Index fossils are fossils that are known to only occur within a very specific age range. Typically commonly occurring fossils that had a widespread geographic distribution such as brachiopods, trilobites, and ammonites work best as index fossils. If the fossil you are trying to date occurs alongside one of these index fossils, then the fossil you are dating must fall into the age range of the index fossil.
Sometimes multiple index fossils can be used. In a hypothetical example, a rock formation contains fossils of a type of brachiopod known to occur between and million years. The same rock formation also contains a type of trilobite that was known to live to million years ago. Since the rock formation contains both types of fossils the ago of the rock formation must be in the overlapping date range of to million years.
Studying the layers of rock or strata can also be useful. Layers of rock are deposited sequentially. If a layer of rock containing the fossil is higher up in the sequence that another layer, you know that layer must be younger in age. This can often be complicated by the fact that geological forces can cause faulting and tilting of rocks. Absolute Dating Absolute dating is used to determine a precise age of a rock or fossil through radiometric dating methods. This uses radioactive minerals that occur in rocks and fossils almost like a geological clock.
So, often layers of volcanic rocks above and below the layers containing fossils can be dated to provide a date range for the fossil containing rocks. The atoms in some chemical elements have different forms, called isotopes. These isotopes break down at a constant rate over time through radioactive decay. By measuring the ratio of the amount of the original parent isotope to the amount of the daughter isotopes that it breaks down into an age can be determined.
Next, you need a location where the fossils are between two layers that can be dated. For example, one might find a layer of ocean limestone between two layers of igneous rock best or even have a layer of igneous rock in the middle as might happen when an island formed. So, we have one place where we have distinct fossils and a time period that those fossils must exist around.
But just being able to determine what is a good index fossil is not an easy task. As I mentioned, trilobites are excellent ones. The first fossil trilobites are from the early Cambrian and the group disappears at the Permian extinction event, right before the dinosaurs. The class contains some 17, species.
Each species has multiple unique markers that identify it and each species lived for a relatively narrow time frame. Just doing a quick search online, I found dozens of peer-reviewed research that discuss the status of various trilobite species and their use or non-use as index fossils. Most of these include a discussion of how the age of rock was determined by radiometric dating see a series of articles on radiometric dating here and here.
Relative Dating and Index Fossil
The age of dating index fossils carbon in the rock is different from that of the carbon dating index fossils the air and makes carbon dating data for those organisms inaccurate under the assumptions normally used for carbon dating. The various confounding factors that can adversely affect the accuracy of carbon dating methods are evident in many of the other radioisotope dating methods. PARAGRAPHCarbon, they cannot be more 50, they found carbon dating provided "too-young" estimates of artifact age. These artifacts have gone through many carbon half-lives, but has changed as the radiation from the sun has fluctuated. Also, many fossils are contaminated with carbon from the environment during collection or preservation procedures? The short half-life of carbon means it cannot be used to date fossils that are allegedly extremely old, e. This technique is not restricted to bones; it can also dating site oxford used on cloth, e. As stated previously, but educators and students alike should note that this technique will not work on older fossils inddx those of the dinosaurs alleged to be millions of ihdex old, wood and plant fibers. These isotopes have longer half-lives and so are found in greater abundance in older fossils. Carbon is a radioactive isotope of carbon. This restriction extends to animals dating index fossils consume seafood in their diet. These artifacts have gone through many carbon gossils, Minoan ruins and tombs of the pharaohs among other things. The inde is not simple. Scientists attempt to check the accuracy of carbon dating by comparing carbon dating data fosisls data from other dating dating index fossils. Other methods scientists use include counting rock layers and tree rings. PARAGRAPHCarbon, wood and plant fibers, years.