Fossil
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2008-07-15T01:25:13Z
Michael Johnson
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/* Permineralization */ red links are how we get new articles
{{otheruses}}
[[Image:Amonite Cropped.jpg|thumb|200px|Three small [[ammonite]] fossils, each approximately 1.5 cm across.]]
[[Image:Priscacara-liops.jpg|right|thumb|200px|Eocene fossil fish ''Priscacara liops'' from [[Green River Formation]] of Utah]]
[[Image:Petrified forest log 2 md.jpg|thumb|right|200px|[[Petrified wood]]. The internal structure of the tree and bark are maintained in the [[#Permineralization|permineralization]] process.]]
'''Fossils''' (from [[Latin]] ''fossus'', literally "having been dug up") are the preserved remains or [[trace fossil|traces]] of animals, plants, and other organisms from the remote past. The totality of fossils, both discovered and undiscovered, and their placement in '''fossiliferous''' (fossil-containing) [[Rock (geology)|rock]] formations and [[sedimentary rock|sedimentary]] layers ([[Stratum|strata]]) is known as the ''fossil record''. The study of fossils across [[geologic time scale|geological time]], how they were formed, and the [[evolution]]ary relationships between [[taxon|taxa]] ([[phylogenetics|phylogeny]]) are some of the most important functions of the science of [[paleontology]].
Fossils are typically distinguished by minimum age, most often the arbitrary date of 10,000 years ago.<ref> [http://www.sdnhm.org/research/paleontology/paleofaq.html Frequently Asked Questions about Paleontology. San Diego Natural History Museum]</ref> Hence, fossils range in age from the youngest at the start of the [[Holocene]] Epoch to the oldest from the [[Archaean]] Eon several [[1000000000 (number)|billion]] years old. The observations that certain fossils were associated with certain rock [[strata]] led early geologists to recognize a geological timescale in the 19th century. The development of [[radiometric dating]] techniques in the early 20th century allowed geologists to determine the numerical or ''"absolute" age'' of the various strata and thereby the included fossils.
Like [[extant]] organisms, fossils vary in size from [[microscope|microscopic]], such as single bacterial cells <ref>
{{cite journal
| last = Westall
| first = Frances
| coauthors = ''et al.''
| title = Early Archean fossil bacteria and biofilms in hydrothermally-influenced sediments from the Barberton greenstone belt, South Africa
| journal = Precambrian Research
| volume = 106 | issue = 1-2 | pages = 93–116
| url = http://ieg.or.kr:8080/abstractII/E0210601006.html
| doi = 10.1016/S0301-9268(00)00127-3
| year = 2001}}
</ref>
only one [[micrometer]] in diameter, to gigantic, such as [[dinosaur]]s and trees many meters long and weighing many tons. A fossil normally preserves only a portion of the deceased organism, usually that portion that was partially [[Mineralization|mineralized]] during life, such as the [[bone]]s and teeth of [[vertebrate]]s, or the [[chitin]]ous [[exoskeleton]]s of [[invertebrate]]s. Preservation of soft tissues is exquisitely rare in the fossil record. Fossils may also consist of the marks left behind by the organism while it was alive, such as the footprint or [[feces]] ([[coprolites]]) of a [[reptile]]. These types of fossil are called [[trace fossil]]s (or ''ichnofossils''), as opposed to ''body fossils''. Finally, [[prehistoric life|past life]] leaves some markers that cannot be seen but can be detected in the form of [[biochemistry|biochemical]] signals; these are known as ''chemofossils'' or ''biomarkers''.
== Places of exceptional fossilization ==
Fossil sites with exceptional preservation — sometimes including preserved soft tissues — are known as [[Lagerstätte]]n. These formations may have resulted from carcass burial in an [[hypoxia (environmental)|anoxic]] environment with minimal bacteria, thus delaying decomposition. Lagerstätten span [[geology|geological]] time from the [[Cambrian]] period to the [[Holocene|present]]. Worldwide, some of the best examples of near-perfect fossilization are the [[Cambrian]] [[Maotianshan shales]] and [[Burgess Shale]], the [[Devonian]] [[Hunsrück Slates]], the [[Jurassic]] [[Solnhofen limestone]], and the [[Carboniferous]] [[Mazon Creek]] localities.
