View source for Graptolite

{{Short description|Subclass of Pterobranchs in the phylum Hemichordata}}
{{Distinguish|text=[[Graptolitha]], a genus of moths}}
{{Automatic taxobox
| taxon = Graptolithina
| authority = Bronn, 1849
| name = Graptolites
| fossil_range = {{Geological range|538.8|0|[[Cambrian]] ([[Fortunian]])–[[Holocene|Present]]<ref>{{Cite journal |last=Maletz |first=Jörg |last2=Beli |first2=Elena |date=2018-01-22 |title=Treatise Online no. 101: Part V, Second Revision, Chapter 15: Subclass Graptolithina and incertae sedis family Rhabdopleuridae: Introduction and systematic descriptions |url=https://journals.ku.edu/treatiseonline/article/view/7053 |journal=Treatise Online |language=en |doi=10.17161/to.v0i0.7053 |issn=2153-4012}}</ref>}}
<small>Survive to the present via the living genus ''[[Rhabdopleura]]''.<ref name= Mitchell>{{cite journal|last1=Mitchell |first1=C. E. |last2=Melchin |first2=M. J. |last3=Cameron |first3=C. B. |last4=Maletz |first4=J. |title=Phylogenetic analysis reveals that ''Rhabdopleura'' is an extant graptolite |journal=Lethaia |year=2013 |volume=46 |number=1 |pages=34–56 |doi=10.1111/j.1502-3931.2012.00319.x|bibcode=2013Letha..46...34M}}</ref></small>
| image = ROM28084 Didymograptus.jpg
| image_caption = ''[[Didymograptus]]'', a genus of graptolite from the [[Middle Ordovician]]
| subdivision_ranks = Subgroups
| subdivision =
* Order †[[Dendroidea]]
* Order †[[Graptoloidea]]
* Family [[Rhabdopleuridae]]
* Family †[[Cysticamaridae]]
* Family †[[Wimanicrustidae]]?
* Family †[[Dithecodendridae]]?
* Family †[[Cyclograptidae]]?
}}

'''Graptolites''' are a group of [[Colony (biology)#Modular organisms|colonial animals]], members of the subclass '''Graptolithina''' within the [[class (biology)|class]] [[Pterobranchia]]. These [[Filter feeder|filter-feeding]] organisms are known chiefly from [[fossil]]s found from the [[Middle Cambrian]] ([[Miaolingian]], [[Wuliuan]]) through the [[Lower Carboniferous]] ([[Mississippian (geology)|Mississippian]]).<ref name="Maletzclass">{{cite journal |last=Maletz |first=Jörg| title=The classification of the Pterobranchia (Cephalodiscida and Graptolithina) |journal=Bulletin of Geosciences |year=2014 |volume=89 |issue=3 |pages=477–540 |doi=10.3140/bull.geosci.1465 |issn=1214-1119|doi-access=free }}</ref> A possible early graptolite, ''[[Chaunograptus]]'', is known from the Middle Cambrian.<ref name=Maletz>{{cite journal|last1=Maletz |first1=J. |year=2014 |title=Hemichordata (Pterobranchia, Enteropneusta) and the fossil record |journal=Palaeogeography, Palaeoclimatology, Palaeoecology |volume=398 |pages=16–27 |doi=10.1016/j.palaeo.2013.06.010|bibcode=2014PPP...398...16M }}</ref> Recent analyses have favored the idea that the living pterobranch ''[[Rhabdopleura]]'' represents an extant graptolite which diverged from the rest of the group in the Cambrian.<ref name="Mitchell" />

Fossil graptolites and ''Rhabdopleura'' share a colony structure of interconnected zooids housed in organic tubes (theca) which have a basic structure of stacked half-rings (fuselli). Most extinct graptolites belong to two major orders: the bush-like [[Sessility (motility)|sessile]] [[Dendroidea]] and the [[Plankton|planktonic]], free-floating [[Graptoloidea]]. These orders most likely evolved from encrusting pterobranchs similar to ''Rhabdopleura''. Due to their widespread abundance, planktonic lifestyle, and well-traced evolutionary trends, graptoloids in particular are useful [[index fossils]] for the [[Ordovician]] and [[Silurian]] periods.<ref name="Maletzgp" />

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. [[Carl Linnaeus|Linnaeus]] originally regarded them as '[[pseudofossil|pictures resembling fossils]]' rather than true fossils, though later workers supposed them to be related to the [[hydrozoans]]; now they are widely recognized as [[hemichordates]].<ref name= Maletzgp>{{cite book|last=Maletz|first=Jörg|title=Graptolite Paleobiology|year=2017|publisher=Wiley-Blackwell|url=https://www.wiley.com/en-us/Graptolite+Paleobiology-p-9781118515617|isbn=9781118515617}}</ref>

==History==
The name "graptolite" originates from the genus ''Graptolithus'' ("writing on the rocks"), which was used by [[Carl Linnaeus|Linnaeus]] in 1735 for inorganic [[Mineralization (biology)|mineralization]]s and incrustations 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. In 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).

