Editing Trilobite (section)

===Divergence and extinction===
[[File:Cambrian Trilobite Olenoides Mt. Stephen.jpg|thumb|''[[Ogygopsis|Ogygopsis klotzi]]'' from the Mt. Stephen Trilobite Beds ([[Middle Cambrian]]) near [[Field, British Columbia]], Canada]]
Trilobites saw great diversification over time.<ref>{{Cite web | url=http://paleoart.com/the-evolution-of-trilobites/?v=f9308c5d0596 | title=The evolution of trilobites – Paleoart | access-date=2019-11-26 | archive-url=https://web.archive.org/web/20181212093835/http://paleoart.com/the-evolution-of-trilobites/?v=f9308c5d0596 | archive-date=2018-12-12 | url-status=dead }}</ref> For such a long-lasting group of animals, it is no surprise that trilobite evolutionary history is marked by a number of extinction events where some groups perished, and surviving groups diversified to fill ecological niches with comparable or unique adaptations. Generally, trilobites maintained high diversity levels throughout the [[Cambrian]] and [[Ordovician]] periods before entering a drawn-out decline in the [[Devonian]], culminating in the final extinction of the last few survivors at the end of the [[Permian]] period.<ref name="Clarkson98"/>

====Evolutionary trends====
Principal evolutionary trends from primitive morphologies, such as exemplified by ''[[Eoredlichia]]'',<ref name=Fortey&Owens97a /> include the origin of new types of eyes, improvement of enrollment and articulation mechanisms, increased size of pygidium (micropygy to isopygy), and development of extreme spinosity in certain groups.<ref name="Clarkson98"/> Changes also included narrowing of the thorax and increasing or decreasing numbers of thoracic segments.<ref name=Fortey&Owens97a>{{Citation |last1=Fortey |first1=R. A. |last2=Owens |first2=R. M. |contribution=Evolutionary History |editor-last=Kaesler |editor-first=R. L. |title=Treatise on Invertebrate Paleontology, Part O, Arthropoda 1, Trilobita, revised. Volume 1: Introduction, Order Agnostida, Order Redlichiida |pages=[https://archive.org/details/treatiseoninvert0002unse/page/249 249–287] |publisher=The Geological Society of America, Inc. & The University of Kansas |place=Boulder, CO & Lawrence, KS |year=1997 |isbn=978-0-8137-3115-5 |url=https://archive.org/details/treatiseoninvert0002unse/page/249 }}</ref> Specific changes to the cephalon are also noted; variable glabella size and shape, position of eyes and facial sutures, and hypostome specialization.<ref name=Fortey&Owens97a /> Several morphologies appeared independently within different major taxa (e.g. eye reduction or miniaturization).<ref name=Fortey&Owens97a />

Effacement, the loss of surface detail in the cephalon, pygidium, or the thoracic furrows, is also a common evolutionary trend. Notable examples of this were the [[order (biology)|order]]s [[Agnostida]] and [[Asaphida]], and the [[suborder]] [[Illaenina]] of the [[Corynexochida]]. Effacement is believed to be an indication of either a burrowing lifestyle or a pelagic one. Effacement poses a problem for [[taxonomist]]s since the loss of details (particularly of the [[Glabella (trilobite)|glabella]]) can make the determination of [[phylogenetics|phylogenetic]] relationships difficult.<ref name="efface">{{cite web|url=http://www.trilobites.info/trends.htm|title=Evolutionary Trends in Trilobites|author=Samuel M. Gon III|date=July 20, 2008|publisher=A Guide to the Orders of Trilobites|access-date=April 14, 2011|archive-date=May 17, 2013|archive-url=https://web.archive.org/web/20130517123118/http://www.trilobites.info/trends.htm|url-status=dead}}</ref>