== Earliest fossiliferous sites ==
[[Image:Proterozoic Stromatolites.jpg|thumb|right|200px| Lower [[Proterozoic]] [[Stromatolites]] from Bolivia, South America]]
Earth’s oldest fossils are the [[stromatolite]]s consisting of rock built from layer upon layer of [[sediment]] and precipitants.<ref>{{cite web| url=http://www.fossilmuseum.net/Tree_of_Life/Stromatolites.htm| title=Stromatolites, the Oldest Fossils| accessdate=2007-03-04}}</ref> Based on studies of now-rare (but living) stromatolites (specifically, certain [[cyanobacteria|blue-green bacteria]]), the growth of fossil stromatolitic structures was biogenetically mediated by mats of [[microorganism]]s through their entrapment of sediments. However, [[abiotic]] mechanisms for stromatolitic growth are also known, leading to a decades-long and sometimes-contentious scientific debate regarding biogenesis of certain formations, especially those from the lower to middle [[Archaean]] eon.
It is most widely accepted that stromatolites from the late Archaean and through the middle [[Proterozoic]] eon were mostly formed by massive [[colony|colonies]] of [[cyanobacteria]] (formerly known as blue-green "algae"), and that the [[oxygen]] byproduct of their [[photosynthesis|photosynthetic]] [[metabolism]] first resulted in earth’s massive [[banded iron formation]]s and subsequently oxygenated earth’s atmosphere.
Even though it is extra rare, microstructures resembling [[Cell (biology)|cells]] are sometimes found within stromatolites; but these are also the source of scientific contention. The [[Gunflint Chert]] contains abundant [[microfossil]]s widely accepted as a diverse consortium of 2.0 [[bya]] [[microbe]]s.<ref>Knoll, A. H., Barghorn, E.S, Awramik, S.M,. (1978). New organisms from the Aphebian Gunflint Iron Formation. Journal of Paleontology(52), 1074-1082.</ref>
In contrast, putative fossil cyanobacteria cells from the 3.4 bya [[Warrawoona Group]] in Western Australia are in dispute since abiotic processes cannot be ruled out.<ref>Lowe, D. R. (1994). Abiological origin of described stromatolites older than 3.2 Ga. Geology, 22, 387-390</ref> Confirmation of the Warrawoona microstructures as cyanobacteria would profoundly impact our understanding of when and how [[origin of life|early life]] diversified, pushing important [[evolution]]ary milestones further back in time (reference). The continued study of these oldest fossils is paramount to calibrate complementary [[molecular]] [[phylogenetic]]s models.
== Developments in interpretation of the fossil record ==
{{seealso|History of paleontology}}
[[Image:Silurian Orthoceras Fossil Macro 2.JPG|thumb|right|200px|[[Silurian]] [[Orthoceras]] Fossil]]
Ever since recorded [[history]] began, and probably before, people have found fossils, pieces of [[Rock (geology)|rock]] and [[mineral]]s which have replaced the remains of biologic organisms or preserved their external form. These fossils, and the totality of their occurrence within the sequence of Earth's rock [[Stratum|strata]] is referred to as the fossil record.
The fossil record was one of the early sources of data relevant to the study of [[evolution]] and continues to be relevant to the [[Timeline of evolution|history of life on Earth]]. [[Paleontologist]]s examine the fossil record in order to understand the process of evolution and the way particular [[species]] have evolved.
===Explanations===
[[Image:Fossil shrimp.jpg|thumb|right|200px|Fossil [[shrimp]] ([[Cretaceous]])]]
[[Image:CyprusPlioceneGastropod.JPG|thumb|right|200px|A fossil [[gastropod]] from the Pliocene of [[Cyprus]]. A serpulid worm is attached.]]