Graptolite fossils were later referred to a variety of groups, including other branching colonial animals such as [[Bryozoa|bryozoans]] ("moss animals") and [[Hydrozoa|hydrozoans]]. The term Graptolithina was established by Bronn in 1849, who considered them to represent [[Orthocone|orthoconic]] cephalopods. By the mid-20th century, graptolites were recognized as a unique group closely related to living pterobranchs in the genera ''[[Rhabdopleura]]'' and ''[[Cephalodiscus]]'', which had been described in the late 19th century. ''Graptolithus'', as a genus, was officially abandoned in 1954 by the [[International Code of Zoological Nomenclature|ICZN]].<ref name="Bulman">Bulman, M. (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.</ref>

==Morphology==

=== Colony structure ===
Each graptolite colony originates from an initial individual, called the sicular zooid, from which the subsequent [[zooids]] will develop. They are all interconnected by [[stolons]], a true colonial system shared by ''Rhabdopleura'' but not ''Cephalodiscus''. These zooids are housed within an organic structure comprising a series of tubes secreted by the glands on the [[cephalic shield]]. The colony structure has been known from several different names, including coenecium (for living pterobranchs), rhabdosome (for fossil graptolites), and most commonly '''tubarium''' (for both). The individual tubes, each occupied by a single zooid, are known as '''theca'''.<ref name="Maletzgp" /> The composition of the tubarium is not clearly known, but different authors suggest it is made out of [[collagen]] or [[chitin]]. In some colonies, there are two sizes of theca, the larger autotheca and smaller bitheca, and it has been suggested that this difference is due to [[sexual dimorphism]] of zooids within a colony.<ref name="Maletzgp" />

Early in the development of a colony, the tubarium splits into a variable number of branches (known as '''stipes''') and different arrangements of the theca, features which are important in the identification of graptolite fossils. Colonies can be classified by their total number of theca rows (biserial colonies have two rows, uniserial have one) and the number of initial stipes per colony (multiramous colonies have many stipes, pauciramous colonies have two or one). Each thecal tube is mostly made up by two series of stacked semicircular half-rings, known as '''fuselli''' (sing: fusellum). The fuselli resemble growth lines when preserved in fossils, and the two stacks meet along a suture with a zig-zag pattern. Fuselli are the major reinforcing component of a tubarium, though they are assisted by one or more additional layers of looser tissue, the cortex.[[Image:Graptolites-EncBrit.jpg|thumb|295x295px|Diversity of graptolite colony forms]]

The earliest graptolites appeared in the fossil record during the Cambrian, and were generally [[Sessility (motility)|sessile]] animals, with a colony attached to the sea floor. Several early-diverging families were encrusting organisms, with the colony developing horizontally along a substrate. Extant ''Rhabdopleura'' fall into this category, with an overall encrusting colony form combined with erect, vertical theca. Most of the erect, dendritic or bushy/fan-shaped graptolites are classified as dendroids (order Dendroidea). Their colonies were attached to a hard substrate by their own weight via an attachment disc. Graptolites with relatively few branches were derived from the dendroid graptolites at the beginning of the [[Ordovician]] period. This latter major group, the graptoloids (order Graptoloidea) were [[pelagic]] and [[planktonic]], drifting freely through the water column. They were a successful and prolific group, being the most important and widespread macroplanktonic animals until they died out in the early part of the [[Devonian]] period. The dendroid graptolites survived until the [[Carboniferous]] period.

