 Fossils are found in sedimentary rocks which you can then date using relative and absolute techniques. So there are two main types of clues that life leaves behind in the rock record. Now the first one is morphological evidence. Now that's when you find hard parts of an organism like bones or teeth and you can find a whole skeleton or sometimes they leave impressions behind like a footprint or a cast of a shell. Now morphological fossils are really good because it can help reconstruct what the organism looked like and where it might have lived and what it might have eaten as well. So we can track past life by also looking at the chemical signatures they leave behind. Now some organisms preferentially use certain isotopes of elements and then this causes whole shifts in the isotope record left behind in the rocks. An example of this is carbon or nitrogen and then this can help us piece together what productivity looked like in the past. Now another way we can look for organisms such as bacteria or algae which don't leave behind fossil hard parts by looking for the molecular fossils and these molecular fossils are fossilized molecules and we can use those to figure out what the microbial ecosystem looked like and the composition of it. We can learn a lot about ecosystems by looking at both the morphological and the chemical signatures in the rock record. Then we bring them together with the geochemistry of the sedimentary rock. We can start piecing together what the environment looked like and then try to figure out how the organisms might have actually died. So when we look at the fossil record at the 65.5 million year mark there are massive global changes to Earth's biodiversity. We see the disappearance of major groups of life, whole branches of life just disappear such as the marine and flying reptiles, non-avian dinosaurs and ammonites. We just don't see these fossils again in the rock record. And then things like plankton and land plants they also reduced in diversity. Now this transition in biodiversity we now refer to as the Cretaceous Paleogene Boundary or the K-P-G boundary for short. But what caused 75% of the species to go extinct at this point? Well we can look at the geochemical evidence of the rocks and let's try and figure it out. Now when scientists started measuring some of the elements in the rocks around this K-P-G boundary they found this unusual spike in an element called iridium. Now iridium is really rare in terrestrial rocks but there's a lot of it in extraterrestrial rocks like meteorites and asteroids. And they found this same spike at the K-P-G boundary all over the globe. Which then led to a hypothesis that maybe it was a large meteorite impact that caused the mass extinction. But that much iridium to then spread it all over the globe this meteorite impact must have been huge. But we couldn't find the crater. Where was the crater? So in North America they were looking at these units with the spikes of iridium and they also found large amounts of shocked quartz. And shocked quartz only conformed during rapid increases in heat and pressure. So this helped narrow down the search for the crater to the North America. Scientists finally found the crater under sediments off the Yucatán Peninsula off today's Mexico. And using magnetic and gravity anomaly studies and also using drill cores they figured out that the crater was over 180 kilometers in diameter. Now given the size of the crater and also the concentration of iridium spikes all over the world scientists have been able to estimate that this meteorite that slammed into earth then was over 10 kilometers long. Now with a meteorite impact this big it would have ejected millions of tons of material into the atmosphere. This would have blocked sunlight and resulted in global cooling. And we can actually see evidence for this global cooling using fossilized molecules that have been preserved in the rock record. And using these molecules as a proxy for temperature scientists have been able to estimate that directly after the impact global ocean temperature dropped by five degrees. So during the same period we see massive shifts in the carbon isotopic record and this gives us an indicator of biological productivity. With the blockage of sunlight photosynthetic organisms were not getting the sunlight they need as their energy source which led to the complete collapse of primary productivity in the oceans. So bringing together all these different lines of geochemical and fossil evidence we were able to strengthen the hypothesis that it was a meteorite impact and then all the resulting changes to the earth's environments from this meteorite impact that led to the K-PG mass extinction.