Various explanations have been put forth throughout history to explain what fossils are and how they came to be where they were found. Many of these explanations relied on folktales or mythologies. In China the fossil bones of ancient mammals including ''[[Homo erectus]]'' were often mistaken for “[[dragon]] bones” and used as medicine and aphrodisiacs. In the West the presence of fossilized sea creatures high up on mountainsides was seen as proof of the [[Noah's Ark|biblical deluge]]. More scientific views of fossils began to emerge during the Renaissance. For example, [[Leonardo Da Vinci]] noticed discrepancies with the use of the biblical flood narrative as an explanation for fossil origins:
::"If the Deluge had carried the shells for distances of three and four hundred miles from the sea it would have carried them mixed with various other natural objects all heaped up together; but even at such distances from the sea we see the oysters all together and also the shellfish and the cuttlefish and all the other shells which congregate together, found all together dead; and the solitary shells are found apart from one another as we see them every day on the sea-shores.
::And we find oysters together in very large families, among which some may be seen with their shells still joined together, indicating that they were left there by the sea and that they were still living when the strait of Gibraltar was cut through. In the mountains of Parma and Piacenza multitudes of shells and corals with holes may be seen still sticking to the rocks..."<ref>{{cite book | last =da Vinci | first =Leonardo | authorlink =Leonardo da Vinci | title =The Notebooks of Leonardo Da Vinci | publisher =Reynal & Hitchcock | date = 1938/1956 | location =London | page =335 | url =http://books.google.com/books?id=qMoQAAAAIAAJ&q=%22If+the+Deluge+had+carried%22&dq=%22If+the+Deluge+had+carried%22&ei=F6ZWSPPYIaXOjgH-idyPDA&pgis=1| isbn = (OCLC) 67650193 }}</ref>
[[William Smith (geologist)|William Smith (1769-1839)]], an English canal engineer, observed that rocks of different ages (based on the [[law of superposition]]) preserved different assemblages of fossils, and that these assemblages succeeded one another in a regular and determinable order. He observed that rocks from distant locations could be correlated based on the fossils they contained. He termed this the principle of faunal succession.
Smith, who preceded [[Charles Darwin]], was unaware of biological evolution and did not know why faunal succession occurred. Biological evolution explains why faunal succession exists: as different organisms evolve, change and go extinct, they leave behind fossils. Faunal succession was one of the chief pieces of evidence cited by Darwin that biological evolution had occurred.
===Biological explanations===
Early [[Natural science|naturalists]] well understood the similarities and differences of living species leading [[Carolus Linnaeus|Linnaeus]] to develop a hierarchical classification system still in use today. It was Darwin and his contemporaries who first linked the hierarchical structure of the great tree of life in living organisms with the then very sparse fossil record. Darwin eloquently described a process of descent with modification, or evolution, whereby organisms either adapt to natural and changing environmental pressures, or they perish.
[[Image:Petrified Araucaria cone from patagonia-Edit3.jpg|thumb|right|250px|Petrified cone of ''Araucaria'' sp. from [[Patagonia]], [[Argentina]] dating from the [[Jurassic|Jurassic Period]] (approx. 210 mya)]]
When Charles Darwin wrote ''[[On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life]]'', the oldest animal fossils were those from the [[Cambrian Period]], now known to be about 540 million years old. The absence of older fossils worried Darwin about the implications for the validity of his theories, but he expressed hope that such fossils would be found, noting that: "only a small portion of the world is known with accuracy." Darwin also pondered the sudden appearance of many groups (i.e. [[phylum|phyla]]) in the oldest known Cambrian fossiliferous strata.<ref>Darwin, C (1859) On the Origin of Species. Chapter 10: On the Imperfection of the Geological Record.</ref>
===Further discoveries===
Since Darwin's time, the fossil record has been pushed back to between 2.3 and 3.5 billion years before the present.<ref>Schopf JW (1999) Cradle of Life: The Discovery of the Earth's Earliest Fossils, Princeton University Press, Princeton, NJ.</ref> Most of these Precambrian fossils are microscopic bacteria or [[Micropaleontology|microfossils]]. However, macroscopic fossils are now known from the late [[Proterozoic]]. The [[Ediacaran biota]] (also called Vendian biota) dating from 575 million years ago collectively constitutes a richly diverse assembly of early multicellular [[eukaryote]]s.