=== Zooids ===
[[File:Zooidgrap.jpg|thumb|A hypothetical graptolite zooid inside its theca, reconstructed based on living pterobranchs|left]]A mature zooid has three important regions, the preoral disc or cephalic shield, the collar and the trunk. In the collar, the mouth and anus (U-shaped digestive system) and arms are found; Graptholitina has a single pair of arms with several paired tentacles. As a [[nervous system]], graptolites have a simple layer of fibers between the epidermis and the basal lamina, also have a ''collar ganglion'' that gives rise to several nerve branches, similar to the [[neural tube]] of chordates.<ref name="Satodevbio" /> Proper fossils of the soft parts of graptolites have yet to be found, and it is not known if they had pharyngeal gill slits or not,<ref>[https://books.google.com/books?id=pTygDQAAQBAJ&dq=graptolite+zooid+soft+pharyngeal+gill&pg=PA218 Fundamentals of Invertebrate Palaeontology: Macrofossils]</ref> but based on extant ''[[Rhabdopleura]]'', it is likely that the grapotlite zooids had the same morphology.<ref name="Maletzgp" />

== Taxonomy ==
{{Further|List of graptolite genera}}
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]].<ref name=Fortey>{{cite book |author=Fortey, Richard A. |author-link = Richard Fortey|title= Life: A Natural History of the First Four Billion Years of Life on Earth |publisher=Alfred A. Knopf |location=New York |year=1998 |page= 129 |title-link = Life: A Natural History of the First Four Billion Years of Life on Earth}}</ref> Comparisons are drawn with the modern hemichordates ''[[Cephalodiscus]]'' and ''[[Rhabdopleura]]''. According to recent phylogenetic studies, rhabdopleurids are placed within the Graptolithina. Nonetheless, they are considered an ''[[incertae sedis]]'' family.<ref name="Maletzclass"/>

On the other hand, [[Cephalodiscida]] is considered to be a sister subclass of Graptolithina. One of the main differences between these two groups is that Cephalodiscida species are not colonial organisms. In Cephalodiscida organisms, there is no common canal connecting all zooids. Cephalodiscida zooids have several arms, while Graptolithina zooids have only one pair of arms. Other differences include the type of early development, the gonads, the presence or absence of [[gill slits]], and the size of the zooids. In the fossil record, where mostly tubaria (tubes) are preserved, it is complicated to distinguish between groups.

Graptolithina includes several minor families as well as two main extinct orders, '''Dendroidea''' ([[benthic]] graptolites) and '''Graptoloidea''' ([[planktic]] graptolites). The latter is the most diverse, including 5 suborders, where the most assorted is '''Axonophora''' (biserial graptolites, etc.). This group includes '''Diplograptids''' and '''Neograptids''', groups that had great development during the Ordovician.<ref name="Maletzclass"/> Old taxonomic classifications consider the orders Dendroidea, Tuboidea, Camaroidea, Crustoidea, Stolonoidea, Graptoloidea, and Dithecoidea but new classifications embedded them into Graptoloidea at different taxonomic levels.

Taxonomy of Graptolithina by Maletz (2014):<ref name="Maletzclass"/><ref name="Maletzgp"/>

