Editing Trilobite (section)

====Dorsal sutures====
The dorsal surface of the trilobite cephalon (the frontmost [[tagma (biology)|tagma]], or the 'head') can be divided into two regions—the [[cranidium]] and the [[librigena]] ("free cheeks"). The cranidium can be further divided into the [[glabella]] (the central lobe in the cephalon) and the [[fixigena]] ("fixed cheeks").<ref name="black">{{Citation|author=Rhona M. Black|title =The elements of palaeontology|publisher =Cambridge University Press|edition=2|year =1988|pages= 151–152|isbn =978-0-521-34836-2|url=https://books.google.com/books?id=cEh5Yn0neiUC}}</ref> The facial sutures lie along the anterior edge, at the division between the cranidium and the librigena.

Trilobite facial sutures on the dorsal side can be roughly divided into five main types according to where the sutures end relative to the [[gena]]l angle (the edges where the side and rear margins of the cephalon converge).<ref name="kipping">{{cite web|url=http://www.trilobita.de/english/molting.htm|title=Change of suit|author=Michael Kipping|publisher=www.trilobita.de|access-date=April 13, 2011}}</ref>
*'''Absent''' – Facial sutures are lacking in the [[Olenellina]]. This is considered a primitive state, and is always combined with the presence of eyes. 
*'''Proparian''' – The facial suture ends in front of the genal angle, along the lateral margin.<ref name="black"/> Example genera showing this type of suture include ''[[Dalmanites]]'' of [[Phacopina]] ([[Phacopida]]) and ''[[Ekwipagetia]]'' of [[Eodiscina]] ([[Agnostida]]).
*'''Gonatoparian''' – The facial suture ends at the tip of the genal angle.<ref name="rich">{{Citation|author=Pat Vickers Rich|author2=[[Mildred Adams Fenton]]|author3=[[Carroll Lane Fenton]]|author4=Thomas Hewitt Rich|title =The fossil book: a record of prehistoric life|series=Dover books on animals|publisher =Courier Dover Publications|year =1989|isbn =978-0-486-29371-4|url=https://books.google.com/books?id=_ntSspji0LYC|page =204}}</ref> Example genera showing this type of suture include ''[[Calymene]]'' and ''[[Trimerus]]'' of [[Calymenina]] ([[Phacopida]]).<ref name="gon"/>
*'''Opisthoparian''' – The facial suture ends at the posterior margin of the cephalon.<ref name="gon"/> Example genera showing this type of suture include ''[[Peltura]]'' of [[Olenina]] ([[Ptychopariida]]) and ''[[Bumastus]]'' of [[Illaenina]] ([[Corynexochida]]). This is the most common type of facial suture.<ref name="gon"/>
* '''Hypoparian''' or '''marginal''' – In some trilobites, dorsal sutures may be secondarily lost. Several exemplary time series of species show the "migration" of the dorsal suture until it coincides with the margins of the cephalon.<ref name="black"/> As the visual surface of the eye is on the diminishing free cheek (or librigena), the number of lenses tends to go down, and eventually the eye disappears. The loss of dorsal sutures may arise from the proparian state, such as in some [[Eodiscina]] like ''[[Weymouthia (trilobite)|Weymouthia]]'', all [[Agnostina]], and some [[Phacopina]] such as ''[[Ductina]]''. The marginal sutures exhibited by the [[Harpetida|harpetids]] and [[Asaphida|trinucleioids]] are derived from opisthoparian sutures.<ref name="blind">{{Citation|author1=Euan Clarkson |author2=Riccardo Levi-Setti |author3=Gabor Horvath |name-list-style=amp |year=2006|title=The eyes of trilobites: The oldest preserved visual system|journal=Arthropod Structure & Development|volume=35|pages=247–259|doi=10.1016/j.asd.2006.08.002|issue= 4 |pmid=18089074|bibcode=2006ArtSD..35..247C }}</ref> On the other hand, blindness is not always accompanied by the loss of facial sutures.

[[File:Trilobite facial suture types.png|center|600px]]
The primitive state of the dorsal sutures is proparian. Opisthoparian sutures have developed several times independently. There are no examples of proparian sutures developing in [[Taxon|taxa]] with opisthoparian ancestry. Trilobites that exhibit opisthoparian sutures as adults commonly have proparian sutures as instars (known exceptions being ''[[Yunnanocephalus]]'' and ''[[Duyunaspis]]'').<ref>{{cite journal|last1= Dai|first1= T.|last2= Zhang|first2= X.|year= 2008|title= Ontogeny of the trilobite ''Yunnanocephalus yunnanensis'' from the Chengjiang lagerstätte, lower Cambrian, southwest China|journal= Alcheringa|volume= 32|issue= 4|pages= 465–468|issn= 0311-5518|doi=10.1080/03115510802418057|bibcode= 2008Alch...32..465D|s2cid= 129582955}}</ref> Hypoparian sutures have also arisen independently in several groups of trilobites.

The course of the facial sutures from the front of the visual surface varies at least as strongly as it does in the rear, but the lack of a clear reference point similar to the genal angle makes it difficult to categorize. One of the more pronounced states is that the front of the facial sutures do not cut the lateral or frontal border on its own, but coincide in front of the glabella, and cut the frontal border at the midline. This is, inter alia, the case in the Asaphida. Even more pronounced is the situation that the frontal branches of the facial sutures end in each other, resulting in yoked free cheeks. This is known in ''[[Triarthrus]]'', and in the Phacopidae, but in that family the facial sutures are not functional, as can be concluded from the fact that free cheeks are not found separated from the cranidium.

There are also two types of sutures in the dorsal surface connected to the [[compound eye]]s of trilobites.<ref name="gon">{{cite web|url=http://www.trilobites.info/sutures.htm|title=Trilobite Facial Sutures|author=Samuel M. Gon III|date=February 3, 2009|publisher=A Guide to the Orders of Trilobites|access-date=April 13, 2011|archive-date=May 17, 2011|archive-url=https://web.archive.org/web/20110517134011/http://www.trilobites.info/sutures.htm|url-status=dead}}</ref><ref name="clarkson"/> They are:
*'''Ocular sutures''' – are sutures surrounding the edges of the compound eye. Trilobites with these sutures lose the entire surface of the eyes when molting. It is common among Cambrian trilobites.
*'''Palpebral sutures''' – are sutures which form part of the dorsal facial suture running along the top edges of the compound eye.

=====Ventral sutures=====
[[File:Trilobite cephalon ventral anatomy.png|thumb|right]]
Dorsal facial sutures continue downward to the ventral side of the cephalon where they become the '''Connective sutures''' that divide the doublure. The following are the types of ventral sutures.<ref name="clarkson">{{Citation|author=Euan Neilson Kerr Clarkson|title =Invertebrate palaeontology and evolution|publisher =Wiley-Blackwell|year =1998|url=https://books.google.com/books?id=g1P2VaPQWfUC|isbn =978-0-632-05238-7}}</ref>

*'''Connective sutures''' – are the sutures that continue from the facial sutures past the front margin of the cephalon.
*'''Rostral suture''' – is only present when the trilobite possesses a rostrum (or rostral plate). It connects the rostrum to the front part of the dorsal cranidium.
*'''Hypostomal suture''' – separates the hypostome from the doublure when the hypostome is of the attached type. It is absent when the hypostome is free-floating (i.e. natant). it is also absent in some coterminant hypostomes where the hypostome is fused to the doublure.
*'''Median suture''' – exhibited by [[Asaphida|asaphid]] trilobites, they are formed when instead of becoming connective sutures, the two dorsal sutures converge at a point in front of the cephalon then divide straight down the center of the doublure.