, and it's, it's the best information that we have in the world about that extinction event.

♪ ♪ ATTENBOROUGH: This dig site lies in the northeastern corner of the Hell Creek Formation.

66 million years ago, instead of today's dusty prairies, there were sandy, silty riverbanks.

Instead of rocky cliffs, there were forests.

♪ ♪ And instead of the wildlife we know today... (creatures bellowing) Well, scientists are trying to find out what that was like.

BAMFORTH: One of the great things about paleontology is also one of its most frustrating elements, and that is that you can never be sure.

Until somebody builds a time machine and we can go back in time and actually observe dinosaurs in their natural environment, we will never know for sure whether our inferences are correct or not.

So it'll always be open to a bit of interpretation and uncertainty because fundamentally trying to pinpoint something that happened on a given day 66 million years ago is really, really tough.

BAMFORTH: And so a lot of paleontology is putting forth theories and then other paleontologists coming forward and saying, "No, that doesn't make sense because here's another theory."

ATTENBOROUGH: Every paleontologist can only hope their site might uncover something new to debate.

♪ ♪ A sand bank lying between a river and a forest would one day become what Robert now calls Tanis.

♪ ♪ The site had been explored by others in the past.

(wind whipping) But it wasn't until after Robert and his team started digging here in 2012... DEPALMA: So somewhere from between there and down here is where that came from, it's coming from up above.

Hey, look at this.

What?

ATTENBOROUGH: That anyone would know how important this site could be... DEPALMA: Here we've got this freshwater environment of the Hell Creek formation, and this shocking red, green colors coming from the shells of ammonites, a marine organism, kind of like a coiled snail in appearance.

So we've got this marine organism that's been thrown up into this freshwater environment and they do not belong here.

ATTENBOROUGH: How they got there is a mystery, but even more intriguing.

DEPALMA: I'm just gonna go ahead and plane down some of this rock.

ATTENBOROUGH: Sitting above the ammonite shells is something that holds a crucial clue about the age of these rocks.

So this orange layer right here is composed 100% of impact-related debris.

It is enriched in iridium.

ATTENBOROUGH: Iridium is an element that's rare in the Earth's crust, but it's common in asteroids.

The layer it's in marks the K-Pg boundary.

The boundary is made up of dust and debris from a huge asteroid impact.

It's been dated to 66 million years ago, the time when dinosaurs disappeared.

Look at that.

That's what you need?

Yeah, yeah, yeah, yeah, yeah, that's what we want.

Okay, so it's coming from this area here.

So somewhere within that region is where these pieces are coming from.

ATTENBOROUGH: And it has been found all over the world.

SONIA TIKOO: In this layer, the concentration of iridium is a hundred times higher than the baseline for the rest of the Earth's crust.

So, perhaps the simplest answer to that is that it came from outer space.

SEAN GULICK: And so we have this wonderful marker that is the iridium layer, that coincides with the extinction event.

So this is one of those few cases where you can really tie what is often a fuzzy thing and kind of bring it into focus because you have this moment in time represented by that layer.

♪ ♪ ATTENBOROUGH: Having the K-Pg boundary here at Tanis dates the site to around the time dinosaurs went extinct.

No rattlesnakes.

BRUSATTE: Once you see that layer, once you identify it, it really does stand out because it is a thin layer of rock that caps one world, the world of dinosaurs.

And it ushers in another world, a world where you never find a single dinosaur bone or tooth or footprint again.

♪ ♪ ATTENBOROUGH: What makes this site even more exciting is the rock layer right beneath the boundary, in which Robert and his team found the ammonites.

The rock here is really not quite rocky, and it just falls apart in your hands.

ATTENBOROUGH: This crumbly rock isn't unique, especially in Hell Creek.

But it's rarely found in layers like this one.

♪ ♪ Over four feet thick, this layer contains several geological features which, to an expert, signify that it was deposited very rapidly.

As in a storm or a flood, burying anything within it in an instant.

(rocks tumbling, water gurgling) ATTENBOROUGH: Which could mean that anything in this layer would have been quickly entombed, like the bodies in the volcanic ash of Pompeii.

BAMFORTH: Generally speaking, the faster you get buried after you die, or even if the burial is what actually kills the animal, that's one of the best scenarios for fossilization.

