You will listen to part of a lecture and answer a question about it. After you h

Now listen to part of a lecture in an archeology class.
    Fossils provide us with our only direct record of prehistoric life. Studying them can help us to reconstruct the anatomy, behavior and evolution of long-extinct organisms. Perhaps less obviously, fossils are also among the most important sources of information for scientists attempting to learn about past climates and environments. By analyzing chemical makeup and mineral proportions found in fossilized organisms, scientists can determine whether past climates were warmer or cooler than present-day climates.
    An example of this can be seen in studying shellfish fossils. It turns out studying the shells of shellfish has shed some interesting light on what ancient climates were like. When an organism becomes fossilized, very important environmental data can be read in the mineral proportions and chemical makeup of the fossil. The shells of shellfish are particularly useful in painting an accurate picture of past climates. Specific climatic conditions, such as temperatures, humidity, and even greenhouse gas concentrations, can alter shell form, structure or features. Simply put, the shellfish shells develop according to the environment in which they live. It has been discovered that by examining magnesium levels found in shellfish fossils and comparing that data to the magnesium levels found in modern-day shellfish living in the same area, scientists are able to determine whether past climates were warmer or cooler than the climates of today. Lower levels of magnesium indicate that past climates were cooler, while higher levels of magnesium indicate that past climates were warmer.