Graptolite
62177
218193757
2008-06-09T15:57:55Z
Black Tusk
1722754
/* Preservation */ link
{{Taxobox
| name = Graptolites
| fossil_range = {{fossil range|510|350}}Mid [[Cambrian]] to Lower [[Carboniferous]]
| image = Graptolite tetragraptus.jpg
| image_width = 260px
| image_caption = Fossil graptolite ''Tetragraptus fruticosus'' from the Ordovician of Australia
| regnum = [[Animal]]ia
| phylum = [[Hemichordata]]
| classis = '''Graptolithina'''
| phylum_authority =
| subdivision_ranks = [[Order (biology)|Order]]s
| subdivision =
*[[Dendroidea]]
*[[Tuboidea]]
*[[Camaroidea]]
*[[Crustoidea]]
*[[Stolonoidea]]
*[[Graptoloidea]]
*[[Dithecoidea]]
}}
'''Graptolites''' (Graptolithina) are [[fossil]] colonial [[animal]]s known chiefly from the Upper [[Cambrian]] through the Lower [[Carboniferous]] ([[Mississippian]]). A possible early graptolite, ''Chaunograptus'', is known from the Middle Cambrian.
The name graptolite comes from the [[Greek language|Greek]] ''graptos'', meaning "written", and ''lithos'', meaning "rock", as many graptolite fossils resemble [[Logogram|hieroglyphs]] written on the rock. [[Linnaeus]] originally regarded them as 'pictures resembling fossils rather than true fossils', though later workers supposed them to be related to the [[hydrozoa]]ns.
==Taxonomy==
The name originates from the genus ''Graptolithus'', which was used by [[Linneus]] in 1735 for inorganic [[Mineralization (biology)|mineralization]]s and crustations which resembled actual fossils. In 1768, in the 12th volume of [[Systema Naturae]], he included ''G. sagittarius'' and ''G. scalaris'', respectively a possible [[plant]] fossil and a possible graptolite. His 1751 ''Skånska Resa'', he included a figure of a "fossil or graptolite of a strange kind" currently thought to be a type of ''Climacograptus'' (a [[genus]] of biserial graptolites). Later workers used the name to refer to a specific group of organisms. ''Graptolithus'' was officially abandoned in 1954 by the [[ICZN]], partly because of its original purpose as a grouping for inorganic mimicries of fossils. (Bulman, 1970: V 6)
Since the 1970s, as a result of advances in [[electron microscopy]], graptolites have generally been thought to be most closely allied to the [[pterobranch]]s, a rare group of modern marine animals belonging to the [[phylum]] [[Hemichordata]] ([[hemichordate]]s). Comparisons are drawn with the modern hemichordates ''[[Cephalodiscus]]'' and ''[[Rhabdopleura]]''.<ref name=Fortey>{{cite book |author=Fortey, Richard A. |title=Life: a natural history of the first four billion years of life on earth |publisher=Alfred A. Knopf |location=New York |year=1998 |pages=129-134 |isbn=0-375-40119-9 |oclc= |doi= |accessdate=}}</ref> ''Cephalodiscus'' numbers about 18 species, and was first discovered in 1882 .
==Graptolites as zone fossils==
Graptolites are common fossils and have a worldwide distribution.
The preservation, quantity and gradual change over a [[geologic time scale]] of graptolites allows the fossils to be used to date [[strata]] of rocks throughout the world.<ref name=Fortey/> They are important [[index fossil]]s for dating [[Palaeozoic]] rocks as they evolved rapidly with time and formed many different species. [[Great Britain|British]] geologists can divide the rocks of the [[Ordovician]] and [[Silurian]] periods into graptolite biozones; these are generally less than one million years in duration. A world-wide [[ice age]] at the end of the Ordovician eliminated the majority of the then-living graptolite; species present during the Silurian period were the result of diversification from only a one or two species that survived the Ordovician [[Glacier|glaciation]].<ref name = Fortey/>
Graptolites are also used to estimate [[water]] [[depth]] and [[temperature]] during the graptolites lifetimes.
