We have now moved out of the Natural History Museum in Copenhagen and out to the locality of Roedvig at Stevns Klint in east Denmark. Stevns Klint is a world-renowned geological locality. Here it is possible to directly study the course and effects of a great mass extinction some 65 million years ago, where upwards of two-thirds of the species on Earth perished. We will get back to the details of exactly what happened 65 million years ago in the last video of this lecture. However, for now I will use this section to show how geologists and palaeontologists in general go about investigating mass extinction events. Let's get closer to the cliff-face and look at the geological layers. Here we can see the geological layers of the cliff. We can think of it as a piece of prehistoric time, with each layer representing a slice of time within that piece. The oldest layer is at the bottom and the layers get younger and younger as we move towards the top. The bottom layer is made up of white chalk. Chalk is a rock consisting almost exclusively of the shells and shell fragments of microscopic marine algae called coccoliths. On top of the white chalk we find a quite different layer: that is a thin grey clay layer, which is almost devoid of carbonate. On top of the grey clay layer we find another kind of limestone - a more yellowish one. And finally, on top of that layer, we find another kind, yet a different kind of limestone, which this time consists of the shells and shell fragments of thousands of small marine colonial animals called bryozoans. This is called a bryozoan limestone. The reason that geologists look for changes in the layers, is that a change in geological layers means a change in environment. Different kinds of environment result in the deposition, the laying down of different kinds of geological layers. And during a global mass extinction environments might be so drastically changed that it is reflected worldwide, and that localities around the world show changes in layers. Here at Stevns Klint, the layer standing out as being very different from the others is the grey clay layer, without any carbonate in it. However, before I start to closely study the sequence further in order to find out if a possible mass extinction took place, I must make sure that everything is present - that there is nothing missing. Remember that the layers here represent a piece of geological time with each layer being a slice of that time. I must make sure to a close investigation that there is not something missing, that there is in fact not a slice of time missing, so therefore I will study the four layers and I will study the earth inside the layers and the boundaries between the layers to make sure that they are continuous - that one evolves smoothly into the other. Now researchers have done several studies of the layers here at Roedvig and they have found out that the lowest three layers, the chalk, the grey clay layer and the limestone on top of the grey layer in fact form a continuous succession. There's nothing missing. However, there is a gap missing between the limestone on top of the grey layer and the bryozoan limestone on top. Here there is a hiatus, a gap in time when no rocks were deposited. Studying mass extinction events is all about studying changes in the ecology and life of the animals here on Earth. Species become extinct and new species appear afterwards. In order to do that we have to look for the most natural thing of all, the remains and the traces of animals living in the past and that is done by looking for their fossils. However, not all fossils are as good as others when it comes to elucidating this structure and sequence of a mass extinction event. For example, I might consider myself really lucky if I had just discovered the tooth of a mosasaur in the chalk here at Stevns Klint. Mosasaurs were large marine reptiles by the end of the age of dinosaurs. Unfortunately mosasaurs and their fossils are not very good for studying extinction events. The same goes in fact for just about every large vertebrate which has been living on the Earth, be they dinosaurs or mammals, birds, flying reptiles, large fishes, any large vertebrate. The reason for that is that they are very rare. Their fossils are far and few between and in order to really study a mass extinction event and the sequence of events during the mass extinction in fine detail, we need fossils which are much more abundant. So if one is out to study mass extinction, one should not focus on vertebrates or for that matter for plant fossils such as fossil leaves or tree trunks. What we need is something much more abundant and much more common. And to do that adequately, we have to study deposits, geological layers from marine environments or the sea. And that is because most fossils are preserved in marine environments in what was once the sea. Fossils that are rather better for studying mass extinction events are for example the shells of marine invertebrates. Invertebrates with shells that lived in the sea are rather abundant and produced a lot of fossils - for example the shells of bivalves or this ammonite would be good shells for studying a mass extinction event. Another kind of marine animal that is easy to study in a mass extinction event would be sharks and that is especially sharks' teeth. Sharks literally go through thousands of teeth during a lifetime, continually shedding them and growing new ones. The lost teeth end up at the sea bottom where they can be buried and preserved quite easily because teeth are made of a hard material. Now, if I was to go into a land environment, I might similarly look for the microscopic teeth of small mammals. Small mammals are also quite abundant, and their teeth are easily preserved because they are made of a hard material - enamel. Therefore in some terrestrial environments you can study mass extinction events by looking for fossil mammal teeth. Getting back to our deposits here at Stevns Klint, I will use ammonites and sharks' teeth as an example of how one might go about studying a possible mass extinction event. Now, the first thing to do when studying mass extinctions is to find out how various animal species appeared or disappeared, perhaps became extinct during the course of geological time. That is done by collecting as many fossils of as many different species as possible from the geological layers. For each fossil you find, carefully note what species it belongs to and at which level in each layer it is found. If we can find the same fossil at several levels within one layer or maybe in two different layers, we can safely assume that the fossil was existing in between the time represented by these two layers or within one layer. The lowermost or oldest appearance of the fossil the first time you find it represents the time that the very species moved into the area or perhaps originated. The last time or uppermost appearance of a fossil will represent the time that it either disappeared from the area or perhaps became extinct. Let us assume that I have been looking for fossils of a specific species of ammonite. Let's say I have found fossils of the species at this level, at this level, at this level and that the topmost fossil I find of the species, I find up here, right below the grey clay