To determine the ages of these specimens, scientists need an isotope with a very long half-life.
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.
The oldest fossils (of bacteria) are 3.8 billion years old.
The most widely known form of radiometric dating is carbon-14 dating.
See also index fossil, microfossil, trace fossil.fossilize verb The evidence in rock of the presence of a plant or an animal from an earlier geological period.Most plants are terrestrial (meaning that they live on land).They are generally incapable of moving around, and so are totally dependent on the atmosphere and the soil or rock (substrate) on which they grow.Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. 1979, 1986 © Harper Collins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012 Cite This Source (fŏs'əl) The remains or imprint of an organism from a previous geologic time.A fossil can consist of the preserved tissues of an organism, as when encased in amber, ice, or pitch, or more commonly of the hardened relic of such tissues, as when organic matter is replaced by dissolved minerals.
Fossils can't form in the igneous rock that usually does contain the isotopes.