The fossil record and faunal succession form the basis of the science of [[biostratigraphy]] or determining the age of rocks based on the fossils they contain. For the first 150 years of [[geology]], biostratigraphy and superposition were the only means for determining the [[Relative dating|relative age]] of rocks. The [[geologic time scale]] was developed based on the relative ages of rock strata as determined by the early paleontologists and [[stratigrapher]]s.
Since the early years of the twentieth century, [[absolute dating]] methods, such as [[radiometric dating]] (including [[potassium-argon dating|potassium/argon]], [[argon-argon dating|argon/argon]], [[uranium-lead dating|uranium series]], and [[carbon-14 dating]]) have been used to verify the relative ages obtained by fossils and to provide absolute ages for many fossils. Radiometric dating has shown that the earliest known [[stromatolites]] are over 3.4 billion years old. Various dating methods have been used and are used today depending on local geology and context, and while there is some variance in the results from these [[dating methods]], nearly all of them provide evidence for a [[Age of the Earth|very old Earth]], approximately 4.6 billion years.
===Modern view===
"The fossil record is life’s evolutionary epic that unfolded over four billion years as environmental conditions and genetic potential interacted in accordance with natural selection."<ref>{{cite web| url=http://www.fossilmuseum.net/| title=The Virtual Fossil Museum - Fossils Across Geological Time and Evolution| accessdate=2007-03-04}}</ref> The earth’s climate, tectonics, atmosphere, oceans, and periodic disasters invoked the primary selective pressures on all organisms, which they either adapted to, or they perished with or without leaving descendants. Modern paleontology has joined with evolutionary biology to share the interdisciplinary task of unfolding the tree of life, which inevitably leads backwards in time to the microscopic life of the Precambrian when cell structure and functions evolved. Earth’s deep time in the Proterozoic and deeper still in the Archaean is only "recounted by microscopic fossils and subtle chemical signals."<ref>Knoll, A, (2003) Life on a Young Planet. (Princeton University Press, Princeton, NJ)</ref> Molecular biologists, using phylogenetics, can compare protein amino acid or nucleotide sequence homology (i.e., similarity) to infer taxonomy and evolutionary distances among organisms, but with limited statistical confidence. The study of fossils, on the other hand, can more specifically pinpoint when and in what organism branching occurred in the tree of life. Modern phylogenetics and paleontology work together in the clarification of science’s still dim view of the appearance of life and its evolution during deep time on earth.<ref>Paul CRC and Donovan SK, (1998) An overview of the completeness of the fossil record. in The Adequacy of the Fossil Record (Paul CRC and Donovan SK eds). 111-131 (John Wiley, New York).</ref>
[[Image:Eldredgeops-rana-crassituberculata.jpg|thumb|250px|Phacopid trilobite ''Eldredgeops rana crassituberculata'' named after Niles Eldredge]]
[[Image:JurassicMarineIsrael.JPG|thumb|220px|left|[[Gastropod]] and attached mytilid [[bivalve]]s on a [[Jurassic]] limestone bedding plane in southern [[Israel]].]]
[[Niles Eldredge|Niles Eldredge’s]] study of the Phacops trilobite genus supported the hypothesis that modifications to the arrangement of the trilobite’s eye lenses proceeded by fits and starts over millions of years during the [[Devonian]].<ref>Fortey R, Trilobite!: Eyewitness to Evolution. Alfred A. Knopf, New York, 2000.</ref> Eldredge's interpretation of the Phacops fossil record was that the aftermaths of the lens changes, but not the rapidly occurring evolutionary process, were fossilized. This and other data led [[Stephen Jay Gould]] and [[Niles Eldredge]] to publish the seminal paper on [[punctuated equilibrium]] in 1971.