Subclass '''Graptolithina''' <small>Bronn, 1849</small>
*''[[Incertae sedis]]''
**Family [[Rhabdopleuridae]] <small>Harmer, 1905</small>
**Family †[[Cysticamaridae]] <small>Bulman, 1955</small>
**Family †[[Wimanicrustidae]] <small>Bulman, 1970</small>
**Family †[[Dithecodendridae]] <small>Obut, 1964</small>
**Family †[[Cyclograptidae]] <small>Bulman, 1938</small>
*Order †Dendroidea <small>Nicholson, 1872</small>
**Family †[[Dendrograptidae]] <small>Roemer, 1897 in Frech, 1897</small>
**Family †[[Callograptidae]] <small>Bulman, 1938</small><ref>{{Cite journal |last=Maletz |first=Jörg |date=2020-10-01 |title=Treatise Online no. 139: Part V, Second Revision, Chapter 17: Order Dendroidea: Introduction, morphology, and systematic descriptions |url=https://journals.ku.edu/treatiseonline/article/view/14679 |journal=Treatise Online |language=en |doi=10.17161/to.vi.14679 |issn=2153-4012|doi-access=free }}</ref>
**Family †[[Mastigograptidae]] <small>Bates & Urbanek, 2002</small>
*Order †Graptoloidea <small>Lapworth, 1875 in Hopkinson & Lapworth, 1875</small> (planktic graptolites)
**Suborder †[[Graptodendroidina]] <small>Mu & Lin, 1981 in Lin (1981)</small>
***Family †[[Anisograptidae]] <small>Bulman, 1950</small>
**Suborder †[[Sinograpta]] <small>Maletz ''et al.'', 2009</small>
***Family †[[Sigmagraptidae]] <small>Cooper & Fortey, 1982</small>
***Family †[[Sinograptidae]] <small>Mu, 1957</small>
***Family †[[Abrograptidae]] <small>Mu, 1958</small>
**Suborder †[[Dichograptina]] <small>Lapworth, 1873</small>
***Family †[[Dichograptidae]] <small>Lapworth, 1873</small>
***Family †[[Didymograptidae]] <small>Mu, 1950</small>
***Family †[[Pterograptidae]] <small>Mu, 1950</small>
***Family †[[Tetragraptidae]] <small>Frech, 1897</small>
**Suborder †[[Glossograptina]] <small>Jaanusson, 1960</small>
***Family †[[Isograptidae]] <small>Harris, 1933</small>
***Family †[[Glossograptidae]] <small>Lapworth, 1873</small>
**Suborder †[[Axonophora]] <small>Frech, 1897</small> (biserial graptolites, and also retiolitids and monograptids)
***Infraorder †[[Diplograptina]] <small>Lapworth, 1880</small>
****Family †[[Diplograptidae]] <small>Lapworth, 1873</small>
*****Subfamily †[[Diplograptinae]] <small>Lapworth, 1873</small>
*****Subfamily †[[Orthograptinae]] <small>Mitchell, 1987</small>
****Family †[[Lasiograptidae]] <small>Lapworth, 1880e</small>
****Family †[[Climacograptidae]] <small>Frech, 1897</small>
****Family †[[Dicranograptidae]] <small>Lapworth, 1873</small>
*****Subfamily †[[Dicranograptinae]] <small>Lapworth, 1873</small>
*****Subfamily †[[Nemagraptinae]] <small>Lapworth, 1873</small>
***Infraorder †[[Neograptina]] <small>Štorch ''et al.'', 2011</small>
****Family †[[Normalograptidae]] <small>Štorch & Serpagli, 1993</small>
****Family †[[Neodiplograptidae]] <small>Melchin ''et al.'', 2011</small>
*****Subfamily †[[Neodiplograptinae]] <small>Melchin ''et al.'', 2011</small>
*****Subfamily †[[Petalolithinae]] <small>Bulman, 1955</small>
****Superfamily †[[Retiolitoidea]] <small>Lapworth, 1873</small>
*****Family †[[Retiolitidae]] <small>Lapworth, 1873</small>
******Subfamily †[[Retiolitinae]] <small>Lapworth, 1873</small>
******Subfamily †[[Plectograptinae]] <small>Bouček & Münch, 1952</small>
****Superfamily †[[Monograptoidea]] <small>Lapworth, 1873</small>
*****Family †[[Dimorphograptidae]] <small>Elles & Wood, 1908</small>
*****Family †[[Monograptidae]] <small>Lapworth, 1873</small>

==Ecology==
Graptolites were a major component of the early [[Paleozoic]] ecosystems, especially for the [[zooplankton]] because the most abundant and diverse species were planktonic. Graptolites were most likely suspension feeders and strained the water for food such as plankton.<ref>{{Cite web|url=http://samnoblemuseum.ou.edu/common-fossils-of-oklahoma/invertebrate-fossils/graptolites/|title=Graptolites|website=samnoblemuseum.ou.edu|access-date=2018-12-28}}</ref>

Inferring by analogy with modern pterobranchs, they were able to migrate vertically through the water column for feeding efficiency and to avoid predators. With ecological models and studies of the [[facies]], it was observed that, at least for Ordovician species, some groups of species are largely confined to the [[epipelagic]] and [[mesopelagic]] zone, from inshore to open ocean.<ref name= Cooper2012>Cooper, R., [[Sue Rigby|Rigby, S.]], Loydell, D. & Bates, D. (2012) Palaeoecology of the Graptoloidea. ''Earth-Science Reviews'', 112(1):23-41.</ref> Living rhabdopleura have been found in deep waters in several regions of Europe and America but the distribution might be biased by sampling efforts; colonies are usually found as [[epibiont]]s of shells.