(creature grunting in distance) ATTENBOROUGH: Robert knows from the geology that anything he finds could be so well preserved that it could reveal new evidence that will bring this time period to life in a way no one has ever done before.

BLACK: When you think about it for a second, it's actually incredibly amazing that we have any fossils at all, much less a fossil record.

So 99.9% the animals that we have don't get preserved as fossils, because you have scavengers and you have other animals that tear away the skeleton as it's being deposited.

To become a fossil, you need certain conditions for fossils to form.

(dinosaurs grunting) And so a lot of the fossil record is really missing.

ATTENBOROUGH: So, for fossil hunters, this site is particularly interesting.

Such rapidly deposited sediment so close to the K-Pg boundary could be evidence that what happened to the last dinosaurs here was as swift as it was destructive.

Yet the story of that devastating day begins long before...

Millions of miles away and billions of years earlier.

♪ ♪ Most scientists think it all started in a ring of dust, rocks, and debris known as the asteroid belt.

♪ ♪ It's usually an uneventful place.

♪ ♪ But sometimes, a rock can get bumped into a new orbit.

♪ ♪ And diverted onto a collision course with planet Earth.

♪ ♪ TIKOO: Jupiter, in particular, is a big bully in our solar system, because it's the largest planet, it has the most gravity.

And it doesn't just take one orbital pass for an asteroid to be influenced.

This is a slow build-up over tens of millions of years, interacting with Jupiter over and over and over in its orbit.

Another thing that can change asteroid orbits is collisions within the asteroid belt.

And what happens is, over time, the asteroid's orbit can be nudged until it becomes a near-Earth orbiting asteroid.

And it has to be pretty bad luck for both the asteroid and the Earth to be in the same place at the same time.

But it does occasionally happen.

♪ ♪ ATTENBOROUGH: Robert and his team dig at this site in North Dakota each summer, the only time the weather allows them to do so.

Come on down, check out this lens over here.

In order to understand how the impact affected life on Earth, you really need to get a very clear picture of what the world was like right before.

That is a critical part of the story.

♪ ♪ ATTENBOROUGH: Paleontologist Dr. David Burnham and Loren Gurche have been digging with Robert for years.

♪ ♪ (chuckles) Oh, wow.

See, see the brown?

Yeah.

That might be a tubercle right there.

ATTENBOROUGH: And it seems today is their lucky day.

Oh, my God, look at that!

Look at that!

Look, the scales are preserved.

They're, like, doing a freaking dissection.

Oh, my God...

Biology of Tanis.

Oh, the scale-- look, look the wrinkles continue down that way.

BURNHAM: Mine's all nice and wet so far.

The scales are getting smaller in that direction.

How big are they there?

I got a, I got a one with the, the projection over here.

What?

Oh!

Yeah.

DEPALMA: Yeah, there's the protuberance right there.

BURNHAM: I've only seen that on one other specimen in my life.

Yeah.

This is the closest thing to getting to touch a living, breathing dinosaur.

It is.

ATTENBOROUGH: They've found something extraordinary-- Dinosaur skin.

And they've uncovered it right next to another fossil.

This is obviously horn.

The gnarliest horn I've ever seen.

ATTENBOROUGH: Which helps them piece together the creature they're from, a triceratops.

DEPALMA: It is so exceedingly rare, a piece of triceratops skin in the Hell Creek formation.

♪ ♪ ATTENBOROUGH: The skin that they have found may look like an impression in the rock, but this is skin that has been fossilized, and over millions of years has turned to stone.

♪ ♪ Triceratops bones are relatively common finds in Hell Creek, but skin in such condition as this is very rare indeed.

The size and the patterning of the scales together with the age and location of the rocks where it was found, strongly suggests that this is from a triceratops.

The presence of the horn where the skin was found supports this.

The brown color contains traces of organic material.

So it might even be possible from this to work out which pigments were in it.

Finding and studying such well-preserved fossils as this helps paleontologists build a much more detailed picture of how these creatures lived.

Combining this information with insights from scientists around the world makes it possible to speculate about what life in the Late Cretaceous might have been like.

♪ ♪ (dinosaur bellowing, footsteps crunching) We know from bones that adult triceratops could reach 30 feet in length and ten feet in height.

(dinosaur grunting) ♪ ♪ Marks on the fossil also show us that this one was badly scared.

(dinosaur grunts) ♪ ♪ Triceratops were plant eaters.

