The Day the Mesozoic Died: The Asteroid That Killed the Dinosaurs — HHMI BioInteractive Video

[Music] picture a world of giant reptiles flying pterosaurs with 40 foot wingspans while beneath the waves fierce predators called mosasaurs fouled the seeds and dinosaurs roamed the land for more than a hundred million years creatures like these ruled the planet and then they were gone lost in the shadows of time their extinction a mystery for the ages [Music] this is all we have left of those magnificent creatures bones and a lot of them we've collected enormous numbers of bones and from them we can tell how these creatures lived what they look like when they lived

but we don't know why they disappeared to solve that mystery required some of the greatest scientific detective work ever and the trail began almost halfway around the world from here this is Gubbio a sleepy Italian town not a single giant reptile in sight still the past is everywhere you look in the medieval palaces and churches the homes and narrow streets while an even deeper past lies nearby [Music] just outside of Gubbio alessandro montón re is looking for that distant past he's a time traveler moving towards the lost world of the dinosaurs it's ancient history written

in these limestone cliffs millions of years ago these mountains were at the bottom of a deep sea collecting layers of sediments and deposited slowly through time and eventually pushed by tectonic forces folded and uplifted so all of this rock and all of these layers were once at the bottom of the ocean exactly and then all these other forces have brought this old rock up to you yes that we can actually study layer by layer like pages of a book the eastery of the earth Rubio's deep-sea limestone now exposed by the side of the road became

a magnet for scientists especially this site that may seem ordinary but holds one of the strangest features to confront the geologists a thin layer of dark clay in the 1970s the clay captured the attention of an American geologist Walter Alvarez he was trying to determine the relative ages of the limestone layers by analyzing the fossils left by tiny shelled sea creatures called foraminifera or forums for short they're among the most common ocean plankton when they die their shells become part of the sediment since many different species have evolved over the ages that can serve as

markers of geologic time but as Alvarez studied them he noticed something striking well what a season it was puzzled by after the very top of that white layer all these very diverse micro fossils species would disappear okay so Sandra when Walter Alvarez looked in these rocks he saw all these species of foreign yeah and they're plentiful there are many species for thousands and thousands and thousands of years but after this little seam of clay here all the way up through these rock layers those forums are missing those pieces are gone disappeared there's a band that's

a fundamental mystery how what would make these little creatures disappear yes so Alvarez was stumped what did this thin line of clay represent it was laid down 65 million years ago the same time when the dinosaurs disappeared was there any link the same questions were being asked 1,500 kilometers away in Spain [Music] on the Atlantic coast outside the town of Zumaya dutch geologist John Smith was studying the forums from a different ancient sea their fossilized shells form these limestone rocks which like goobie owes mountains were once at the bottom of the ocean now exposed they

represent over a million years of a geological time period known as the Cretaceous like Alvarez in Italy Schmitt had also found a strange clay layer formed at the same time which told a dramatic story yawn what can we tell about Earth's history from just looking at these rocks well you can see at the end of the one world and the beginning of the next one you see there is a very sharp extremely sharp dividing line between the two that gray band that's that gray band over there at the bottom of that gray bones you see

it's a razor sharp and this all this reddish maroon rock the oceans are healthy and steaming are healthy they're steady they don't change at all and all of a sudden receive dramatic changes which we call the KT boundary the KT boundary found at the bottom of this gray layer marks not only the end of a period the Cretaceous but also on the Mesozoic era a much longer stretch of time the history of animal life on earth has been divided into three such errands early life a Paleozoic the age of dinosaurs or Mesozoic and the era

of mammals or Cenozoic the cretaceous-tertiary boundary lies right in between these two eras marked here by this layer of clay what drew you to this boundary yard well shown we're looking here at the very top most rocks of the Cretaceous these rocks are literally packed with foraminifera so we call this the Cretaceous and these rocks here are the tertiary and the base of the tertiary consists of a dark grey clay layer and there the life has almost disappeared and the KT boundary is just in between I can point at it it's right here we see

a contrast between the purple and the dark it's an extremely sharp boundary and just across that thin little boundary there's this huge change in what you see in the forums and why is that so stunning to you it's so stunning because there is no preceding evidence of anything happening there so it doesn't matter if I take a piece here a piece deer or just underneath what we call the KT boundary the foramen ephra will remain the same so they don't change overnight and then bang they're gone so what's that tell you that tells me the

