Changes

Because fossil species evolved through time in genetically related lineages and because extinction events were followed by new assemblages of species, fossils provide the best criteria for dividing the geological record into time intervals characterized by the first-appearance datums (FADs) and last-appearance datums (LADs) of key species ([[:file:paleontology_fig1.png|Figure 1]]). Fossil tops and bases may be synchronous in sections where the strata record continuous deposition and where suitable environmental conditions persisted.

Because fossil species evolved through time in genetically related lineages and because extinction events were followed by new assemblages of species, fossils provide the best criteria for dividing the geological record into time intervals characterized by the first-appearance datums (FADs) and last-appearance datums (LADs) of key species ([[:file:paleontology_fig1.png|Figure 1]]). Fossil tops and bases may be synchronous in sections where the strata record continuous deposition and where suitable environmental conditions persisted.

[[file:paleontology_fig2.png|thumb|{{figure number|2}}Example of geological time using the stratigraphic column of Cook Inlet basin, Alaska. Geological time table modified after van Eysinga<ref name=pt05r158>van Eysinga, F. W. B., 1975, Geologic Time Table: Amsterdam, Elsevier (chart).</ref>. Tertiary stages are from Wolfe<ref name=pt05r172>Wolfe, J. A., 1977, Paleogene floras from the Gulf of Alaska region: U., S. Geological Survey Professional Paper 997, p. 108.</ref>. (From <ref name=pt05r108>Magoon, L. B., Claypool, G. E., 1981, Petroleum geology of Cook Inlet Basin—an exploration model: AAPG Bulletin, v. 65, p. 1043–1061.</ref>.)]]

[[file:paleontology_fig2.png|thumb|{{figure number|2}}Example of geological time using the stratigraphic column of Cook Inlet basin, Alaska. Geological time table modified after van Eysinga<ref name=pt05r158>van Eysinga, F. W. B., 1975, Geologic Time Table: Amsterdam, Elsevier (chart).</ref>. Tertiary stages are from Wolfe<ref name=pt05r172>Wolfe, J. A., 1977, Paleogene floras from the Gulf of Alaska region: U., S. Geological Survey Professional Paper 997, p. 108.</ref>. (From <ref name=pt05r108>Magoon, L. B., Claypool, G. E., 1981, [http://archives.datapages.com/data/bulletns/1980-81/data/pg/0065/0006/1000/1043.htm Petroleum geology of Cook Inlet Basin—an exploration model]: AAPG Bulletin, v. 65, p. 1043–1061.</ref>.)]]

Key fossil datums in reference sections provide the basic division of the geological section into systems, series, and stages.<ref name=pt05r72>Hancock, J. M., 1977, The historic development of biostratigraphic correlation, in Kauffman, E. G., Hazel, J. E., eds., Concepts and Methods of Biostratigraphy: Stroudsburg, PA, Dowden, Hutchinson and Ross, p. 3–22.</ref> However, reference sections may not contain a complete record of sedimentation because the contacts between many strata are unconformable. Therefore, the continuous span of time is divided into eras, epochs, and ages ([[:file:paleontology_fig2.png|Figure 2]]). The intervals of geological time are calibrated to absolute time by means of radiometric or isotopic ages from interbedded or cross-cutting rocks such as volcanic flows and ash beds.

Key fossil datums in reference sections provide the basic division of the geological section into systems, series, and stages.<ref name=pt05r72>Hancock, J. M., 1977, The historic development of biostratigraphic correlation, in Kauffman, E. G., Hazel, J. E., eds., Concepts and Methods of Biostratigraphy: Stroudsburg, PA, Dowden, Hutchinson and Ross, p. 3–22.</ref> However, reference sections may not contain a complete record of sedimentation because the contacts between many strata are unconformable. Therefore, the continuous span of time is divided into eras, epochs, and ages ([[:file:paleontology_fig2.png|Figure 2]]). The intervals of geological time are calibrated to absolute time by means of radiometric or isotopic ages from interbedded or cross-cutting rocks such as volcanic flows and ash beds.