Some of the isotopes used for this purpose are uranium-238, uranium-235 and potassium-40, each of which has a half-life of more than a million years.Unfortunately, these elements don't exist in dinosaur fossils themselves.to time that is used by geologists, paleontologists and other earth scientists to describe the timing and relationships between events that have occurred during the history of the Earth.The table of geologic time spans presented here agrees with the dates and nomenclature proposed by the International Commission on Evidence from radiometric dating indicates that the Earth is about 4.570 billion years old.
Different spans of time on the time scale are usually delimited by major geological or period is defined by the Cretaceous Tertiary extinction event, which marked the demise of the dinosaurs and of many marine species.
Dinosaur bones, on the other hand, are millions of years old -- some fossils are billions of years old.
To determine the ages of these specimens, scientists need an isotope with a very long half-life.
If, in the year AD 1600, you had asked an educated European how old the planet Earth was and to recount its history he would have said that it was about 6000 years old and that its ancient history was given by the biblical account in Genesis.
If you asked the same question of an educated European in AD 1900 you would have received a quite different answer.