What wiped out the dinosaurs?

Modern humans have only been around for a relatively short time - just 60,000 years or so, and mass extinctions have been happening for much longer. So what else wipes out species, and how do scientists do a palaeontological post mortem to find out what happened?

Sara Sjosten went behind the scenes at the Sedgwick Museum of Earth Sciences in Cambridge to meet dinosaur specialist Dave Norman. He explained what fossils can tell us about mass extinctions, and why the dinosaurs disappeared abruptly about 60 million years ago...

Dave - Finding fossils like this remarkable jaw here, this is just a fragment of a jaw and it's 20 cm long with sort of 7 or 8 cm long teeth which are huge and spiked. I know that's a lizard because I'm enough of an anatomist to be able to recognise the characteristics of the bone, the shapes of the teeth. This is an animal that lived a long time ago, towards the end of the Cretaceous period and I can say that it was lizard-like but it had flippers instead of conventional legs and it swam through the sea and was a giant predator. This isn't the world of the present day. This is a very different world. Studying fossils allows you to look at the way in which the world has changed, and how different the world was in the past. Now rather curiously, the animals that you find in rocks of Cretaceous, that's 65 million years ago, that particular population of types of animals which included these giant lizard-like creatures, which are known as mosasaurs, and a whole variety of dinosaurs, very, very suddenly disappeared. If you look at rocks that are younger than 65.5 million years of age, look as you might for these gigantic and dramatic looking creatures, you can't find any. Within 10 or so million years, you start to get rather different looking creatures, utterly unlike the ones that lived in that previous world. Life had changed, something very dramatic had happened that in a sense wiped away the mosasaurs, the dinosaurs and indeed, a whole variety of other sorts of organisms and in a sense, made a space. It's like a punctuation in the history of life.

Sara - The world changed dramatically from a time of giant reptiles like the mosasaur with the jaw the size of my arm to the world we know, where mammals and birds cover the land. What happened? Was it down to some cataclysmic event?

Dave - What we've got is in a sense, a whole range of different lines of evidence that are all pointing towards something very dramatic happening about 65.5 million years ago. There were various hints from the fossils that animals disappeared rather dramatically at 65 million years ago and were replaced by different types of animals and plants. Therefore, there's a lot of focus on rocks of exactly that sort of age and whether they have any particular chemical or geological characteristics. One of the things that started to be discovered was that the rocks at that time zone tended to have what are called tectites, lots of little glass beads, which could've been the result of an explosive event rather like a huge explosive volcano. Obviously, the nearer to this event, the more of these glassy beads you would find and the further away, the fewer you would find. The focus of attention gradually became southern North America and northern South America, and then some astrophysicists came along.

Sara - Louis Alvarez was an astrophysicist at this time. He was brought into this problem when his son asked him to date a strange layer of clay from Panama. Often, rocks can be dated using radioactive elements. These decay at steady rate, so by measuring their relative abundances in the minerals in a rock, it's possible to work out the age of the rock. But this clay didn't have any minerals to date, so Alvarez suggested they look at the other elements in the clay. He found that there was an unusually large amount of the trace metal iridium. Iridium rains down on the Earth's surface at a constant rate. So, any extra iridium could only come from one place - space. Geologists now had an idea of what to look for and the glass bead distribution told them where to look. They took large boats off the coast of Central America, equipped with something like sonar for imaging the subsurface in what's called seismic profiling. There, they found the key evidence and solved the mystery of the disappearing dinosaurs.

Dave - Finally, seismic profiling allowed us to identify a ring structure in the Yucatan Peninsula. That ring structure seemed to reflect the fact that there was actually a very large meteorite impact that left this huge crater under the ground. And that everything was now beginning to fall into place. We had this extraordinary coming together of evidence from an astrophysicist, evidence from the fossil record, evidence from the chemistry, the nature of the geological sediments of the time, which led to a sort of synthetic theory which was, "Hey, the life on Earth at 65 million years ago was reset by an extraordinary event. This event was unpredictable. It was extraterrestrial in origin. It's reset the way in which life has evolved on Earth.

Chris - Fascinating! Dave Norman from Cambridge University on his paleontological post-mortem of what wiped out the dinosaurs. If you'd like to know more about how hidden fossils are revealed in rocks, you can listen to a special podcast we'll be putting out tomorrow featuring Sarah Finney, also from the Sedgwick Museum, and that's with Sara Sjosten who you also heard in that piece.