base of the food chain of the oceans disappeared and everything which is dependent on it is totally zapped right at the boundary [Music] in Amsterdam change at the boundary becomes clearer when Schmitt takes a closer look at the evidence in his lab he analyzes four amps extracted from the limestone under the microscope the rich diversity of fossilized shells from the Cretaceous snaps into focus [Music] some four dozen species appear below the boundary but above it there's a different world [Music] these are the foremen ephra from just below the boundary and these are from just above

the buyer only a few species have survived into the tertiary and they're much smaller as soon as you see the extinction and you realize at the same time that the dinosaurs are disappearing you know you're looking at something very important but you see it that nobody has witnessed it so we're looking for silent witnesses in the rock and the first thing that comes to mind is this beautiful play layer a layer varying in color and thickness and found around the world Walter Alvarez also believed it was a silent witness to the end of an era

the key question was how long did it take to form for the world to change to find out he sought help at the University of California at Berkeley from a Nobel prize-winning physicist someone he knew pretty well his father Louie Alvarez Louie loved a good mystery no matter what field it was in and that's how physics joined geology in the quest to explain the KT extinction Alvarez looked at this layer tried to figure out how you could determine the time scale he brought in his knowledge of astrophysics his knowledge of nuclear physics and realized that

there's an element that's relatively rare in the crust of the earth that occurs in meteorites the element was iridium which falls steadily in an invisible rain of cosmic space dust if the lair had taken thousands of years to form Alvarez thought there might be just enough iridium to measure but when the clay was tested the scientists were stunned to find it contained 30 times more iridium than the surrounding rock moreover samples from other KT sites had similar levels too much to come from ordinary space dust what could explain so much iridium deposited around the world

perhaps catastrophic event in outer space Alvarez wondered if a supernova exploding nearby might be responsible so he asked me if that was possible and I concluded that there was only one chance in a billion that such a supernova would occurred that close in 100 million years supernova would have also deposited a rare isotope plutonium 244 but testing revealed there wasn't any well I suggested alternatively that it could have been an asteroid or a comet there are hundreds of asteroids whose paths crossed the Earth's orbit their sizes range from a few meters to hundreds of kilometers

across so Louis Alvarez had this hypothesis that an asteroid or comet would cause this destruction he had the clue the amount of iridium at Gubbio under this hypothesis would be spread all around the world so now he could calculate how much iridium there had been laid down over the entire earth now you also know how much iridium there is in asteroids and comets so he can now calculate the size of the object the answer was sobering an asteroid ten kilometers in diameter as large as Mount Everest and weighing hundreds of billions of tons still how

could something that size wreak havoc on a large planet because traveling through the vacuum of space it would have slammed into the Earth's atmosphere at 80 thousand kilometers per hour 20 times faster than a bullet heating the air to several times the temperature of the Sun at impact the energy released would be equal to about 100 million nuclear bombs exploding at once a huge mass of pulverized debris would have been blasted into space some of it orbiting the Earth before raining back down the debris may have blocked out the Sun for months photosynthesis would have

stopped plants plant eaters and then meat-eaters would have died this was the asteroid impact hypothesis for how the Mesozoic era ended a big idea that was just too big for some when the Alvarez hypothesis was first proposed it was difficult for many scientists who accept because for almost two centuries geologists had crafted their worldview around a gradual picture a slow but steady change in the earth without major catastrophes now they were hearing a proposal that something had come from outer space and rewritten the history of life in almost an instant Louie Alvarez got very frustrated

when the paleontologist didn't say yes sir thank you for solving our problem many of the paleontologists just looked on him as someone who didn't know their field and was stepping into this just because it was such a big important famous problem the controversy would continue for years to convince the skeptics more evidence was needed [Music] one criticism of the kt hypothesis was the lack of a crater the right age type and size alvarez thought it should be 200 kilometers across bigger than the state of connecticut how could you miss that they look they look for

craters that was 65 million years old all over the earth many of these things had been discovered and had been measured and find any two-thirds of the planet is covered by water if the asteroid landed in the ocean crater might never be found even so it would still be a trail of debris ejecta blasted from the crater so attention focused on finding this evidence geologist Jean Smith discovered glass like beads and Katie boundaries called spherules formed when vaporized Rock cools and rains back down [Music] another key clue was rock that had been so shocked by

