Each of the Eras ends with a mass extinction. Period boundaries coincide with smaller extinction events, followed by appearances of new species. Fossils can be used to recognize the approximate age of a unit and its place in the stratigraphic column. They can also be used to correlate strata from place to place. It is determined by recording the occurrence of the fossils in numerous stratigraphic sequences from hundreds of locations.
Ranges are well known for some species, and poorly known for others.
Corals indicate tropical climates Plant fossils with aerial roots, which lack yearly rings, and which have large wood cell structure indicate tropical climates. Planktonic organisms vary in size and coiling direction according to temperature, for example, the foraminifer. Globorotalia Compositions of the skeletons, for example, shells in warmer waters have higher magnesium contents Oxygen isotope ratios in shells. Shells that are enriched in O-18 indicate times of glaciation. For more information on the Fossil Record, click here.