{{Graptolite timeline}}
==Morphology==
[[Image:Graptolites-EncBrit.jpg|thumb|right|300px|Graptolite morphology illustrated from the [[Encyclopædia Britannica]]{{Expand|caption|date=May 2007}}]]
Each graptolite colony is known as a '''[[rhabdosome]]''' and has a variable number of branches (called [[stipes]]) originating from an initial individual (called a [[sicula]]). Each subsequent individual ([[zooid]]) was housed within a tubular or cup-like structure (called a [[theca]]). In some colonies, there are two sizes of theca, and it has been suggested that this difference was due to [[sexual dimorphism]].{{Fact|date=May 2007}}
The number of branches and the arrangement of the thecae are important features in the identification of graptolite fossils.
Most of the dendritic or many-branched types are classified as dendroid graptolites (order Dendroidea). They appear earlier in the fossil record (in the Cambrian period), and were generally benthic animals (attached to the sea-floor by a root-like base). Graptolites with relatively few branches were derived from the dendroid graptolites at the beginning of the [[Ordovician]] period. This latter type (order Graptoloidea) were pelagic, drifting freely on the surface of ancient seas or attached to floating [[seaweed]] by means of a slender thread. They were a successful and prolific group, being the most important animal members of the plankton until they died out in the early part of the [[Devonian]] period. The dendroid graptolites survived until the Carboniferous period.
==Preservation==
Graptolite fossils are often found in [[shale]]s and mud rocks where sea-bed fossils are rare, this type of rock having formed from [[sediment]] deposited in relatively deep water that had poor bottom circulation, was deficient in [[oxygen]], and had no scavengers. The dead planktonic graptolites, having sunk to the sea-floor, would eventually become entombed in the sediment and are thus well preserved.
Graptolites are also found in [[limestone]]s and [[chert]]s, but generally these rocks were deposited in conditions which were more favorable for bottom-dwelling life, including scavengers, and undoubtedly most graptolite remains deposited here were generally eaten by other animals.
Graptolite fossils are often found flattened along the bedding plane of the rocks in which they occur, though may be found in three dimensions when they are infilled by [[iron]] [[pyrite]]. They vary in shape, but are most commonly [[Dendrite|dendritic]] or branching (such as ''Dictoyonema''), saw-blade like, or "tuning fork" shaped (such as ''Didymograptus murchisoni''). Their remains may be mistaken for fossil [[plant]]s by the casual observer.
Graptolites are normally preserved as a black [[carbon]]ized film on the rock's surface or as light grey clay films in [[tectonics|tectonically]] distorted rocks. They may be sometimes difficult to see, but by slanting the specimen to the light they reveal themselves as a shiny marking. Pyritized graptolite fossils are also found.
A well known locality for graptolite fossils in Britain is [[Abereiddy Bay]], [[Dyfed]], [[Wales]] where they occur in rocks from the Ordovician period.
==Footnotes==
{{reflist}}
==References==
Bulman, 1970. ''In'' Teichert, C. (ed.). ''Treatise on Invertebrate Paleontology. Part V. Graptolithina , with sections on Enteropneusta and Pterobranchia.'' (2nd Edition). Geological Society of America and University of Kansas Press, Boulder, Colorado and Lawrence, Kansas, xxxii + 163 pp.
==External links==
*Classification of the Cephalodiscoidea (Graptolithoidea) - [http://pterobranchia.graptolite.net/Cephalodiscoidea.html Graptolite.net - ''Cephalodiscus'']
*BIG G - The British & Irish Graptolite Group - [http://www.graptolites.co.uk/ British and Irish Graptolite Group (BIG-G)]
*Graptolite Mudrocks - [http://www.le.ac.uk/geology/aap8/index.htm The palaeoecology and palaeoenvironment of graptolitic mudrocks]
*[http://www.earth.rochester.edu/ees207/Graptolites/caplangrap1.html Written by Josh Caplan for Professor Bill Chaisson's Invertebrate Paleontology course in December 2000.]
[[Category:Prehistoric invertebrates]]
[[Category:Hemichordates]]
[[de:Graptolithen]]
[[eo:Graptolito]]
[[fr:Graptolite]]
[[gl:Graptolito]]
[[it:Graptolithina]]
[[ka:გრაპტოლითები]]
[[lt:Graptolitai]]
[[hu:Graptoliták]]
[[nl:Graptoliet]]
[[no:Graptolitter]]
[[pl:Graptolity]]
[[pt:Graptólito]]
[[ru:Граптолиты]]
[[sr:Граптолити]]
[[fi:Graptoliitit]]
[[sv:Graptoliter]]
[[uk:Граптоліти]]
[[zh:笔石纲]]