We have looked at ten different bodyplans out of the 35 that we think exist today ranging from 1 to more than a million species. Most of the examples that I have given you are modern, animals that we can see around us today. But we know that the distribution of animals changes. We have records that there were wild boars and wolves around until fairly recently. If we go back before written history we find bones in caves belonging to lemmings and reindeer, of which we have no historical records.
Animals move about.
As you dig deeper the bones no longer look like old bones, they become heavier, a different colour and more mineralised as the calcium and phosphorous is leached out and replaced by salts from the surrounding rocks and soils - fossilised. As you go deeper you find more exotic animal bones appearing, hippos, lions, elephant-like animals, big cats. When these bones were deposited animals lived here that no longer live here today, and animals lived here that don't live anywhere today. As well as moving about animals have changed. We can therefore put together a chronology for the earth.