Their locomotion was relative to the water mass in which they lived but the exact mechanisms (such as turbulence, [[buoyancy]], active swimming, and so forth) are not clear yet. One proposal, put forward by Melchin and DeMont (1995), suggested that graptolite movement was analogous to modern free-swimming animals with heavy housing structures. In particular, they compared graptolites to "sea butterflies" ([[Sea butterfly|Thecostomata]]), small swimming [[Pteropoda|pteropod]] [[Snail|snails]]. Under this suggestion, graptolites moved through [[rowing]] or swimming via an undulatory movement of paired muscular [[appendages]] developed from the cephalic shield or feeding tentacles. In some species, the thecal aperture was probably so restricted that the appendages hypothesis is not feasible. On the other hand, buoyancy is not supported by any extra thecal tissue or gas build-up control mechanism, and active swimming requires a lot of energetic waste, which would rather be used for the tubarium construction.<ref name="Cooper2012" />

There are still many questions regarding graptolite locomotion but all these mechanisms are possible alternatives depending on the species and its habitat. For benthic species, that lived attached to the sediment or any other organism, this was not a problem; the zooids were able to move but restricted within the tubarium. Although this zooid movement is possible in both planktic and benthic species, it is limited by the [[stolon]] but is particularly useful for feeding. Using their arms and tentacles, which are close to the mouth, they filter the water to catch any particles of food.<ref name="Cooper2012" />

==Life cycle==
The study of the [[developmental biology]] of Graptholitina has been possible by the discovery of the species ''R. compacta'' and ''[[Rhabdopleura normani|R. normani]]'' in shallow waters; it is assumed that graptolite fossils had a similar development as their extant representatives. The life cycle comprises two events, the [[ontogeny]] and the [[astogeny]], where the main difference is whether the development is happening in the individual organism or in the [[Modularity#Modularity in biology|modular]] growth of the colony.

The life cycle begins with a planktonic [[planula]]-like [[larva]] produced by sexual reproduction, which later becomes the ''sicular zooid'' who starts a colony. In ''[[Rhabdopleura]]'', the colonies bear male and female zooids but fertilized eggs are incubated in the female tubarium, and stay there until they become larvae able to swim (after 4–7 days) to settle away to start a new colony. Each larva surrounds itself in a protective cocoon where the metamorphosis to the zooid takes place (7–10 days) and attaches with the posterior part of the body, where the stalk will eventually develop.<ref name="Maletzgp"/>

The development is indirect and [[lecithotrophic]], and the larvae are ciliated and pigmented, with a deep depression on the ventral side.<ref>Röttinger, E. & Lowe, C. (2012) Evolutionary crossroads in developmental biology: hemichordates. ''Development'', 139:2463-2475.</ref><ref name=Satodevbio>Sato, A., Bishop, J. & Holland, P. (2008). Developmental Biology of Pterobranch Hemichordates: History and Perspectives. ''Genesis'', 46:587-591.</ref> Astogeny happens when the colony grows through [[asexual reproduction]] from the tip of a permanent terminal zooid, behind which the new zooids are budded from the stalk, a type of budding called [[monopodial]]. It is possible that in graptolite fossils the terminal zooid was not permanent because the new zooids formed from the tip of latest one, in other words, [[sympodial]] budding. These new organisms break a hole in the tubarium wall and start secreting their own tube.<ref name="Maletzgp"/>

==Graptolites in evolutionary development==

In recent years, living graptolites have been used as a hemichordate model for [[Evo-Devo]] studies, as have their sister group, the [[acorn worms]]. For example, graptolites are used to study asymmetry in hemichordates, especially because their gonads tend to be located randomly on one side. In ''[[Rhabdopleura normani]]'', the [[testicle]] is located asymmetrically, and possibly other structures such as the oral lamella and the [[gonopore]].<ref name= Sato8>Sato, A. & Holland, P. (2008). Asymmetry in a Pterobranch Hemichordate and the Evolution of Left-Right Patterning. ''Developmental Dynamics'', 237:3634 –3639)</ref> The significance of these discoveries is to understand the early vertebrate left-right [[asymmetry]] due to [[chordates]] being a sister group of hemichordates, and therefore, the asymmetry might be a feature that developed early in [[deuterostomes]]. Since the location of the structures is not strictly established, also in some [[enteropneusta|enteropneusts]], it is likely that asymmetrical states in hemichordates are not under a strong developmental or evolutionary constraint. The origin of this asymmetry, at least for the gonads, is possibly influenced by the direction of the basal coiling in the tubarium, by some intrinsic biological mechanisms in pterobranchs, or solely by environmental factors.<ref name="Sato8" />