layer. All my ammonite fossils so far of this species have been found in the chalk. I will then assume that the species appeared or entered the area at this level the first time I find it and disappeared or perhaps turned extinct at the topmost level, just before the grey layer. I can continue by looking for other species of ammonite, and I will find that some species of ammonites disappear some time below the grey layer at the top of the chalk and that quite a number of ammonite species exist right up until just below the grey clay layer. However, for my studies and my search for ammonites at Stevns Klint I will find that there are no ammonite fossils what-so-ever to be found in the grey layer or in the two limestone layers above the grey layer. I can then make a preliminary conclusion that ammonites either disappeared completely or perhaps suffered a mass extinction event at the end of the white chalk just before the grey layer. If I continue to look at what other researchers have done studying ammonites from other countries like North America, Australia, Israel or Italy, I can read their papers and find that everybody finds that ammonites are buried in abundance in the time represented by the white chalk. However, there are no ammonites to be found in the time represented by the grey clay layer or the limestones above it anywhere in the world. From that I can make a conclusion that ammonites probably suffered from a world-wide mass extinction right at the boundary between the white chalk and the grey clay layer. In order to understand the effects of a mass extinction and of the course of events that led to it, it is impossible to study the pattern of extinction and evolutions within as many different groups of organisms as possible. The reason for that is that different animals have different ecologies, i.e. that they are adapted and tolerate different kinds of environments and if we study which animal groups survive or for that matter go extinct during a mass extinction event, we can get an idea of how the extinction event affected different ecologies and different kinds of life. Now, in that vein here at Stevns Klint, I could look at sharks' teeth just in the same way as I studied ammonites a little ago. So for example by carefully looking for fossils of specific shark species, I might find that many shark species exist in the upper part of the chalk, but that many of them disappear at the bottom, just below the grey clay layer just as it happened with the ammonites. However, I would also find that a number of shark species actually exist both within the white chalk and can be found in the grey clay layer or in the limestones above them. These would be species of sharks that actually survived through the possible mass extinction that took place at the boundary between the white chalk and the grey clay layer. Again I might find that in the limestone layers above the grey clay layer a number of new shark species appear, species that cannot be found below the grey clay layer, below in the white chalk. This would indicate that sharks as a group went through a species turnover - some species disappeared and probably became extinct at the boundary between the chalk and the grey clay layer at that time, and other species managed it through and new species would appear again after the time of the grey clay layer and so forth. Again this would indicate that sharks went through a species turnover when new species replaced old ones. And again I could continue this study for many different kinds of organisms: sea urchins, bivalves, brachiopods and so forth in order to get the full picture of how the extinction event at the time of the grey clay layer affected different groups of animals and affected different ecologies. However, the very best fossils for studying marine extinction events are those you probably need a microscope to study and find: microfossils. Microfossils include the tiny, tiny shells of marine planktonic organisms, animals and plants, it includes shark micro teeth, pollen and spores, anything that is small and extremely abundant. These have existed in billions in the sea and on land and are therefore very common, and easily become preserved as fossils. Because they are so abundant, they are easy to understand, and it is easy to follow thin, thin time slices showing their evolution diversity and extinction. Also, many of these extinct micro-organisms have close living relatives - species that can be found today and whose ecologies can be used to understand that of their extinct relatives and hence the effects of the mass extinction. All in all, studies all over the world of sections similar to here at Stevns Klint about the time period around the deposition of the grey clay layer show the same pattern. At the boundary, the time represented by the grey clay layer, around two thirds of all species on Earth disappear and become extinct. That includes many groups, such as the ammonites that disappear completely. An extinction level of two thirds of all species - upwards at 65% - stands out well above the background extinction level here on Earth and indicates a major mass extinction affecting all life and most ecologies on Earth. But there is more that can be done here. I need to know the approximate age of each layer, and also the duration of time - "the slice of time" - that each layer represents. In geology, the only way to get absolute ages is to use radiometric dating on volcanic deposits such as lavas or ash layers. Radiometric dating uses radioactive isotopes from lavas and ashes to determine the age of the volcanic eruption. Here I am at a loss - there are no traces of any ash layers within my section at Stevns. But by using the principle of fossil correlation, I can compare the fossil species within my layers with those of other places in the world. If the same species are found in a section somewhere else that has datable volcanic layers, I can correlate or transfer those ages back to this section. And by correlating the fossils of other places with Stevns Klint, we find that the time of the grey clay layer was approximately 65.5 million years ago - at the border between the Cretaceous and Tertiary periods. Further areas of study are opened up with the use of geochemistry - investigations of the chemical composition of the layers and the fossils within them. For example, by studying the relative content of carbonates within a deposit, we can get an idea of how many carbonate-shelled organisms were about and how productive they were. Also isotopes such as Oxygen-18 or Carbon-13 can give information about the temperature and environment at the time. I could also look for traces of rare chemical elements, which may give further clues to the nature of the event. Just as with the fossil data, this is done by carefully taking samples and recording their level within each layer, before taking them back to the laboratory and analyzing them. But we will return in the final video of this lecture, to look in detail at what happened exactly at the great mass extinction some 65 million years ago. Now, we will return to the Natural History Museum in Copenhagen to look closer at another mass extinction event, which took place some 251 million years ago and which took away almost 90% of all species on Earth.