(leaves rustling) (dinosaur grunting) Other fossils tell us they had sharp beaks and hundreds of teeth, which enabled them to shred hundreds of pounds of tough vegetation.

(dinosaur growls) Almost all adult triceratops fossils ever found were on their own.

So it's possible that the adults were solitary, a pattern observed in many modern-day animals.

(dinosaur grunting) If you look at American bison, for example, they herd through much of their youth and much of their young adulthood, but especially old males will be by themselves.

So that's not to say that all the triceratops you find by themselves are these old bulls, but there might be something similar at play.

So they were probably territorial, fighting rivals away.

(both growling) These were very large animals that probably had very large territorial ranges.

CAMERON MUSKELLEY: There actually is fossil evidence of puncture wounds in the frills of these dinosaurs, but they were probably using their horns just like modern caribou, where they lock their horns together to compete for mates and other territorial places.

ATTENBOROUGH: A solitary animal would perhaps mark its territory.

♪ ♪ (spraying) If you weigh more than an African elephant, there's not much that can bother you.

(animal chittering, dinosaur grunting) Except perhaps a little mammal.

♪ ♪ (dinosaur roaring) ♪ ♪ (animals chittering, squeaking) ATTENBOROUGH: Robert found these jawbones in the fossilized burrow.

The shape of this tiny bone and tooth means it's most likely come from what's known as a pediomyid, an early mammal... and a type of marsupial.

The team also discovered fossilized nuts and seeds in the burrow, so we have an idea of what it might have eaten.

(animal bellowing in distance) BRUSATTE: We think of mammals oftentimes as the new kids on the block...

But what we often don't appreciate is that mammals and dinosaurs, their legacies go back to the same time.

ANUSUYA CHINSAMY-TURAN: Some of them, we think, may have been opportunistic because there's even evidence of a small mammal that actually has the remains of a baby dinosaur within its belly.

(animals grunting in distance) ATTENBOROUGH: The team's finds are adding to our knowledge of the complex world at the very end of the Late Cretaceous.

And it's not just the fossilized creatures.

If you walk on damp sand, you'll leave a trace behind.

A footprint.

The same was true 66 million years ago.

And very, very occasionally, such traces were preserved.

GURCHE: We wont to foil the backside.

Right.

We'll just put the plaster right on.

ATTENBOROUGH: The dig team has discovered a number of footprints.

DEPALMA: Yeah, let's see.

Looks like a good print.

Yeah.

ATTENBOROUGH: Their shape gives them an idea of what might have made them.

♪ ♪ If the team is right, they were made by a winged creature that might well have liked a small mammal... for lunch.

(animal chittering, beak thudding) (dinosaur grunting, animal chittering) The footprints are long and narrow with four toe prints.

Two are slightly longer than the others.

And that suggests they were made by... A pterosaur.

♪ ♪ (pterosaur bellows) Pterosaurs are not dinosaurs, but flying reptiles on a different branch of the evolutionary tree.

♪ ♪ UNWIN: There is nothing like a flying reptile around today.

Pterosaurs got to enormous sizes.

A group of pterosaurs known as azhdarchids, which include the pterosaur known as Quetzalcoatlus, is a pterosaur that grew up to around 40 feet.

This was an animal that had a 40-foot-long wingspan.

♪ ♪ ATTENBOROUGH: Some evidence shows that some pterosaurs might have lived in large groups, much as flamingos do today.

(pterosaurs bellowing) ATTENBOROUGH: Male pterosaurs usually had crests, while females didn't.

So, crests may have been used in courtship displays.

♪ ♪ (beaks clattering) (pterosaur bellows) And we have a clue about where females laid their eggs because evidence suggests that at least one pterosaur laid hers in the soft, sandy banks of the river at Tanis.

♪ ♪ ♪ ♪ UNWIN: The fossil record of pterosaur eggs is really small.

Um, so far we have a, a couple of eggs from northeastern China, and we also have an extraordinary trove of eggs from western China, from Xinjiang Province.

The only other record of eggs is a single egg that comes from Argentina.

So our record is very, very small indeed.

ATTENBOROUGH: This is the fossilized egg of a pterosaur that Robert and his team found in the crumbly layer, the only one ever discovered in North America.

If you look at it with the naked eye, all you see is a jumble of lines.

