Relative and numerical dating techniques
The law of superimposition that the oldest fossils are found on the bottom is superseded by the law of decent with modification that the simplest fossils are the oldest and the youngest fossils are the most complex.
This is the basis of relative dating Fossils Out of Sequence Palaios June 1990 page 227 "We define stratigraphic disorder as the departure from perfect chronological order of fossils in a stratigraphic sequence, in which an older fossils occurs above a younger one. stratigraphic disorder at some scale is probably a common feature of the fossil record.
Comparing the quantity of the parent form and the byproduct will give a numerical value for the age of the material containing such isotopes.
Example include carbon14-nitrogen, uranium-led, uranium-thorium. Relative dating instead allows for identifying the sequential order of geological events one relative to the other.
Absolute dating is based on radioactive decay half lives.
The fossils which are buried deep inside the earth are more ancient.
While in the absolute dating, isotopes of carbon are used for dating fossils.
Fossil Frustrations New Scientist Nov 1983 " Ager Derek I think of no cases of radioactive decay being used to date fossils.
" Ultimately fossils are dated using Darwinian evolutionary theory.