====Cambrian====
Although it has historically been suggested that trilobites originated during the [[Precambrian]]<ref name="lieberman02" /><ref name="Fortey-etal06">{{Citation |last1=Fortey |first1=R. A. |title=The Cambrian evolutionary "explosion": decoupling cladogenesis from morphological disparity |journal=[[Biological Journal of the Linnean Society]] |volume=57 |pages=13–33 |year=1996 |doi=10.1111/j.1095-8312.1996.tb01693.x |last2=Briggs |first2=D. E. G. |last3=Wills |first3=M. A. |doi-access=free}}</ref> this is no longer supported, and it is thought that trilobites originated shortly before they appeared in the fossil record.<ref name="Holmes-2022" /> Very shortly after trilobite fossils appeared in the lower Cambrian, they rapidly diversified into the major orders that typified the Cambrian—[[Redlichiida]], [[Ptychopariida]], [[Agnostida]], and [[Corynexochida]]. The first major crisis in the trilobite fossil record occurred in the Middle [[Cambrian]]; surviving orders developed isopygius or macropygius bodies and developed thicker cuticles, allowing better defense against predators (see [[#Thorax|Thorax]] below).<ref name="Nedin1999" /> The Late Cambrian marks the beginning of the apex of trilobite diversity.<ref name=":0">{{Cite journal |last1=Bault |first1=Valentin |last2=Balseiro |first2=Diego |last3=Monnet |first3=Claude |last4=Crônier |first4=Catherine |date=July 2022 |title=Post-Ordovician trilobite diversity and evolutionary faunas |url=https://linkinghub.elsevier.com/retrieve/pii/S0012825222001192 |journal=Earth-Science Reviews |language=en |volume=230 |pages=104035 |doi=10.1016/j.earscirev.2022.104035|bibcode=2022ESRv..23004035B }}</ref> The end-[[Cambrian]] mass extinction event marked a major change in trilobite fauna; almost all Redlichiida (including the Olenelloidea) and most Late Cambrian stocks became extinct.<ref name="Clarkson98" /> A continuing decrease in [[Laurentia]]n continental shelf area<ref name="Rudkin03" /> is recorded at the same time as the extinctions, suggesting major environmental upheaval.

Notable trilobite genera appearing in the Cambrian include:<ref name="PREHISTORIC LIFE">{{cite book|title=Prehistoric Life: The Definitive Visual History of Life On Earth|year=2009|publisher=Dorling Kindersley|location=London|isbn=9780756655730|page=76,88,89,90,91,104,105,127,161,180,181}}</ref>
*''[[Abadiella]]'' (Lower Cambrian)
*''[[Buenellus]]'' (Lower Cambrian)
*''[[Judomia]]'' (Lower Cambrian)
*''[[Olenellus]]'' (Lower Cambrian)
*''[[Ellipsocephalus]]'' (Middle Cambrian)
*''[[Elrathia]]'' (Middle Cambrian)
*''[[Paradoxides]]'' (Middle Cambrian)
*''[[Peronopsis]]'' (Middle Cambrian)
*''[[Xiuqiella]]'' (Middle Cambrian)
*''[[Yiliangella]]'' (Middle Cambrian)
*''[[Yiliangellina]]'' (Middle Cambrian)
*''[[Olenus (trilobite)|Olenus]]'' (Late Cambrian)

====Ordovician====
[[File:Worlds largest trilobite.jpg|thumb|Cast of ''[[Isotelus|Isotelus rex]]'', the largest-known trilobite, from the middle to upper [[Ordovician]] of [[North America]]]]
[[File:Trilobite Ordovicien 8127.jpg|thumb|''[[Cheirurus]]'' sp., middle [[Ordovician]] age, [[Volkhov River]], [[Russia]] ]]
The Early [[Ordovician]] is marked by vigorous radiations of articulate brachiopods, bryozoans, bivalves, echinoderms, and graptolites, with many groups appearing in the fossil record for the first time.<ref name="Clarkson98" /> Although intra-species trilobite diversity seems to have peaked during the Cambrian,<ref name="webster07">{{Citation |first=Mark |last=Webster |title=A Cambrian peak in morphological variation within trilobite species |journal=[[Science (journal)|Science]] |volume=317 |pages=499–502 |year=2007 |doi=10.1126/science.1142964 |pmid=17656721 |issue=5837|bibcode = 2007Sci...317..499W |s2cid=36290256 |doi-access= }}</ref> trilobites were still active participants in the Ordovician radiation event, with a new fauna taking over from the old [[Cambrian]] one.<ref name="Adrain98" /> [[Phacopida]] and [[Trinucleioidea]] are characteristic forms, highly differentiated and diverse, most with uncertain ancestors.<ref name="Clarkson98" /> The Phacopida and other "new" [[clade]]s almost certainly had Cambrian forebears, but the fact that they have avoided detection is a strong indication that novel morphologies were developing very rapidly.<ref name="Clowes06a" /> Changes within the trilobite fauna during the Ordovician foreshadowed the mass extinction at the end of the Ordovician, allowing many families to continue into the [[Silurian]] with little disturbance.<ref name=Adrain98>{{Citation |last1=Adrain |first1=Jonathan M. |last2=Fortey |first2=Richard A. |author-link2=Richard Fortey |last3=Westrop |first3=Stephen R. |title=Post-Cambrian trilobite diversity and evolutionary faunas |journal=[[Science (journal)|Science]] |volume=280 |year=1998 |doi=10.1126/science.280.5371.1922 |pmid=9632387 |issue=5371 |pages=1922–5|bibcode = 1998Sci...280.1922A |url=http://doc.rero.ch/record/14799/files/PAL_E1948.pdf }}</ref> Ordovician trilobites were successful at exploiting new environments, notably [[reef]]s. The Ordovician mass extinction did not leave the trilobites unscathed; some distinctive and previously successful forms such as the [[Telephinidae]] and [[Agnostida]] became extinct. The Ordovician marks the last great diversification period amongst the trilobites: very few entirely new patterns of organisation arose post-Ordovician. Later evolution in trilobites was largely a matter of variations upon the Ordovician themes. By the [[Ordovician-Silurian extinction events|Ordovician mass extinction]], vigorous trilobite radiation has stopped, and gradual decline is foreshadowed.<ref name="Clarkson98" /> The Ordovician marks the apex of trilobite morphological and species diversity.<ref name=":0" />

