passage: Fossils can be used to match up rock layers .
  As organisms change over time , they look different .
  Older fossils will look different than younger fossils .
  Some organisms only survived for a short time before going extinct .
  Knowing what organisms looked like at certain times also helps date rock layers .
  Some fossils are better than others for this use .
  The fossils that are very distinct at certain times of Earths history are called index fossils .
  Index fossils are commonly used to match rock layers .
  You can see how this works in Figure 2.30 .
  If two rock layers have the same index fossils , then they 're probably about the same age .
query_and_answer: What helps to date rock layers ? || Artifacts in the rock
evidences: Fossils can be used to match up rock layers . Knowing what organisms looked like at certain times also helps date rock layers .