Editing Fossil (section)

== Dating/Age ==

=== Stratigraphy and estimations ===
{{Main|Geochronology|Relative dating}}
[[File:Montañita-Olón locality (stratigraphy) - Dos Bocas Formation.png|thumb|Stratigraphy of the Montañita-Olón locality of the [[Dos Bocas Formation]]. Stratigraphy is a useful branch when it comes to the understanding of the successive layers of rock and their fossiliferous content, giving insight into the relative age of fossils]]
Paleontology seeks to map out how life evolved across geologic time. A substantial hurdle is the difficulty of working out fossil ages. Beds that preserve fossils typically lack the radioactive elements needed for [[radiometric dating]]. This technique is our only means of giving rocks greater than about 50&nbsp;million years old an absolute age, and can be accurate to within 0.5% or better.<ref name="Martin2000">{{Cite journal | author1 = Martin, M.W. | author2 = Grazhdankin, D.V. |author3=Bowring, S.A. |author4=Evans, D.A.D. |author5=Fedonkin, M.A. |author6=Kirschvink, J.L. | s2cid = 1019572 | date = 5 May 2000 | title = Age of Neoproterozoic Bilaterian Body and Trace Fossils, White Sea, Russia: Implications for Metazoan Evolution | journal = Science | volume = 288 | issue = 5467 | pages = 841–5 | doi = 10.1126/science.288.5467.841 | pmid = 10797002 |bibcode = 2000Sci...288..841M}}</ref> Although radiometric dating requires careful laboratory work, its basic principle is simple: the rates at which various radioactive elements [[radioactive decay|decay]] are known, and so the ratio of the radioactive element to its decay products shows how long ago the radioactive element was incorporated into the rock. Radioactive elements are common only in rocks with a volcanic origin, and so the only fossil-bearing rocks that can be dated radiometrically are volcanic ash layers, which may provide termini for the intervening sediments.<ref name="Martin2000" />

Consequently, palaeontologists rely on [[stratigraphy]] to date fossils. Stratigraphy is the science of deciphering the "layer-cake" that is the [[sediment]]ary record.<ref>{{Cite journal |author1=Pufahl, P.K. |author2=Grimm, K.A. |author3=Abed, A.M. |author4=Sadaqah, R.M.Y. |name-list-style=amp | title=Upper Cretaceous (Campanian) phosphorites in Jordan: implications for the formation of a south Tethyan phosphorite giant | journal=[[Sedimentary Geology (journal)|Sedimentary Geology]] | volume=161 |date=October 2003 | pages=175–205 | doi=10.1016/S0037-0738(03)00070-8 | issue=3–4 | bibcode=2003SedG..161..175P}}</ref> Rocks normally form relatively horizontal layers, with each layer younger than the one underneath it. If a fossil is found between two layers whose ages are known, the fossil's age is claimed to lie between the two known ages.<ref>{{cite web | url=http://pubs.usgs.gov/gip/geotime/radiometric.html | access-date=20 September 2008 | title=Geologic Time: Radiometric Time Scale | publisher=U.S. Geological Survey | archive-url=https://web.archive.org/web/20080921135337/http://pubs.usgs.gov/gip/geotime/radiometric.html | archive-date=21 September 2008 | url-status=live }}</ref> Because rock sequences are not continuous, but may be broken up by [[fault (geology)|faults]] or periods of [[erosion]], it is very difficult to match up rock beds that are not directly adjacent. However, fossils of species that survived for a relatively short time can be used to match isolated rocks: this technique is called ''biostratigraphy''. For instance, the conodont ''Eoplacognathus pseudoplanus'' has a short range in the Middle Ordovician period.<ref>{{Cite journal | author = Löfgren, A. | year = 2004 | title = The conodont fauna in the Middle Ordovician ''Eoplacognathus pseudoplanus'' Zone of Baltoscandia | journal = Geological Magazine | volume = 141 | issue = 4 | pages = 505–524
| doi = 10.1017/S0016756804009227 | doi-broken-date = 21 January 2025 | bibcode = 2004GeoM..141..505L | s2cid = 129600604 }}</ref> If rocks of unknown age have traces of ''E. pseudoplanus'', they have a mid-Ordovician age. Such [[index fossil]]s must be distinctive, be globally distributed and occupy a short time range to be useful. Misleading results are produced if the index fossils are incorrectly dated.<ref name=Gehling2001>{{Cite journal | last1 = Gehling | first1 = James | last2 = Jensen | first2 = Sören | last3 = Droser | first3 = Mary | last4 = Myrow | first4 = Paul | last5 = Narbonne | first5 = Guy | title = Burrowing below the basal Cambrian GSSP, Fortune Head, Newfoundland | journal = Geological Magazine | volume = 138 | issue = 2 | pages = 213–218 | date = March 2001 | doi = 10.1017/S001675680100509X | bibcode = 2001GeoM..138..213G | s2cid = 131211543 }}</ref> Stratigraphy and biostratigraphy can in general provide only relative dating (''A'' was before ''B''), which is often sufficient for studying evolution. However, this is difficult for some time periods, because of the problems involved in matching rocks of the same age across [[continent]]s.<ref name="Gehling2001" /> Family-tree relationships also help to narrow down the date when lineages first appeared. For instance, if fossils of B or C date to X&nbsp;million years ago and the calculated "family tree" says A was an ancestor of B and C, then A must have evolved earlier.