Some of the genera of Trilobites appearing in the Ordovician include:<ref name="PREHISTORIC LIFE" />

*''[[Cyclopyge (trilobite)|Cyclopyge]]'' (Early to Late Ordovician)
*''[[Selenopeltis]]'' (Early to Late Ordovician)
*''[[Parabolina]]'' (Early Ordovician)
*''[[Cheirurus]]'' (Middle Ordovician)
*''[[Eodalmanitina]] ''(Middle Ordovician)
*''[[Trinucleus]]'' (Middle Ordovician)
*''[[Triarthrus]]'' (Late Ordovician)

====Silurian and Devonian====
[[File:Trilobite diversity EN (cropped).png|thumb|upright|Number of families, Middle Devonian to Upper Permian. Proetida – brown, Phacopida – steel blue, Lichida – clear blue, Harpetida – pink, Odontopleurida – olive and Corynexochida – purple]]Most [[Llandovery epoch|Early Silurian]] families constitute a subgroup of the Late Ordovician fauna. Few, if any, of the dominant Early Ordovician fauna survived to the end of the Ordovician, yet 74% of the dominant Late Ordovician trilobite fauna survived the Ordovician. Late Ordovician survivors account for all post-Ordovician trilobite groups except the [[Harpetida]].<ref name="Adrain98" /> Silurian and [[Devonian]] trilobite assemblages are superficially similar to Ordovician assemblages, dominated by [[Lichida]] and [[Phacopida]] (including the well-known [[Calymenina]]).<ref name="Fortey&Owens97a" /> The Silurian diversity of trilobites was high during the [[Llandovery Epoch|Llandovery]] and [[Wenlock Epoch|Wenlock]], though there was a sharp drop during the [[Pridoli Epoch|Pridoli]] at the end of the period, followed by a diversification during the Early Devonian, reaching a highpoint of 180 trilobite genera during the [[Emsian]] stage.<ref name=":0" />

The Middle-Late Devonian was a decisive turning point in trilobite history, with the [[Taghanic event]] during the [[Givetian]] sharply decreasing trilobite diversity, particularly in shallow water environments, which was followed by the [[Late Devonian mass extinction]]/Kellwasser event (involving a combination of [[Past sea level|sea level change]] and [[Anoxic event|marine anoxia]]) at the [[Frasnian]]-[[Famennian]] boundary, widely regarded as one of the most significant mass extinction events in Earth's history, decimating the groups diversity including the extinction of the orders [[Corynexochida]], [[Harpetida]] and [[Odontopleurida]], with the low trilobite diversity in its aftermath in the Famennian, consisting only of the orders [[Phacopida]] and [[Proetida]], being again strongly impacted by the [[Hangenberg event]] (also called the end-Devonian mass extinction) at the end of the Devonian, with both shallow water and deep water trilobites being affected.<ref name=":0" /> Only a single order, the Proetida, survived into the Carboniferous.<ref name="Clarkson98" />

Genera of trilobites during the Silurian and Devonian periods include:<ref name="PREHISTORIC LIFE" />

*''[[Dalmanites]]'' (Early to Late Silurian)
*''[[Calymene]]'' (Silurian)
*''[[Encrinurus]]'' (Silurian)
*''[[Exallaspis]]'' (Middle to Late Silurian)
*''[[Paralejurus]]'' (Early Devonian)
*''[[Lioharpes]]'' (Early-Middle Devonian)
*''[[Phacops]]'' (Middle to Late Devonian)

