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Paleontology seeks to map out how life evolved across geologic time.
There are several different methods for estimating the ages of fossils, including: Paleontologists rely on stratigraphy to date fossils.
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.
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.
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.