Episode 240: The End of the Dinosaurs

Sign up for our mailing list! We also have t-shirts and mugs with our logo! Here we go. It's the big one, the Cretaceous-Paleogene extinction event! Further reading: How Birds Survived the Asteroid Impact That Wiped Out the Dinosaurs How an asteroid ended the age of dinosaurs Extinction event that wiped out dinosaurs cleared way for frogs How life blossomed after the dinosaurs died 66-million-year-old deathbed linked to dinosaur-killing meteor Show transcript: Welcome to Strange Animals Podcast. I’m your host, Kate Shaw. Here it is, the extinction event episode that everyone’s been waiting for, or at least that everyone knows about. It’s the one that killed off the dinosaurs and ushered in the age of mammals. It’s probably the one we know most about and it’s certainly the one we have the most paintings of, usually of a T. rex staring into the sky at an approaching comet. In episode 227 we talked about the end-Permian extinction event, which took place about 250 million years ago. The Cretaceous-Paleogene extinction event, or end-Cretaceous, took place just over 66 million years ago, which means that for almost 200 million years there was more or less smooth sailing in the world. Dinosaurs evolved during that time, and I think we can all agree that dinosaurs are fascinating animals. The largest terrestrial animals ever to live were dinosaurs, specifically the sauropods. Sauropods were just unimaginably huge. They were like walking buildings that ate plants, and even that doesn’t give a good idea of their size. Some sauropods had extremely long tails as well as very long necks, which increased their length. Right now the largest sauropod known was probably Argentinosaurus that might have grown as long as 118 feet, or 36 meters, but paleontologists keep finding bigger and bigger sauropods. Some sauropods had extremely long necks that they held up like a giraffe. The tallest was probably Barosaurus, estimated as being 72 feet tall, or 22 meters. And we won’t even get into estimates of how much these massive animals weighed. They make the biggest elephant that ever lived look like a toy elephant. Sauropods ate plants, with the low-necked species eating low-growing plants and the high-necked species eating tree leaves, although even saying that much is controversial. There’s a lot we don’t know about sauropods in general, since most sauropod fossils are incomplete and many species are only known from one or a few bones. But we do know some surprising things about sauropods. We have a lot of sauropod tracks, which helps us understand how their feet looked and whether they had claws, but it also tells us that some species of sauropod traveled in herds. Paleontologists do generally agree that many sauropods migrated, since animals that big would soon exhaust all the food in one area if they didn’t. Sauropods were extremely successful and lived all over the world. There were plenty of sauropods alive 66 ½ million years ago, and then…there were no sauropods alive ever again. These days, there’s so much evidence that a massive asteroid killed off the dinosaurs that pretty much everyone agrees, but when the idea was first proposed in 1980, it was extremely controversial. When I was a kid I remember reading dinosaur books that still said the extinction of the dinosaurs was a mystery but that many scientists thought it was due to disease or volcanoes. The asteroid strike hypothesis was proposed by the physicist Luis Alvarez and his son, Walter. They worked with a small team of other scientists, including two chemists, Helen Michel and Frank Asaro, to investigate a strange anomaly in rock strata. Rocks dating to the end of the Cretaceous period and the beginning of the Paleogene period are separated by a thin layer of clay that’s visible throughout the world, or at least wherever the rocks remain and can be examined. It’s called the Cretaceous-Paleogene boundary, or K-Pg boundary, although in older books and websites it’s called the K-T boundary. It occurred just over 66 million years ago. The Alvarezes were curious about this layer, and during their investigations they found out that the clay is full of an element called iridium. Iridium is a silvery-white metal chemically related to platinum, and it’s rare. At least, it’s rare on Earth. It’s a common component of asteroids, which was one of the main reasons why the Alvarezes came to their hypothesis that the K-Pg boundary was the result of a massive asteroid impact. Other scientists had made similar suggestions in the decade or so leading up to the Alvarezes’ theory, but the iridium discovery provided the proof everyone wanted. Iridium wasn’t the only thing found in the K-Pg boundary layer, either. There were other platinum-group metals present in high concentrations—much higher than found on Earth, and in fact these elements are referred to as rare-earth metals for that reason. In some places, the K-Pg boundary contains