definitions:
petrification
the process where an organism's remains are replaced by minerals, preserving them.
principle of faunal succession
sedimentary rock strata contain fossilized flora and fauna in a predictable, chronological order
index fossil
the remains of a plant or animal that was abundant, widely distributed geographically, and existed for only a short geological time span
radioactive isotope
isotope that eventually decays and becomes a different atom