Index fossils
are remains of plants and animals known to have lived in a certain geologic time frame and are used to date rock layers that may not have clear dating from purely geological data. This is often referred to as biostratigraphy.
Characteristics
These fossils are typically species that had widespread distribution while extant and are easily identifiable evolutionary intermediaries. If a species showed quick evolution with derived features that are obvious, then each clear intermediate represents a short span of geologic time.
Common Index Fossils
Ammonites exemplify good index fossils; these extinct cephalopods were diverse in the Mesozoic, but only one known species survived from the Triassic to the Jurassic. They then show renewed diversification during the Jurassic until the Cretaceous, after which they stop appearing in the fossil record. This demarcated evolutionary history allows a geologist to quickly determine the general age of a stratum containing ammonite fossils.
Strengths
Their usefulness lies in the quickness with which a rock layer can be dated just with the confirmation of their presence. Finding index fossils in a rock layer in the field can save a researcher from having to spend the time and effort to remove a sample and take it to a laboratory for analysis.
Limitations
No matter how narrow the span of geologic time the appearance of an index fossil represents, they can only give a general sign as to the age of a stratum. More precise dating requires different, more involved techniques such as radiometric dating of isotopes in the rock sample.
Industry
Oil companies often use the presence of microfossils in exploration to determine the suitability of a field for drilling.
Tags: geologic time, index fossils, rock layer, span geologic, span geologic time