But if you examine it with the latest technology, you can find out a wealth of information from the chemistry of the bones, to the composition of the shell.

And that, in turn, can tell us a lot about how these incredible creatures lived.

To investigate the pterosaur egg, Robert has been given access to the Diamond Light Source synchrotron.

Situated in Oxfordshire, in the U.K., it's a powerful research tool that acts like a giant microscope.

♪ ♪ ATTENBOROUGH: By accelerating electrons in this huge ring, the synchrotron creates beams of light billions of times brighter than the sun.

♪ ♪ Robert and paleobiologist Dr. Victoria Egerton now want to turn that beam onto the egg fossil to discover more about its chemical makeup.

♪ ♪ We're pretty much lined up on the skeleton, but we might have to move the stage a little bit to get to the right part.

Sure.

ATTENBOROUGH: Each synchrotron scan can take several hours.

Meanwhile, Robert can reveal the creature inside.

Who made this wonderful thing?

DEPALMA: I got replicas of the bones from inside that egg and I restored the remainder and put together what the skeleton would've looked like when it hatched.

That's how big the creature would've been outside the egg, if it had hatched.

So this is the baby.

How big was it gonna grow?

These very long neck vertebrae here are what really gave part of the story away to us, because those long bones match very, very closely with the azhdarchoid pterosaurs.

That is the giant pterosaurs.

Oh, they were the whoppers, weren't they?

I mean, what, 25 feet wingspan?

Some of them.

This probably had a wingspan, maybe 15 feet?

Well, it looks as though it could take off, really.

It's easy to picture something like that just hatching out of the egg and fluttering out almost like a little bat.

DAVID MARTILL: A lot of birds are utterly dependent on the parents bringing them food for a long time.

But there are precocious birds, and there are some that simply stand up after a few minutes and start foraging for food themselves.

Well, pterosaurs might have taken that a stage further, and they simply flew away.

♪ ♪ ATTENBOROUGH: They've scanned the egg here and in America... Victoria has the results.

♪ ♪ So what have you learned from the synchrotron image?

What we have here is a chemical map of calcium directly within the bones of this animal.

That tells us that these bones were already hardened.

So it might be ready to fly not long after it hatches.

Can you see any sign of the shell and what sort of shell was it?

We can.

What I can show you... Ah.

We can see the rim of the egg in sulphur.

Does that tell you whether it was a hard shell or a soft shell?

We have been looking at this.

We can see folding occurring and this unusual undulation.

If it were a hard egg, we would expect splintered bits and broken bits, just like a chicken egg.

This helped to tell us that it was soft.

So it was perhaps like a turtle?

Absolutely.

That's not the case, is it, with dinosaurs?

Many dinosaurs laid hard-shelled eggs.

Yes.

So this is a new discovery about azhdarchoid pterosaurs?

Absolutely.

This is something that we are confirming for the first time.

Huh.

Some flying pterosaurs had eggs like turtles.

Yes.

Much more reptilian-like than bird-like.

And that can potentially tell us more about the environment in which these eggs were laid.

How interesting, yeah.

♪ ♪ Creatures that lay soft eggs tend to bury them in order to protect them.

♪ ♪ (pterosaur bellowing) So female pterosaurs probably looked for places like this to lay their eggs.

Because the sandy soil here is just soft enough for the hatchling to dig itself out.

♪ ♪ Now, the pterosaur just has to make sure that the hole... is perfect.

(animal chittering) ♪ ♪ (pterosaur bellows) ♪ ♪ (pterosaur grunting) ♪ ♪ (pterosaur grunts) ATTENBOROUGH: Success.

But it's not over yet.

Pterosaurs had two ovaries (pterosaur grunts) and they laid their eggs in pairs.

(pterosaur bellows) ♪ ♪ UNWIN: So clearly, this method, this way of reproducing for pterosaurs was incredibly successful.

What it kind of says is, "Hey, everything's normal" until the moment when the impact happens and it all goes horribly wrong, basically.

♪ ♪ ATTENBOROUGH: Here on the sand bank, sandwiched between the river and these glorious trees, life at Tanis seemed to be thriving.

(gasps) Whoops.

Never a dull moment.

But all that was about to change.

♪ ♪ Deep in space, a countdown clock is ticking.

♪ ♪ The asteroid's journey would take it through the orbit of our neighboring planet, Mars.

♪ ♪ Had the two collided, a catastrophe on Earth would have been avoided.