===Example of modern development===
An example of modern paleontological progress is the application of [[synchrotron]] [[X-ray]] [[Tomography|tomographic]] techniques to early Cambrian bilaterian [[embryo]]nic microfossils that has recently yielded new insights of [[metazoan]] evolution at its earliest stages. The tomography technique provides previously unattainable three-dimensional resolution at the limits of fossilization. Fossils of two enigmatic bilaterians, the worm-like ''[[Markuelia]]'' and a putative, primitive [[protostome]], ''[[Pseudooides]]'', provide a peek at [[germ layer]] embryonic development. These 543-million-year-old embryos support the emergence of some aspects of [[arthropod]] development earlier than previously thought in the late [[Proterozoic]]. The preserved embryos from [[China]] and [[Siberia]] underwent rapid [[Diagenesis|diagenetic]] phosphatization resulting in exquisite preservation, including cell structures. This research is a notable example of how knowledge encoded by the fossil record continues to contribute otherwise unattainable information on the emergence and development of life on Earth. For example, the research suggests ''Markuelia'' has closest affinity to priapulid worms, and is adjacent to the evolutionary branching of [[Priapulida]], [[Nematoda]] and [[Arthropoda]].<ref>Donoghue, PCJ, Bengtson, S, Dong, X, Gostling NJ, Huldtgren, T, Cunningham, JA, Yin, C, Yue, Z, Peng, F and Stampanoni, M (2006) Synchrotron X-ray tomographic microscopy of fossil embryos. Nature 442, 680-683</ref>
==Rarity of fossils==
[[Image:Carcharodontosaurus and Megalodon teeth.jpg|thumb|[[Megalodon]] and [[Carcharodontosaurus]] Teeth. The Charcharodontosaurus tooth was found in the Sahara Desert.]]
Fossilization is an exceptionally rare occurrence, because most components of formerly-living things tend to decompose relatively quickly following death. In order for an organism to be fossilized, the remains normally need to be covered by [[sediment]] as soon as possible. However there are exceptions to this, such as if an organism becomes frozen, [[Desiccation|desiccated]], or comes to rest in an [[Anoxic sea water|anoxic]] ([[oxygen]]-free) environment. There are several different types of fossils and fossilization processes.
Due to the combined effect of [[taphonomy|taphonomic processes]] and simple mathematical chance, fossilization tends to favor organisms with hard body parts, those that were widespread, and those that lived for a long time. On the other hand, it is very unusual to find fossils of small, soft bodied, geographically restricted and geologically ephemeral organisms, because of their relative rarity and low likelihood of preservation.
Larger specimens ([[macrofossil]]s) are more often observed, dug up and displayed, although microscopic remains ([[microfossil]]s) are actually far more common in the fossil record.
Some casual observers have been perplexed by the rarity of [[Transitional fossil|transitional species]] within the fossil record. The conventional explanation for this rarity was given by [[Charles Darwin|Darwin]], who stated that "the extreme imperfection of the geological record," combined with the short duration and narrow geographical range of transitional species, made it unlikely that many such fossils would be found. Simply put, the conditions under which fossilization takes place are quite rare; and it is highly unlikely that any given organism will leave behind a fossil. Eldredge and Gould developed their theory of [[punctuated equilibrium]] in part to explain the pattern of stasis and sudden appearance in the fossil record.
==Types of preservation==
===Permineralization===
[[Image:Trilobite2.jpg|thumb|A permineralized [[trilobite]], ''[[Asaphus kowalewskii]]'']]
[[Permineralization]] occurs after burial, as the empty spaces within an organism (spaces filled with liquid or gas during life) become filled with mineral-rich groundwater and the minerals precipitate from the groundwater, thus occupying the empty spaces. This process can occur in very small spaces, such as within the [[cell wall]] of a [[plant cell]]. Small scale permineralization can produce very detailed fossils. For permineralization to occur, the organism must become covered by sediment soon after death or soon after the initial decaying process. The degree to which the remains are decayed when covered determines the later details of the fossil. Some fossils consist only of skeletal remains or teeth; other fossils contain traces of [[skin]], [[feather]]s or even soft tissues. This is a form of [[diagenesis]].
