Fossils only contain radioactive Carbon 14 and not any other form of radioactivity. Carbon 14 can only be used for very recent fossils.
For example a fossil that has experienced 10 half lives has only 1/1024 of the original Carbon 14. This is .098 percent of the original carbon 14. So after 10 half lives approximately 57,000 years Carbon can no longer be used to date the rocks where it is found.
Fossils where the original organic has been replaced with minerals (stone) there is no radioactive material left to be used for dating.
Fossils can only be used to establish relative ages of fossil bearing strata. It is assumed that all life started out simple and has been increasing in complexity over time. This theory says that the rocks with the simpler fossils are the oldest and the rocks with more complex fossils are younger.
Also some fossils are declared to be index fossils. These fossils are believed to have only existed during certain relative ages of the earth. So if the index fossils is found the age of the rock is assumed to be of a certain age because of the index fossil. Some of these index fossils have been found to be still living.
Fossils can be used to determine a relative age of the strata. Fossils can provide evidence to indicate the age of the earth. Fossils can not prove the age of a rock except for fossils less than 50,000 years.