♪ ♪ But it didn't happen and the fate of life on Earth was sealed.

♪ ♪ New evidence is helping to build a vivid picture of Late Cretaceous life, here in this corner of North Dakota.

And the team have found some more well-preserved footprints.

♪ ♪ BURNHAM: So these are animals that were actually walking in the water?

DEPALMA: These guys would've been essentially on a mushy riverbank going down to drink at some point, you know, animals tend to congregate around the rivers.

ATTENBOROUGH: This footprint is about a foot long.

So I think this is from a type of dinosaur that we call a duck-billed dinosaur.

And they would've been very common in the Cretaceous.

They ate the plants in the area and they got very large, 30 feet long.

ATTENBOROUGH: And there are more.

This track, you see all the toes are very well preserved.

You even see a nail print at the tips of the toes.

So the little toenails dug into the mud.

I love this one.

ATTENBOROUGH: This is the team's prize footprint.

It has three toes, and it's longer than it is wide.

So it's very likely to be a carnivorous dinosaur.

It's so well preserved that you can see the mark left by its sharp claw there.

Hell Creek is well known for one carnivore in particular, T. rex.

This footprint is too small for an adult T. rex, but it's possible that it was made by a young one.

♪ ♪ (dinosaur growls) (growls) Robert also found this, the crown of a tooth.

Its shape and its serrated edge are indications that it comes from an adult T. rex.

(animal chittering) (dinosaur growls) ♪ ♪ ♪ ♪ ATTENBOROUGH: Bite marks found on T. rex bones how that they may have eaten each other.

And a youngster would make an easy catch.

♪ ♪ (roars, snaps jaw) ♪ ♪ ATTENBOROUGH: But not this time.

♪ ♪ Very few footprints are preserved as fossils in Hell Creek.

So if you find several in one place, as Robert has done, it's a reasonable assumption that there would've been many more nearby.

MARTILL: When one dinosaur leaves a track, the next one that comes along obliterates that track.

And eventually you end up with a ploughed field effect.

If we think about the actual extent of the rock in which we're making our excavations, our excavations are tiny, tiny samples.

So it's entirely possible there are more out there.

♪ ♪ ATTENBOROUGH: And that supports the idea... (dinosaur squawks) ♪ ♪ ...that dinosaurs and pterosaurs were thriving at Hell Creek shortly before the impact.

(dinosaurs growl, squawk) And if they were thriving... (pterosaur bellows) ATTENBOROUGH: They must have been reproducing.

♪ ♪ (snorts, growls) ATTENBOROUGH: No one has ever found a T. rex's nest, but fossils from similar dinosaurs showed that they may have laid around 20 eggs in a circular nest.

♪ ♪ ATTENBOROUGH: It's possible that like crocodiles, they partially covered their eggs with vegetation to keep them warm.

(dinosaur snorts) ATTENBOROUGH: Looking after eggs must've been a tricky business when you weigh seven tons.

♪ ♪ (dinosaur grunts) ♪ ♪ ATTENBOROUGH: As the team's dig continues, a vision of the prehistoric world here is emerging.

It seems the sand bank was full of life.

T. rex, triceratops, little mammals alongside the footprints of other dinosaurs and pterosaurs, all in a very small area.

♪ ♪ (blows) You see the scales?

GURCHE: I do, oh, my God.

That excites me just looking at it.

(chuckles) ATTENBOROUGH: In 2019, Robert finds something truly remarkable.

♪ ♪ DEPALMA: See the cracks already forming?

Look at that.

So we're gonna have to really monitor that before we glue it.

'Cause this is getting vulnerable now.

♪ ♪ ATTENBOROUGH: An almost complete creature.

After 66 million years, finding anything intact is extremely rare.

(speaking indistinctly) Get the consolidant, and to get this block out, we're freezing it.

ATTENBOROUGH: To keep the fossil in one piece as they remove it from the crumbly layer, the team decides to use a potentially tricky technique.

They've covered the fossil in plaster to protect it.

Steady.

ATTENBOROUGH: Freeing it means they have to flash freeze the crumbly rock surrounding it... ...using liquid nitrogen, at around minus-300 degrees Fahrenheit.

DEPALMA: Watch your footing.

Loren, I'm worried about brittleness here.

Get that hammer.

Give this a couple whacks with the hammer.