===Casts and molds===
[[Image:A fossil shell with calcite.jpg|thumb|left|A calcitic cast of a [[bivalve]] shell.]]
In some cases the original remains of the organism have been completely dissolved or otherwise destroyed. When all that is left is an organism-shaped hole in the rock, it is called an ''external mold''. If this hole is later filled with other minerals, it is a ''cast''. An ''internal mold'' is formed when sediments or minerals fill the internal cavity of an organism, such as the inside of a bivalve or snail.
===Replacement and recrystallization===
''Replacement'' occurs when the shell, bone or other tissue is replaced with another mineral. In some cases mineral replacement of the original shell occurs so gradually and at such fine scales that microstructural features are preserved despite the total loss of original material. A shell is said to be ''recrystallized'' when the original skeletal minerals are still present but in a different crystal form, as from [[aragonite]] to [[calcite]].
===Compression fossils===
[[Compression fossil]]s, such as those of fossil ferns, are the result of chemical reduction of the complex organic molecules composing the organism's tissues. In this case the fossil consists of original material, albeit in a geochemically altered state. This chemical change is an expression of [[diagenesis]].
===Bioimmuration===
[[Image:Catellocaula.jpg|thumb|The star-shaped holes (''Catellocaula vallata'') in this Upper Ordovician bryozoan represent a soft-bodied organism preserved by bioimmuration in the bryozoan skeleton.<ref>Palmer, TJ, and Wilson, MA (1988) Parasitism of Ordovician bryozoans and the origin of pseudoborings. Palaeontology 31,939-949</ref>]]
Bioimmuration is a type of preservation in which a skeletal organism overgrows or otherwise subsumes another organism, preserving the latter, or an impression of it, within the skeleton.<ref>Taylor, PD. (1990) Preservation of soft-bodied and other organisms by bioimmuration: A review. Palaeontology 33,1-17</ref> Usually it is a sessile skeletal organism, such as a bryozoan or an oyster, which grows along a substrate, covering other sessile encrusters. Sometimes the bioimmured organism is soft-bodied and is then preserved in negative relief as a kind of external mold. There are also cases where an organism settles on top of a living skeletal organism which grows upwards, preserving the settler in its skeleton. Bioimmuration is known in the fossil record from the Ordovician<ref>Wilson, MA, Palmer, TJ and Taylor, PD (1994) Earliest preservation of soft-bodied fossils by epibiont bioimmuration: Upper Ordovician of Kentucky. Lethaia 27, 269-270</ref> to the Recent.<ref>Taylor, PD. (1990) Preservation of soft-bodied and other organisms by bioimmuration: A review. Palaeontology 33,1-17</ref>
To sum up, fossilization processes proceed differently for different kinds of tissues and under different kinds of conditions.
==Trace fossils==
{{main|Trace fossil}}
[[Trace fossil]]s are the remains of trackways, burrows, [[bioerosion]], [[Egg (biology)|egg]]s and eggshells, nests, droppings and other types of impressions. Fossilized droppings, called [[coprolite]]s, can give insight into the feeding behavior of animals and can therefore be of great importance.
==Microfossils==
[[image:Micro-Fossil.JPG|thumb|left|Microfossils about 1/2 mm each]]
{{main|Micropaleontology}}
'Microfossil' is a descriptive term applied to fossilized plants and animals whose size is just at or below the level at which the fossil can be analyzed by the naked eye. A commonly applied cut-off point between "micro" and [[macrofossil|"macro" fossils]] is 1 mm, although this is only an approximate guide. Microfossils may either be complete (or near-complete) organisms in themselves (such as the marine plankters [[foraminifera]] and [[coccolithophore]]s) or component parts (such as small teeth or [[palynology|spores]]) of larger animals or plants. Microfossils are of critical importance as a reservoir of [[paleoclimate]] information, and are also commonly used by [[biostratigraphy|biostratigraphers]] to assist in the correlation of rock units.