grains of shocked quartz and microtektites. We’ll discuss those in a minute. As we’ve discussed before in various episodes, the earth’s surface is always moving around. It’s slow to us, with continents moving around at the same dizzying speed that our fingernails grow, but over millions of years that adds up. Continents move around and crash into each other, forming new mountain ranges that then wear down to plains, and where continental plates pull apart or push together the crust can weaken and allow magma to erupt through as volcanoes. Ocean levels rise and fall. In other words, a crater made 66 million years ago might have disappeared as all this geologic activity goes on. But then, we found the crater. The crater. The Chicxulub crater is in Mexico, specifically the Yucatán Peninsula at the southern portion of the Gulf of Mexico. You can’t see it when you’re walking around because it’s buried under 2,000 feet of soil, or 600 meters, that has built up over the last 66 million years. Two geophysicists found it in the 1970s while surveying for petroleum, but it wasn’t until 1990 that they were able to verify that it was a crater. Asteroid impacts leave clues behind that the geophysicists recognized. The first clue is shocked quartz. Quartz is a common crystalline mineral throughout the world, and it has a certain structure that’s familiar to geologists. In shocked quartz, that structure has been deformed by intense pressure, but not high temperature. It was first noticed after nuclear bomb tests, and after that scientists recognized it in meteor craters. The second clue is little pieces of glass called tektites. They’re different from obsidian, which is a type of glass formed by volcanic activity. Tektites are usually shaped like droplets or little blobs, but sometimes they’re round. They’re only found around big impact sites and only for relatively recent meteor impacts, because they don’t last forever. The Chicxulub crater is actually kind of old for its tektites to still be around, except for two things. First, the tiniest tektites, the microtektites, ended up in the K-Pg boundary layer, as I mentioned earlier. Second, we actually have a fossil site in North Dakota, in the middle of North America up near Canada, that seems to date to literally the day of the asteroid impact, and there are tektites all over the site, including clogging the gills of fish. The tektites match the chemical signatures of the Chicxulub crater so we know that’s where they came from. Before we talk about the North Dakota fossil bed, let’s discuss what exactly happened on the day the asteroid hit the earth. Because we’ve found the asteroid’s crater, we know a lot about the asteroid itself. Most researchers estimate that it was about 6 miles across, or 10 km. It approached the earth at an angle, traveling about 12 km a second. That’s 7.5 miles per second. It hit the earth right on the coast, partly in the ocean, partly on land, forming a crater about 110 miles across, or 180 km, and 12 miles deep, or 19 km. The asteroid smashed into the Earth so fast that it was completely buried in about the time it takes you to blink. There really wasn’t time for any dinosaurs to look up and wonder what that bright light was, because the time between the asteroid entering earth’s atmosphere and smashing into the earth was maybe five seconds. The megatsunami resulting from the impact would have been unbelievably huge. Waves may have been a mile high, or over 1.5 km. The initial impact would have thrown water more than 7.5 miles into the air, or 12 km, and when that water fell back down it would have set up another megatsunami. Not only that, the impact actually shook the whole earth like a massive earthquake, which caused landslides all over the place and set up even more tsunamis. It’s like shaking a snowglobe to watch the fake snow swirl around and around, only instead of fake snow it was ocean. At the same time, everything near the impact site was instantly on fire. It was on fire because the asteroid was traveling so fast that it was glowing white-hot with incandescent heat just from pushing against air molecules, and when it hit the Earth, all that heat had to go somewhere. Also, everything exploded. The water exploded up and outward, the land exploded up and outward. A lot of water turned instantly to steam. The asteroid itself disintegrated and tiny bits of it were carried high into the atmosphere along with ash, dust, molten glass created by the blast, and anything else that was nearby and not instantly incinerated. The shockwaves from the impact acted as a magnitude 12 earthquake, with follow-up shocks estimated at about magnitude 9 occurring across the entire planet. Volcanoes erupted as a result, pumping even more ash and gases into the atmosphere. All the trees were flattened for about 930 miles around the impact, or 1500 km. Within a few hours of the impact, fireballs of molten rock and glass were falling across the world,...