====Carboniferous and Permian====
The [[Proetida]], the only trilobite order to survive the end of the Devonian, continued through the [[Carboniferous]] period and lasted until the end of the [[Permian]] (when [[Permian–Triassic extinction event|the vast majority of species on Earth were wiped out]]).<ref name="Clarkson98" /> Proetids are generally morphologically homogeneous (similar to each other), having a generally conservative bodyform,<ref name=":2">{{Cite journal |last1=Hopkins |first1=Melanie J. |last2=Wagner |first2=Peter J. |last3=Jordan |first3=Katherine J. |date=2023-04-17 |title=Permian trilobites and the applicability of the "living fossil" concept to extinct clades |journal=Frontiers in Ecology and Evolution |volume=11 |doi=10.3389/fevo.2023.1166126 |doi-access=free |issn=2296-701X}}</ref> and were probably all predators or scavengers. Trilobites rapidly diversified during the earliest Carboniferous ([[Tournaisian|Tournasian]]), reaching diversity levels unseen since prior to the Taghanic event, though most of this diversification was of the family [[Phillipsiidae]], with other trilobite families barely rebounding. During the [[Serpukhovian]] at the end of the Early Carboniferous, trilobite diversity again strongly declined, and trilobite diversity remained stagnantly low throughout the late Carboniferous. Trilobite diversity may have been have been effected by ecological changes during the Carboniferous, such as the rise of [[durophagous]] fish with crushing mouthparts.<ref name=":0" />

By the end of the Carboniferous, the diversity of trilobites had dropped to only 1.8-2.2% (around 7 genera<ref name=":1">{{Cite journal |last=Brezinski |first=David K. |date=January 2023 |title=Biogeographic patterns in Late Paleozoic trilobites |url=https://linkinghub.elsevier.com/retrieve/pii/S0031018222004904 |journal=Palaeogeography, Palaeoclimatology, Palaeoecology |language=en |volume=609 |pages=111319 |bibcode=2023PPP...60911319B |doi=10.1016/j.palaeo.2022.111319}}</ref>) of the peak diversity it had had during the early Paleozoic, with this low diversity continuing into the Permian. During the Permian period, while trilobites were widespread and occurred in a variety of environments, they were typically rare components of local faunas, in sharp contrast to their often great abundance earlier in the Paleozoic.<ref name=":2" /> Permian trilobite diversity reached a peak during the [[Guadalupian]] with diversity sharply dropping by the beginning of the following [[Lopingian]].<ref>{{Citation |last1=Lerosey-Aubril |first1=Rudy |title=Quantitative Approach to Diversity and Decline in Late Palaeozoic Trilobites |date=2012 |work=Earth and Life |pages=535–555 |url=https://doi.org/10.1007/978-90-481-3428-1_16 |access-date=2025-03-16 |place=Dordrecht |publisher=Springer Netherlands |isbn=978-90-481-3427-4 |last2=Feist |first2=Raimund|doi=10.1007/978-90-481-3428-1_16 }}</ref>

Some of the genera of trilobites during the Carboniferous and Permian periods include:<ref name="PREHISTORIC LIFE" />

*''[[Archegonus]] ''(Early to Middle Carboniferous)
*''[[Hesslerides]]'' (Middle Carboniferous)
*''[[Endops]]'' ([[Guadalupian|Middle Permian]])
*''[[Triproetus]]'' (Late Carboniferous to [[Cisuralian|Early Permian]])
*''[[Ditomopyge]] ''(Late Carboniferous to Late Permian)
*''[[Pseudophillipsia]] ''(Late Carboniferous to Late Permian)

====Final extinction====
At the end of the Permian ([[Changhsingian]]), only two genera of trilobites remained extant, ''[[Acropyge]]'' and ''Pseudophillipsia.<ref name=":2" />'' [[Late Permian]] trilobites primarily occurred in shallow marine [[carbonate platform]] environments, but were also found in deep water, and were widespread, ranging towards the poles.<ref name=":2" /> 

Exactly why the trilobites became extinct is not clear; with repeated extinction events (often followed by apparent recovery) throughout the trilobite fossil record, a combination of causes is likely. After the extinction event at the end of the Devonian period, what trilobite diversity remained was bottlenecked into the order Proetida. Decreasing diversity<ref name="Owens03">
{{Citation |last1=Owens |first1=R. M. |contribution=The stratigraphical distribution and extinctions of Permian trilobites. |editor=Lane, P. D. |editor2=Siveter, D. J. |editor3=Fortey R. A. |series=Special Papers in Palaeontology|volume= 70 |title=Trilobites and Their Relatives: Contributions from the Third International Conference, Oxford 2001 |pages=377–397 |publisher=Blackwell Publishing & Palaeontological Association |year=2003}}</ref> of genera limited to shallow-water shelf habitats coupled with a drastic lowering of sea level ([[Marine regression|regression]]) meant that the final decline of trilobites happened shortly before the [[Permian–Triassic extinction event|end Permian mass extinction event]].<ref name="Fortey&Owens97a" /> With so many marine species involved in the Permian extinction, the end of nearly 300{{nbsp}}million successful years for the trilobites would not have been unexpected at the time.<ref name="Owens03" />