75% species go extinct to be considered a mass extinction
Earth changes all the time, making it a <u>dynamic </u>planet.
Fossils can be found in the Earth's crust.
The crust is the thinnest layer of the Earth. The fossils are most probably be found in this layer because it is the outermost layer. Also, the deeper the layer of the Earth, the higher the temperature that it has, and it probably cannot preserve the fossil.