(hammer tapping) Okay... move over five centimeters.

Good.

It's cracked loose.

Yep.

Okay, it's loose.

So we have to get this out in one piece.

One... two... three.

Yee-haw!

Total success.

Total success.

DEPALMA (voiceover): This is a technique used in archaeology for digging up human remains.

We've got enough time to work with the fossil and not damage it.

And I couldn't be happier.

♪ ♪ ATTENBOROUGH: And the creature Robert and his team have found?

A turtle.

♪ ♪ This is the fossil now it's been cleaned up.

It's lying on its side.

Here's the outline of its shell.

The shape of the shell and the sculpt edges here tell us that this will was a baenid turtle.

♪ ♪ This baenid turtle would have looked very similar to modern Cooter turtles and lived in the same sort of freshwater environments.

(water splashes) ♪ ♪ BAMFORTH: The Late Cretaceous period is kind of the heyday of turtles, in at least northern North America.

There were at least 16 species that were known from Saskatchewan.

And compare that to today, we only have three.

So back then, it was a much better time to be a turtle.

♪ ♪ ATTENBOROUGH: The turtle fossil Robert found is almost complete, so we can tell a lot about the way it died.

This is the underside, and this brown material up here is fossilized wood.

It's the end of a stick that passes right through its body and comes out just here.

So the evidence points towards this turtle having been impaled.

Another well-preserved creature amongst those found in the thick rock layer.

♪ ♪ DEPALMA: When I look at the animals and plants preserved in the sediments of Tanis and the footprints beneath it, I see a picture of a vibrant ecosystem, many different dinosaurs, and a thriving, thriving place.

♪ ♪ ATTENBOROUGH: Robert and his team have found so many fossils, it looks as if even at the very end of the Late Cretaceous, this area could have been flourishing.

(all bellowing) ATTENBOROUGH: Full of dinosaurs and reptiles that had dominated the planet for more than 150 million years.

♪ ♪ It's impossible to know how much longer the dinosaurs' reign would have continued... (pterosaur bellows) Because what happened next would bring this to an end.

♪ ♪ ♪ ♪ The asteroid hit the sea in an area that is now the Yucatan Peninsula in Mexico.

♪ ♪ It's called the Chicxulub asteroid after the town nearest to the center of its crater.

♪ ♪ Anything living within 900 miles of the hit is destroyed by the blast.

(rumbling) But what effect does the impact have on Tanis, nearly 2,000 miles away?

Is it possible to link the creatures Robert and the team have found so far with the day of the impact?

BRUSATTE: When we date rocks from the Cretaceous, we can say the end Cretaceous was 66 million years ago, plus or minus a few tens of thousands of years.

That is a huge achievement of modern science.

However, when it comes to the asteroid, that asteroid hit the Earth one day.

(chuckles) And really it hit the Earth at one instant.

And so to date fossils in the rock and to try to tie them to one instant in geological time that happened 66 million years ago, it's just outside of the scope of the chemical methods that we have to date rocks, so other evidence is needed to make a plausible scenario or a plausible story if somebody were to find a fossil and wanted to argue that that fossil came from the very end of the Cretaceous, killed by the asteroid.

♪ ♪ ATTENBOROUGH: To tie the site to the day the asteroid hit is a challenge.

But Robert and his team are following a compelling trail of clues, the first of which lies in a jumble of fossils known as a mass death assemblage.

DEPALMA: We've got some wood, and pressed up against this and all intertangled we've got the carcasses of fish.

That's a beautifully preserved tail, so that fish is gonna be absolutely gorgeous.

♪ ♪ ATTENBOROUGH: Some of the evidence he's found so far has been inside the fishes themselves.

♪ ♪ DEPALMA: In more ways than one, it literally is an operation of a Cretaceous fish, so we're performing surgery on this thing.

ATTENBOROUGH: Robert wants to look inside this fish's skull.

DEPALMA: And very carefully we want to separate this from the rest of the fish.

Okay... ♪ ♪ Here we go.

Opening up the fish.

Got a nice ant that made a home in there.

And beautiful, look at that.

Okay, here we have the gill bars of the fish.

Those are the bars that hold the filaments of the gills.

Between the gill bars, all of these clusters of round objects.

ATTENBOROUGH: Tiny round balls of clay.

But, what are they?

