2 9 15 Biostratigraphy using sets of fossils to organize rocks by age Fossils are the key to correlating rocks worldwide The fossil of any extinct organism marks two points in time first appearance and extinction Range Zone All of the eras periods epochs ages of the time scale are based on range zones of fossil groups William Smith was one of the first people to realize that fossils could be used to place rocks in order He worked as a surveyor planning routes for canal during early stages of industrialization of the UK Made the observation Each rock bed holds a specific SET of fossils If planning a canal route he could predict the rocks below the surface he knew the cost to dig A stratigraphic column is hypothetical not real The result of Smith s work The Principle of Faunal Succession 1 A group of animals will be common for a specific length of time appearance extinction 2 They will be replaced by another group and these will then become common for a time each fossil group assemblage defines that time 3 The order is regular once extinct they never re appear order is the same everywhere Biostratigraphy relies on the use of Index Fossils a fossil that can be assigned to a specific time range To be an index fossil four conditions must be satisfied 1 They must be abundant Most species are known from just a few to a few 100 fossils There are only 30 near complete T Rex fossils so not a good index fossil 2 They must be easy to identify no confusion 3 They have to have a wide geographic distribution 4 They have to evolve quickly so that individual species lived for only a short time span