Editing Graptolite (section)

==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]] 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/>

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|-
| style="text-align:center;" |<small>Ranges of Graptolite taxa.</small>

|}