The Age of Dinosaurs was so many millions of years ago that it is very difficult to date exactly.
Scientists use two kinds of dating techniques to work out the age of rocks and fossils. This considers the positions of the different rocks in sequence (in relation to each other) and the different types of fossil that are found in them.
Index fossils are fossils that can be used to date the rock in which they are found.
There are some radioactive elements in rock that decay by giving off energy and turning into different, more stable elements.
This radioactive decay takes place at a constant rate for each radioactive element.
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.
Ammonites, shelled relatives of today’s octopus, make ideal index fossils.
Suppose a dinosaur fossil has been found in the beds of an ancient delta (the mouth of a river leading to the sea).Using the potassium-argon method, Fitch and Miller were the first to measure the age of the tuff.Their result of 212–230 million years did not agree with the age of the fossils (elephant, pig, ape and tools) so they rejected the date.They said the sample was contaminated with excess argon.Using new samples of feldspar and pumice they ‘reliably dated’ the tuff at 2.61 million years, which agreed nicely.Then I surprised them, “The problem is that six hours is the wrong answer.” They look puzzled and disbelieving. If the result seems okay then he will happily accept it.