Absolute dating fossils
The sediment of this area was laid down after ammonite A appeared 199 million years ago, and before ammonite B became extinct 195 million years ago.
This narrows the date of the delta beds to the four million years between these dates.
The best examples are fossils of animals or plants that lived for a very short period of time and were found in a lot of places.
Different isotopes have different half-lives and sometimes more than one present isotope can be used to get an even more specific age of a fossil.
Below is a chart of commonly used radiometric isotopes, their half-lives, and the daughter isotopes they decay into.
By measuring the ratio of the amount of the original radioactive element to the daughter isotope, scientists can determine how many half-lives the element has undergone and from there can figure out the absolute age of the sample.
The half-lives of several radioactive isotopes are known and are used often to figure out the age of newly found fossils.