Guide to Fossils -Dating of Fossils

Dating of Fossils


Fossils are abundant in cliffs of Dover. This article is part of our Guide to Fossils.

Dating of fossils is done to understand the age of the fossils. Paleontology is the science which studies about fossils and it helps to map out how life evolved across the geologic time. A hurdle however is the difficulty to work out the fossil age.  The beds on which the fossils are preserved usually lack the radioactive elements which are needed for radiometric dating. This is one of the most reliable techniques which gives rocks a proper dating when above 50 million years an absolute age. It could be accurate to within 0.5% or even more accurate.

Radiometric dating needs careful laboratory work and its basic principle remains the same where the rates at which various radioactive elements decay is known. Thus, the ratio of radioactive element to its decay products shows the extent to which radioactive elements were incorporated into the rock. It is only common to see the presence of radioactive elements in rocks which have a volcanic origin, thus the only fossil bearing rocks which could be dated radiometrically are the volcanic ash layers which could provide termini for intervening sediments.

The scientists who study about fossils, paleonatologists rely on stratigraphy to date fossils. This is the science of deciphering the ‘layer cake’ which is a sedimentary record. The rocks generally form relatively horizontal layers with generally each layer being younger than the one underneath it. In case a fossil is found between two layers and the age of each layer is known, then the fossil would have an approximate age between that of the two fossils. As the rock sequences are not continuous then it may be broken by faults of erosion, then it gets very difficult to match up the rock beds which are not directly adjacent. However, the fossils of species which survived for a relatively short time could be used to match isolated rocks. This technique is biostatigraphy. In case rocks have an unknown age and have small amounts of E. pseudoplanus, it means that it could have a mid-Ordovician age. Thus, these kind of index fossils are distinctive and is useful. The results could be misleading if the index fossils have been incorrectly dated. The techniques of stratigraphy and biostratigraphy can only provide relative dating which is needed to study evolution. Challenges however occur when matching rocks of the different continents.

The possibility is also there on how to estimate two living clades diverged and this could be done with the study of DNA mutations which accumulated at a constant rate. However, these molecular clocks are fallible, providing only an approximate timing and may not be precise and reliable to estimate when the groups which feature in the Cambrian explosion first evolved.