Misleading results can occur if the index fossils are incorrectly dated.Stratigraphy and biostratigraphy can in general provide only relative dating (A was before B), which is often sufficient for studying evolution.This is difficult for some time periods, however, because of the barriers involved in matching rocks of the same age across continents.
Stratigraphy is the science of understanding the strata, or layers, that form the sedimentary record.
Strata are differentiated from each other by their different colors or compositions and are exposed in cliffs, quarries, and river banks.
These rocks normally form relatively horizontal, parallel layers, with younger layers forming on top.
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Paleontology seeks to map out how life evolved across geologic time.A substantial hurdle is the difficulty of working out fossil ages.If a fossil is found between two layers of rock whose ages are known, the fossil's age is thought to be between those two known ages.Because rock sequences are not continuous, but may be broken up by faults or periods of erosion, it is difficult to match up rock beds that are not directly adjacent.The layers of sedimentary rock, or strata, can be seen as horizontal bands of differently colored or differently structured materials exposed in this cliff.The deeper layers are older than the layers found at the top, which aids in determining the relative age of fossils found within the strata. Such index fossils must be distinctive, globally distributed, and occupy a short time range to be useful.