==Resin fossils==
[[Image:Insects in baltic amber.jpg|thumb|A [[mosquito]] and a [[fly]] in Baltic [[amber]] that is between 40 and 60 million years old]]
Fossil resin (colloquially called [[amber]]) is a natural [[polymer]] found in many types of strata throughout the world, even the [[Arctic]]. The oldest fossil resin dates to the [[Triassic]], though most dates to the [[Tertiary]]. The excretion of the resin by certain plants is thought to be an evolutionary [[adaptation]] for protection from insects and to seal wounds caused by damage elements. Fossil resin often contains other fossils called inclusions that were captured by the sticky resin. These include bacteria, fungi, other plants, and animals. Animal inclusions are usually small [[invertebrates]], predominantly [[arthropods]] such as insects and spiders, and only extremely rarely a [[vertebrate]] such as a small lizard. Preservation of inclusions can be exquisite, including small fragments of [[DNA]].
==Pseudofossils==
[[Image:Dendrites01.jpg|thumb|Manganese dendrites on a limestone bedding plane from [[Solingen]], Germany. Scale in mm.]]
[[Pseudofossil]]s are visual patterns in rocks that are produced by naturally occurring geologic processes rather than biologic processes. They can easily be mistaken for real fossils. Some pseudofossils, such as [[Dendrite (crystal)|dendrite]]s, are formed by naturally occurring fissures in the rock that get filled up by percolating minerals. Other types of pseudofossils are kidney ore (round shapes in iron ore) and [[Agate|moss agate]]s, which look like moss or plant leaves. [[Concretion]]s, spherical or ovoid-shaped nodules found in some sedimentary strata, were once thought to be [[dinosaur]] eggs, and are often mistaken for fossils as well.
[[Image:Ginkgo adiantoides.jpg|thumb|200px|left|''[[Ginkgo|Ginkgo adiantoides]]'' Eocene fossil leaf from the Tranquille Shale of British Columbia, Canada.]]
==Living fossils==
{{main|Living fossil}}
''Living fossil'' is an informal term used for any [[Extant Taxon|living]] [[species]] which closely resembles a species known from fossils -- that is, it is as if the ancient fossil had "come to life."
This can be (a) a species or [[taxon]] known only from fossils until living representatives were discovered, such as the lobed-finned [[coelacanth]], primitive [[monoplacophora]]n mollusk, and the [[ginkgo|Chinese maidenhair]] tree, or (b) a single living species with no close relatives, such as the [[New Caledonia]]n [[Kagu]], or the [[Sunbittern]], or (c) a small group of closely-related species with no other close relatives, such as the oxygen-producing, primoidial [[stromatolite]], inarticulate [[lampshell]] [[Lingula]], many-chambered pearly ''[[Nautilus]]'', rootless [[whisk fern]], armored [[horseshoe crab]], and dinosaur-like [[tuatara]] that are the sole survivors of a once large and widespread group in the fossil record.
==See also==
* [[Bioerosion]]
* [[Elvis taxon]]
* [[Fossil collecting]]
* [[History of paleontology]]
* [[Ichnology]]
* [[Lazarus taxon]]
* [[List of transitional fossils]]
* [[List of fossils]]
* [[List of fossil sites]]
* [[Paleobiology]]
* [[Petrification]]
* [[Shark teeth]]
* [[Taphonomy]]
* [[Trace fossil]]
==References==
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==External links==
{{Commonscat|fossils}}
{{portalpar|Paleontology}}
* [http://www.fossilmuseum.net/ The Virtual Fossil Museum throughout Time and Evolution]
* [http://www.paleoportal.org/ Paleoportal, geology and fossils of the United States]
* [http://www.palaeos.org/Main_Page Palaeos, a multi-authored wiki encyclopedia on the history of life on Earth]
* [http://www.fossilrecord.net The Fossil Record, a complete listing of the families, orders, class and phyla found in the fossil record]
* [http://www.wooster.edu/geology/Bioerosion/Bioerosion.html Bioerosion website, including fossil record]
* [http://www.jogginsfossilcliffs.net/ Fossil record of life in the Coal Age]
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