♪ ♪ ATTENBOROUGH: After a large asteroid impact, a mix of vaporized and molten rock is propelled into the stratosphere, some of it into space.

There, much of it cools, solidifying into tiny glass droplets.

Some of it is high enough velocity, they can actually leave the Earth's gravitational field.

So it's almost certain that some of the material ejected from Chicxulub would have ended up on, on the moon, which is kind of an exciting thing to think about.

♪ ♪ ATTENBOROUGH: But most of the droplets, known as ejecta spherules, would have been pulled back to Earth by gravity.

Then, over millions of years, pressure and chemical reactions in the ground would turn most of them to clay.

They'd look something like this.

So, finding spherules in the gills of a fish, as Robert has done at Tanis, suggests the fish sucked them in while the spherules were still forming.

So these creatures could have died at the time of an asteroid impact.

(pattering) GULICK: Those have to have come from the impact event.

You can't make spherules in other ways; they're a vapor plume condensate feature.

That shows that these were fish that were alive before the impact.

Those spherules arrived in the next 20 minutes to perhaps hour.

Those fish swallowed them and surely died soon afterwards.

So that's an absolutely amazing discovery.

The fact that there's spherules in the gills of the fish at the Tanis site really brings them as close as really you can possibly get to impact.

ATTENBOROUGH: These ejecta spherules could be evidence of what Robert suspects-- that creatures here died on the day of the asteroid strike.

Once the team begins to look for ejecta spherules, they find more and more, and realize the thick crumbly layer of rock at Tanis is full of them.

DEPALMA: I mean, this stuff is... oh my God, look at that one.

These things are just gorgeous.

(voiceover): Ejecta spherules like this give us a fingerprint of where they came from.

ATTENBOROUGH: If these spherules were connected to the Chicxulub impact, then the whole crumbly layer could be full of evidence of what happened on the day the asteroid hit.

That's a good one.

Oh, is that a droplet right there?

ATTENBOROUGH: But to do the best analysis, they need to find a spherule that hasn't turned to clay.

DEPALMA: Oh, my God, that's a beautiful droplet.

Okay.

ATTENBOROUGH: The small pieces of orange material that Robert and Loren are digging up may be able to help.

They're amber.

♪ ♪ DEPALMA: If there was anything flying through the air at that time, this is where it's gonna get caught.

ATTENBOROUGH: The amber they're collecting was once sticky resin oozing out of a Late Cretaceous tree trunk.

♪ ♪ It's a way for the tree to protect itself, like a scab forming on a cut.

♪ ♪ (thunder crashes) (spherules pattering) ♪ ♪ Anything covered by the resin would be frozen in an amber time capsule.

♪ ♪ ATTENBOROUGH: A well-preserved spherule can be analyzed to see if it came from the asteroid impact.

Loren has found something trapped in there.

GURCHE: So during this batch, we were incredibly lucky that we came across two completely unaltered spherules.

♪ ♪ ATTENBOROUGH: Could this spherule be the evidence to link the site directly with the Chicxulub impact?

There are several lines of evidence that geologists would need in order to definitively say that this ejecta and this ejecta are from the same event.

FLORENTIN MAURASSE: The shape of the spherules, the size of the spherules, the color of the spherules, can be similar for material coming from different sources.

Only the geochemical signature would tell you exactly what the origin of the parent material was.

GULICK: The ability to use trace minerals as a way to diagnose the provenance, the place from which the rocks, or the particles within the rocks, originally came, it is a whole field of geology.

And it's a pretty mature science at this point.

ATTENBOROUGH: If it's a match, Tanis could be something incredibly rare.

TIKOO: If we can match spherules to the impact site geochemically and in terms of radiometric ages, that's pretty accurate.

That's a smoking gun.

ATTENBOROUGH: Does the site Robert and his team have found record the very last day of the Cretaceous, full of fossilized creatures that were alive at the moment the asteroid hit?

The potential for the Tanis site is, is huge.

♪ ♪ ATTENBOROUGH: And might Robert's team find something extraordinary?

That's bone right next to that skin.

ATTENBOROUGH: A dinosaur that died as a direct result of the asteroid impact.

♪ ♪ The day that the asteroid hit would definitely be hell on Earth.

TIKOO: No matter where it is, you're in for a bunch of chaos.

♪ ♪ ♪ ♪ ♪ ♪ ♪ ♪ ♪ ♪