''[[Hedgehog signaling pathway|Hedgehog]]'' (hh), a highly conserved gene implicated in neural developmental patterning, was analyzed in Hemichordates, taking ''Rhabdopleura'' as a pterobranch representative. It was found that hedgehog gene in pterobranchs is expressed in a different pattern compared to other hemichordates as the [[enteropneust]] ''[[Saccoglossus kowalevskii]]''. An important conserved glycine–cysteine–phenylalanine (GCF) motif at the site of autocatalytic cleavage in hh genes, is altered in ''R. compacta'' by an insertion of the [[amino acid]] [[threonine]] (T) in the N-terminal, and in ''S. kowalesvskii'' there is a replacement of [[serine]] (S) for [[glycine]] (G). This mutation decreases the efficiency of the autoproteolytic cleavage and therefore, the signalling function of the protein. It is not clear how this unique mechanism occurred in evolution and the effects it has in the group, but, if it has persisted over millions of years, it implies a functional and genetic advantage.<ref name= Sato2009>Sato, A., White-Cooper, H., Doggett, K. & Holland, P. 2009. Degenerate evolution of the hedgehog gene in a hemichordate lineage. ''Proceedings of the National Academy of Sciences'', 106(18):7491-7494.</ref>

==Geological relevance==

=== Preservation ===
[[Image:TetragraptusfruticosusBendigonian.jpg|thumb|''Pendeograptus fruticosus'' from the Bendigonian Australian Stage (Lower Ordovician; 477–474 mya) near [[Bendigo]], [[Victoria (Australia)|Victoria]], [[Australia]]. There are two overlapping, three-stiped rhabdosomes.]]

Graptolites are common fossils and have a worldwide distribution. They are most commonly found in [[shale]]s and mudrocks 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 planktic graptolites, having sunk to the sea floor, would eventually become entombed in the sediment and were thus well preserved.

These colonial animals 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.
[[File:Cyrtograptus canadensis.jpg|thumb|''[[Cyrtograptus|Cyrtograptus canadensis]]'' from the middle Silurian of the [[Cape Phillips Formation]], [[Nunavut]]. ]]
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]] or some other minerals. They vary in shape, but are most commonly [[wiktionary:dendritic|dendritic]] or branching (such as ''[[Dictyonema (graptolite)|Dictyonema]]''), [[sawblade]]-like, or "[[tuning fork]]"-shaped (such as ''[[Didymograptus]] murchisoni''). Their remains may be mistaken for fossil [[plant]]s by the casual observer, as it has been the case for the first graptolite descriptions.

Graptolites are normally preserved as a black [[Carbonaceous film|carbon film]] on the rock's surface or as light grey clay films in [[tectonics|tectonically]] distorted rocks. The fossil can also appear stretched or distorted. This is due to the strata that the graptolite is within, being folded and compacted. 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]]. Sites in the [[Southern Uplands]] of Scotland, the [[Lake District]] and [[Welsh Borders]] also yield rich and well-preserved graptolite faunas. A famous graptolite location in Scotland is [[Dob's Linn]] with species from the boundary Ordovician-Silurian. Since the group had a wide distribution, fossils are also abundant in several parts of the United States, Canada, Australia, Germany and China, among others.

=== Stratigraphy ===
Graptolite fossils have predictable preservation, widespread distribution, and gradual change over a [[geologic time scale]]. This allows them to be used to date [[stratum|strata]] of rocks throughout the world.<ref name="Fortey" /> They are important [[index fossils]] for dating [[Palaeozoic]] rocks as they evolved rapidly with time and formed many different distinctive species. 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 worldwide [[ice age]] at the end of the Ordovician eliminated most graptolites except the neograptines. Diversification from the neograptines that survived the Ordovician [[Glacier|glaciation]] began around 2 million years later.<ref>Bapst, D., Bullock, P., Melchin, M., Sheets, D. & Mitchell, C. (2012) Graptoloid diversity and disparity became decoupled during the Ordovician mass extinction. ''Proceedings of the National Academy of Sciences'', 109(9):3428-3433.</ref>

