Changes

Because fossil species evolved through time in genetically related lineages and because extinction events were followed by new [[Fossil assemblage|assemblages]] of [http://evolution.berkeley.edu/evosite/evo101/VADefiningSpecies.shtml 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 [http://en.wiktionary.org/wiki/isochronous 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 [[Fossil assemblage|assemblages]] of [http://evolution.berkeley.edu/evosite/evo101/VADefiningSpecies.shtml 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 [http://en.wiktionary.org/wiki/isochronous synchronous] in sections where the strata record continuous deposition and where suitable environmental conditions persisted.

[[file:paleontology_fig2.png|thumb|300px|{{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 Magoon & Claypool.<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>)]]

[[file:paleontology_fig2.png|thumb|300px|{{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 Magoon & Claypool.<ref name=pt05r108>Magoon, L. B., and G. E. Claypool, 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 [[stratigraphic system|system]]s, [[stratigraphic series|series]], and [[stratigraphic stage|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 [[Unconformity|unconformable]]. Therefore, the continuous span of time is divided into [[Wikipedia:Era (geology)|eras]], [[Wikipedia:Epoch (geology)|epochs]], and [[Wikipedia:Age (geology)|ages]] ([[:file:paleontology_fig2.png|Figure 2]]). The intervals of geological time are calibrated to absolute time by means of [http://dictionary.reference.com/browse/radiometric+dating radiometric] or [http://geology.about.com/od/geotime_dating/a/timeyardstick.htm isotopic ages] from interbedded or cross-cutting rocks such as [http://geology.campus.ad.csulb.edu/people/bperry/IgneousRocksTour/VolcanoesAndLavaFlows.html volcanic flows] and [[Wikibooks:Historical_Geology/Volcanic_ash|ash beds]].

Key fossil datums in reference sections provide the basic division of the geological section into [[stratigraphic system|system]]s, [[stratigraphic series|series]], and [[stratigraphic stage|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 [[Unconformity|unconformable]]. Therefore, the continuous span of time is divided into [[Wikipedia:Era (geology)|eras]], [[Wikipedia:Epoch (geology)|epochs]], and [[Wikipedia:Age (geology)|ages]] ([[:file:paleontology_fig2.png|Figure 2]]). The intervals of geological time are calibrated to absolute time by means of [http://dictionary.reference.com/browse/radiometric+dating radiometric] or [http://geology.about.com/od/geotime_dating/a/timeyardstick.htm isotopic ages] from interbedded or cross-cutting rocks such as [http://geology.campus.ad.csulb.edu/people/bperry/IgneousRocksTour/VolcanoesAndLavaFlows.html volcanic flows] and [[Wikibooks:Historical_Geology/Volcanic_ash|ash beds]].