the impact it had criss-crossing bands of dislocated minerals this was shocked quartz and we know if you set off a nuclear bomb the damage done to the surrounding rocks will produce you a shock quartz so if you put two and two together we find shocked quartz at the KT boundary and shocked quartz and nuclear craters you know it is an explosion which deforms your quartz crystals and quartz is only found on land so there was the big clue we have to look for a crater somewhere on land [Music] the search for fresh clues led here

to Texas along the Brazos River some 300 kilometers from the Gulf of Mexico [Music] sixty-five million years ago this was the bottom of the sea instead of grazing land in the early 1980s scientists noted unusual deposits of sediments across the river basin intrigued Alan Hildebrand then a graduate student in geology came to investigate [Music] along the banks of the river in his tributaries the examined exposures of the KT boundary different from those seen elsewhere I saw something on top of the Cretaceous mud and rock that have once accumulated on the seafloor [Music] so this is

the typical mud just founded in this area exactly all we see is this rain cretaceous mud and and there's seven million years of this mud here so I mean we're talking boring right you know there was nothing going on but see rate here the seafloor has been eroded rate at this point an untrained eye might see this sediment and never looked twice at the protruding boulders but hildebrand saw evidence of a catastrophic event so something happened here it eroded the seafloor and we start saying these very coarse sediments and this first unit is really quite

extraordinary because you'd trace round over here you see here's a boulder in it it's like 50 centimetres across and you come over here this is another one it was weak so it's weathered out but it's even bigger here's another boulder but notice this Boulder is different stuff than these so you had this really regular mud layer and then all of a sudden this this area that's just full of this whole mixture of boulders and and from different places it isn't just the rock that was here in the sea floor maybe some of it got pulled

from deep water maybe something get pulled from chawla it all got jumbled together and dumped here all right so what could have happened here that would explain something so dramatic well had to be a hugely energetic wave in the ocean a giant tsunami if an asteroid ten kilometers across landed in the sea or at the edge of a continent it would displace incredible amounts of rock and water causing tsunamis over 100 metres high these giant waves would have crossed the sea with the speed of a jet ripping up the seafloor moving tons of sediment as

that debris came to rest in what today is the Brazos River Basin it would have mixed with ejecta falling from the sky okay so now we're here in Texas is this making you think that you're a little closer to the crater in Texas and you might be in Italy for example exactly because we can see the products of the impact mixed into this konami deposit so we're getting warm Hildebrand was hot on the trail of every new piece of evidence next stop Haiti where he investigated a report of volcanic rocks as he suspected they were

actually ejecta full of shock minerals and spherules they also contained melted rock called tektites another telltale sign more evidence that a strike had occurred somewhere around the Gulf of Mexico but somewhere wasn't precise enough ironically a key clue discovered by another geologist had long been overlooked years before the blended field had hunted for oil on the Yucatan Peninsula in Mexico from the air and field saw nothing unusual but as instruments measured differences in gravitational fields and revealed the features of a giant buried crater it was Hildebrand who eventually followed up on Penfield's work rock samples

from the area Penfield identified showed all the signs of a high-energy impact and it was full of shocked quartz - so this evidence finally convinced everybody that indeed there was a big crater buried on the Akutan Peninsula after years of speculation the crater had finally been found it was named the Chicxulub crater after a village built over its center the discovery the Chicxulub crater was the ultimate evidence of the asteroid impact and it tied together all of the clues that have been gathered over the previous decade the shock courts the tektites the sphere riyals that

had fallen across the earth moreover the crater was the same age as the KT boundary and it was the size predicted by Louie Alvarez we now knew for certain what happened on that horrible day [Music] [Music] the asteroid crater had finally been found but important questions remained which species were wiped out at the end of the Mesozoic which survived and why the search for those answers led to the Badlands of the Dakotas and Montana in the Hell Creek Formation [Music] it's eroding buttes hold fossils of plants and animals that live during the last million years

of the Cretaceous and beyond [Music] when paleontologists Kirk Johnson discovered this KT boundary with its telltale sphere Ewell's he found the dividing line between two vastly different worlds you're looking at a ball of glass that used to be the bedrock in Chicxulub Mexico Kirk how important was it to find the KT boundary up here in North Dakota if you can put your finger on the battery like you can right here what that means that you can ask the very simple question how its life before the impact different from life after the impact I got to