The [[Great Ordovician Biodiversification Event]] ([[Ordovician radiation|GOBE]]) influenced changes in the morphology of the colonies and thecae, giving rise to new groups like the planktic Graptoloidea. Later, some of the greatest extinctions that affected the group were the [[Hirnantian]] in the Ordovician and the Lundgreni in the Silurian, where graptolite populations were dramatically reduced (see also [[Lilliput effect]]).<ref name="Maletzgp" /><ref name="1993 Urbanek graptoloids">{{cite journal |last1=Urbanek |first1=Adam |date=1993 |title=Biotic Crises in the History of Upper Silurian Graptoloids: A Palaeobiological Model |journal=Historical Biology |volume=7 |issue=1 |pages=29–50 |doi=10.1080/10292389309380442|bibcode=1993HBio....7...29U }}</ref>

Graptolite diversity was greatly reduced during the [[Sedgwickii Event]] in the [[Aeronian]].<ref name=Newins>{{Cite journal |title=New insights on the Silurian graptolite biostratigraphy of the La Chilca Formation, Poblete Norte section, Central Precordillera of San Juan, Argentina: faunal replacement and paleoenvironmental implications |journal=[[Andean Geology]] |last1=Lopez |first1=Fernando Enrique |issue=2 |volume=50 |last2=Kaufmann |first2=Cintia |doi=10.5027/andgeov50n2-3617 |year=2023|page=201 |doi-access=free |bibcode=2023AndGe..50..201L }}</ref> This event has been attested in locations such as today's Canada, Libya as well as in [[La Chilca Formation]] of Argentina (then part of [[Gondwana]]).<ref name=Newins/>

==Researchers==
The following is a selection of graptolite and pterobranch researchers:<ref name="Maletzgp" />

*[[Joachim Barrande]] (1799–1883)
*[[Hanns Bruno Geinitz]] (1814–1900)
*[[James Hall (paleontologist)|James Hall]] (1811–1898)
*[[Frederick M'Coy]] (1817–1899)
*[[Henry Alleyne Nicholson]] (1844–1899)
*[[John Hopkinson]] (1844–1919)
*[[Sven Leonhard Törnquist]] (1840–1920)
*[[Sven Axel Tullberg]] (1852–1886)
*[[Gerhard Holm]] (1853–1926)
*[[Carl Wiman]] (1867–1944)
*[[Thomas Sergeant Hall]] (1858–1915)
*[[Alexander Robert Keble]] (1884–1963)
*[[Noel Benson]] (1885–1957)
*[[William John Harris]] (1886–1957)
*[[David Evan Thomas]] (1902–1978)
*[[Mu Enzhi]] (1917–1987)
*[[Li Jijin]] (1928–2013)
*[[Vladimir Nikolayevich Beklemishev]] (1890–1962)
*[[Michael Sars]] (1805–1869)
*[[George Ossian Sars]] (1837–1927)
*[[William Carmichael M'Intosh]] (1838–1931)
*[[Nancy Kirk (geologist)|Nancy Kirk]] (1916–2005)
*[[Roman Kozłowski]] (1889–1977)
*[[Gertrude Elles]] (1872–1960)
*[[Jörg Maletz]]
*[[Denis E. B. Bates]]
*[[Alfred C. Lenz]]
*[[Chris B. Cameron]]
*[[Adam Urbanek]]
*[[David K. Loydell]]
*Hermann Jaeger (1929–1992)

==See also==
{{Portal|Paleontology}}
*[[List of graptolite genera]]

==References==
{{Reflist}}

==External links==
{{Commons category|Graptolithina}}
*Classification of the Graptolithoidea - [http://www.graptolite.net/graptolites.html Graptolites and Pterobranchs]
*Podcast on Graptolites by David Bapst - [http://www.palaeocast.com/episode-71-graptolites/ Palaeocast]
*Graptolites gallery by Michael P. Klimetz - [http://earthphysicsteaching.homestead.com/Graptolites.html Graptolites]
*What are Fossil Graptolites and why are they useful in geology? - [https://www.youtube.com/watch?v=8i68jyBurtc Youtube]
*Writing on the rocks - [http://www.earthsciences.hku.hk/shmuseum/earth_evo_05_01_8.php Stephen Hui Geological Museum]

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[[Category:Graptolites| ]]
[[Category:Paleozoic invertebrates]]
[[Category:Cambrian invertebrates]]
[[Category:Carboniferous invertebrates]]
[[Category:Devonian invertebrates]]
[[Category:Ordovician invertebrates]]
[[Category:Permian invertebrates]]
[[Category:Silurian invertebrates]]
[[Category:Cambrian first appearances]]
[[Category:Carboniferous extinctions]]