Older fossils usually view available hint(s)
are found above younger fossils
are found in the deepest strata
have the longest half-lives
contain more radioactive isotopes than younger fossils

Older fossils usually are found in the deepest strata.  

Explanation: The deeper the fossils are in the ground, the older that they are.  The reason is because layers of dirt, sand, and rock cover the fossils, and because they have been around for millions of years, it would give it all the time to cover up the fossils.  

organ system-an organ system is a group of organs that work together to perform one or more functions. each does a particular job in the body, and is made up of certain tissues.