do is look for the fossils below just compare them the fossils above and that's what we've done here for the last 30 years [Music] this arid landscape was once a wet lush forest crack open some rocks and you'll find the leaves of plants and trees that flourished here over 65 million years ago you can even tell what insects ate them let's see there's two different kinds of insect damage on this leaf as well instead of a hole feeding us in the leaf and there's a margin feeding on the it'd take a close look at the

ground and you can pick up fossils of small animals that thrived in lakes rivers and forests so in my hand I've got evidence of a turtle fish crocodile fish and man so and then there were the dinosaurs the challenge connecting their fate to the KT boundary the clues their fossilized remains this is an ankle bone of a small meat-eating dinosaur and you find these bones identify the animal and pretty soon you start assembling the list of dinosaurs that are present at any given level and the lower you are in the formation the older you are

and the higher you are the younger you are and the closer to the boundary you are the more we address the question of how long the dinosaur survived so this just nearby we found this bone which is a bone of a much larger meat-eating dinosaur same bone but see the size difference right and here's the exact same bone so when you find bones of different species in the same layer they're living at the same place at the same time [Music] the work takes a great deal of time and patience this is how basically how you

find a dinosaur you know you're walking around looking in these goalies and then you spot a piece of bone you can see it's very porous and as you know that it had to travel down force of gravity you can see the bone trail here's a bone here's a bone follow the trail of bone up and then here you have or shin bone of a ductile dinosaur really broken up and then here is where it would articulate with the knee joint scientists know that 22 types of dinosaurs lived in Hell Creek including Triceratops Tyrannosaurus and this

duckbill some 9 metres or 30 feet long the more complete the skeleton the easier it is to reconstruct the past but discoveries like these are extremely rare you walk around you know and the battle ends out here and you'll pick up numerous just chunks of dinosaur chunk asaurus that's what we call it and and that's about it so usually you find piles of bones that aren't articulated if they're not in the correct order so having the vertebrae in the right order like this is very very rare and what makes this specimen even more important is

its articulated and it's pretty close to the KT boundary now I've found several specimens that are very close to the boundary and really what that's showing us is that even if dinosaurs are rare if you look long enough you'll find them when they were living on the planet what we have not found yet is any dinosaur skeleton anywhere in the world above the KT boundary layer scientists now knew what lived here and their ultimate fate [Music] within a thousand kilometers of the impact death came quickly if you turn your oven on broil open the door

put your hand into the globe our that's what the dinosaurs felt very soon and they were probably broiled alive within an hour or so for dinosaurs further away death may have been delayed but not for long soon vaporized ejecta and smoke from fires filled the air and there may have also been a lot of sulphur blown into the atmosphere because the impact site in the Yucatan Peninsula had a lot of sulfur in it and all of those things were bunted enough to obliterate the Sun without light anything dependent on photosynthesis on land or in the

sea was vulnerable as food chains collapsed giant reptiles still alive died off the Mesozoic era the age of dinosaurs was over and there were other radical changes in Kirk Johnson's Denver lab you can see what happened to plant life [Music] samples taken from below the KT boundary show a high diversity of pollen grains staying here in red but above the boundary this earlier diversity disappears reflecting the extinction of 60% of all plant species where flowering plants once thrived ferns took over first transport unlike pollen fern spores can germinate on a barren landscape devoid of living

plants we have a short period of time where there's only ferns that's the fern spike but after that there's about a million years of time where we have the low diversity disaster recovery flora and after that first million years then things start to pick back up again we start to see animals coming back into the landscape in this new post-impact world the niche that dinosaurs left was waiting to be filled and so the survivors of this on the land were creatures that lived in holes birds mice like creatures turtles frogs things that lived in swamps

or rivers or near the seashore and compared to dinosaurs they had the advantage of size they were small small animals have large population size small animals have higher reproduction rates now it's not saying that small animals didn't suffer mass death but enough survived and that's the key point in that million years of recovery the small inherited the earth it was the beginning of the age of mammals eventually larger mammals dominated the land just as dinosaurs had done before and among them were primitive primates whose evolution would lead in a very promising direction at least for

humans but the asteroid impact taught us about evolution is that it's not always about survival the fittest sometimes it's about survival of the luckiest and there's a profound point there for our species only after the extinction of the giant reptiles did mammals flourish including our primate ancestors without the asteroid there'd be no us [Music] [Music] you