It is also possible to estimate how long ago two living clades diverged (i.e., the age of their [[last common ancestor]]) by assuming that [[mutation]]s accumulate at a constant rate for a given gene. These "[[molecular clock]]s", however, are fallible, and provide only approximate timing: for example, they are not sufficiently precise and reliable for estimating when the groups that feature in the [[Cambrian explosion]] first evolved,<ref>{{Cite journal |author1=Hug, L.A. |author2=Roger, A.J. | title=The Impact of Fossils and Taxon Sampling on Ancient Molecular Dating Analyses | journal=Molecular Biology and Evolution | year=2007 | volume=24 | issue=8 | pages=889–1897 | doi=10.1093/molbev/msm115 | pmid=17556757| doi-access=free }}</ref> and estimates produced by different techniques may vary by a factor of two.<ref name="PetersonEtAl2005">{{Cite journal | doi = 10.1073/pnas.0503660102 | pmid = 15983372 |author1=Peterson, Kevin J. |author2=Butterfield, N.J. | journal = Proceedings of the National Academy of Sciences | volume = 102 | issue = 27 | pages = 9547–52 | year = 2005
| title = Origin of the Eumetazoa: Testing ecological predictions of molecular clocks against the Proterozoic fossil record | pmc = 1172262 |bibcode=2005PNAS..102.9547P| doi-access = free }}</ref>

=== Limitations <span class="anchor" id="Fossil record limitations"></span>===
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{{Further|Ghost lineage|Signor–Lipps effect|Biostratigraphy}}

Organisms are only rarely preserved as fossils in the best of circumstances, and only a fraction of such fossils have been discovered. This is illustrated by the fact that the number of species known through the fossil record is less than 5% of the number of known living species, suggesting that the number of species known through fossils must be far less than 1% of all the species that have ever lived.<ref name=Prothero2007pp5053>{{cite book |author-link=Donald Prothero |last=Prothero |first=Donald R. |date=2007 |title=Evolution: What the Fossils Say and Why It Matters |url=https://archive.org/details/evolutionwhatfos00prot_0/page/50 |url-access=registration |pages=[https://archive.org/details/evolutionwhatfos00prot_0/page/50 50–53] |publisher=[[Columbia University Press]] |isbn=978-0-231-51142-1 }}</ref> Because of the specialized and rare circumstances required for a biological structure to fossilize, only a small percentage of life-forms can be expected to be represented in discoveries, and each discovery represents only a snapshot of the process of evolution. The transition itself can only be illustrated and corroborated by transitional fossils, which are never guaranteed to demonstrate a convenient half-way point.<ref name="CC200">{{cite web|url = http://www.talkorigins.org/indexcc/CC/CC200.html|title = Claim CC200: There are no transitional fossils.|access-date = 30 April 2009|date = 5 November 2006|publisher = [[TalkOrigins Archive]]|last = Isaak|first = M|archive-url = https://web.archive.org/web/20090227065512/http://www.talkorigins.org/indexcc/CC/CC200.html|archive-date = 27 February 2009|url-status = live}}</ref>

The fossil record is strongly biased toward organisms with hard parts, leaving most groups of [[soft-bodied organism]]s with little to no presence.<ref name=Prothero2007pp5053 /> It is replete with [[Mollusca|mollusks]], [[vertebrate]]s, [[echinoderm]]s, [[brachiopod]]s, and some groups of [[arthropod]]s.<ref>{{cite book|editor=Donovan, S. K.|editor2=Paul, C. R. C. |date= 1998 |title= The Adequacy of the Fossil Record |publisher=Wiley |location= New York |page= 312 |isbn=978-0-471-96988-4}}{{page needed|date=August 2014}}</ref>