Typically commonly occurring fossils that had a widespread geographic distribution such as brachiopods, trilobites, and ammonites work best as index fossils.
If the fossil you are trying to date occurs alongside one of these index fossils, then the fossil you are dating must fall into the age range of the index fossil. In a hypothetical example, a rock formation contains fossils of a type of brachiopod known to occur between 410 and 420 million years.
Where igneous rocks are absent, a fossil's "age" is determined by comparing the fossils of one location to those of another, and then comparing those comparisons to charts in books with age assignments provided on the pages.
There are two main methods determining a fossils age, relative dating and absolute dating.
Relative dating is used to determine a fossils approximate age by comparing it to similar rocks and fossils of known ages.
But when this admission of fault occurs, it often accompanies the introduction of a new and supposedly superior dating technique.
The problem is that the new technique has in the end always proven to be unreliable.
Scientists can use certain types of fossils referred to as index fossils to assist in relative dating via correlation.
Index fossils are fossils that are known to only occur within a very specific age range.
Three geologists have reported what they called the first "successful" direct dating of dinosaur bone.
Will this new radioisotope dating (or radiodating) technique solve the problems that plagued older dating methods?
The paper used one of the same excuses that have been used so many times when radiodated "ages" of given samples fail to line up with the accepted dates—that something tampered with the bone and thus skewed the results.
But what can assure readers that the particular test results chosen by the authors as acceptable were not themselves the result of tampering?
Using relative dating the fossil is compared to something for which an age is already known.