Fossils and Time
Fossils are the remains or traces of prehistoric organisms that have been preserved within the rock record. Although most fossils are recovered from sedimentary rocks, on rare occasions some fossils may also be preserved in igneous materials, such as volcanic ash, or in low-grade metamorphic rock.
Fossils are categorized into two broad groups; body fossils and trace fossils.
Body fossils consist mostly of the hard, skeletal parts such as bones, shells, and teeth. In some rare cases soft body parts such as tissue and hair may also be preserved through freezing or mummification. Unlike body fossils trace fossils do not contain any remnants of the organism itself but are an indication of organic activity, such as tracks, trails, or burrows.
Body fossils may be preserved as either altered remains or unaltered remains. Unaltered remains, meaning the fossil retains their original composition, are produced when the organism is preserved in a material that does not allow for bacterial decay, such as in amber, tar, or ice. Altered remains form when the process of preservation causes some change in the composition or structure of the original biologic material.
In order to form either body or trace fossils the environment in which they form must allow for therir preservation. For body fossils this means that the organism must not be destroyed by bacterial decay or scavenging and trace fossils, such as tracks, must remain within a given sediment. In both cases preservation occurs most often through rapid burial processes and as such does not occur in every environment. As a result fossilization is an extremely rare occurrence.
Fossils and Time
The use of fossils in relative dating was first demonstrated in the 1800's by the English civil engineer William Smith. Through his work Smith noticed that the same vertical sequence of fossils could be found at many locations. Just like Steno in the past, Smith reasoned that in any vertical sequence of rock the oldest layer was on the bottom. He then concluded that the same must apply for any fossil found within the strata. Since the fossils once belonged to living organisms they must have been deposited at the same time as the sediment that produced the rock. Furthermore, sediment deposited during a certain time period can only contain fossils of organisms that were alive during that same time period. Eventually this observation would become the basis for the Principle of Fossil Succession. The principle of fossil succession states that fossil assemblages (groups of fossil found in a given strata) succeed one another in a regular and determinable order.
So why is this useful? In order for us to recreate what the world looked like during certain time periods we must be able to show that strata from distant locations were deposited during the same time periods. Although relative dating, as discussed thus far, allows us to determine the ages of events in a single vertical section of rock it does not allow us to compare the ages of rock in different distant locations. To further complicate this proccess and as discussed in the previous section, as environments change, such as in a marine transgression, the same sediment, such as the sandstone facies of the near shore environment, can be deposited as the facies moves across the surface of the Earth but it's deposition at each location occurs at different time periods. For example the Tapeats Sandstone, located at the base of the Grand Canyon, was deposited along the shore of a transgressing sea. This sandstone can be traced all the way from Arizona through to South Dakota, however it was deposited in S. Dakota millions of years after it was deposited in Arizona.
Making this even more difficult is our understanding of sedimentary facies which tells us that different sediments will be deposited simultaneously--meaning we can't just match up like rock types. Fortunately, living organisms are not particularly concerned with what type of sediment is deposited at the surface--dead organisms are even less concerned. What controls where organisms exist is the availability of resources and the environmental conditions of the region.