Editing Agnostida

{{Short description|Extinct order of arthropods}}
{{Use dmy dates|date=October 2021}}
{{Automatic taxobox
| taxon = Agnostida
| fossil_range = {{fossil_range|Cambrian Stage 3 |Late Ordovician}}
| image = Itagnostus interstrictus (White, 1874) - 8 mm 1.JPG
| image_caption = ''Itagnostus interstrictus''
| display_parents = 3
| authority = [[John William Salter|Salter]], 1864
| synonyms = Isopygia Gürich, 1907<br />
Miomera Jækel, 1909
| subdivision_ranks = Families
| subdivision =
'''Suborder [[Agnostina]]'''
* '''Superfamily Agnostoidea'''
** [[Agnostidae]]
** [[Ammagnostidae]]
** [[Clavagnostidae]]
** [[Diplagnostidae]]
** [[Doryagnostidae]]
** [[Glyptagnostidae]]
** [[Metagnostidae]]
** [[Peronopsidae]]
** [[Ptychagnostidae]]
** [[Spinagnostidae]]
* '''Superfamily [[Condylopygoidea]]'''
** [[Condylopygidae]]
'''Suborder [[Eodiscina]]'''
* '''Superfamily [[Eodiscoidea]]'''
** [[Calodiscidae]]
** [[Eodiscidae]]
** [[Hebediscidae]]
** [[Tsunyidiscidae]]
** [[Weymouthiidae]]
** [[Yukoniidae]]
}}

'''Agnostida''' are an [[order (biology)|order]] of extinct [[arthropod|arthropods]] which have classically been seen as a group of highly modified [[Trilobite|trilobites]], though some recent research has doubted this placement. Regardless, they appear to be close relatives as part of the [[Artiopoda]].<ref name=":0">{{cite journal | vauthors = Moysiuk J, Caron JB | title = Burgess Shale fossils shed light on the agnostid problem | journal = Proceedings. Biological Sciences | volume = 286 | issue = 1894 | pages = 20182314 | date = January 2019 | pmid = 30963877 | pmc = 6367181 | doi = 10.1098/rspb.2018.2314 }}</ref> They are present in the [[Lower Cambrian]] fossil record along with [[trilobites]] from the [[Redlichiida]], [[Corynexochida]], and [[Ptychopariida]] orders, and were highly diverse throughout the [[Cambrian]]. Agnostidan diversity severely declined during the Cambrian-[[Ordovician]] transition, and the last agnostidans went [[extinct]] in the Late Ordovician.<ref>{{cite journal | vauthors = Barnes BD, Sclafani JA, Zaffos A | title = Dead clades walking are a pervasive macroevolutionary pattern | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 118 | issue = 15 | date = April 2021 | pmid = 33827921 | doi = 10.1073/pnas.2019208118 | pmc = 8053996 | s2cid = 233184869 | doi-access = free }}</ref>

==Systematics==
The Agnostida are divided into two suborders&nbsp;— [[Agnostina]] and [[Eodiscina]]&nbsp;— which are then subdivided into a number of [[family (biology)|families]]. As a group, agnostids are isopygous, meaning their [[pygidium]] is similar in size and shape to their [[cephalon (arthropod anatomy)|cephalon]]. Most agnostid species were eyeless.

The systematic position of the order Agnostida within the class Trilobita remains uncertain, and there has been continuing debate whether they are trilobites or a [[stem group]]. The challenge to the status has focused on Agnostina partly due to the [[juvenile (organism)|juveniles]] of one genus have been found with legs differing dramatically from those of adult trilobites,<ref>{{cite journal | vauthors = Müller KJ, Walossek D | date = 1987 | title = Morphology, ontogeny, and life habit of ''Agnostus pisiformis'' from the Upper Cambrian of Sweden | journal = Fossils and Strata | volume = 19 | pages = 1–124 | doi = 10.18261/8200075117-1987-01 | isbn = 8200075117 }}</ref> suggesting they are not members of the [[lamellipedia]]n [[clade]], of which trilobites are a part. Instead, the limbs of agnostids closely resemble those of stem group crustaceans, although they lack the [[proximal endite]], which defines that group. The study suggested that they were likely the [[sister taxon]] to the crustacean stem lineage, and, as such, part of the clade, [[Crustaceomorpha]].<ref>{{cite book | vauthors = Bergström J, Hou XG | date = 2005 | chapter = Early Palaeozoic non-lamellipedian arthropods | veditors = Koenemann S, Ronald AJ | title = Crustacea and Arthropod Relationships | volume = | pages = 75–93 | publisher = Taylor and Francis Group | isbn = 978-1-4200-3754-8 }}</ref> Other researchers have suggested, based on a [[cladistic]] analyses of [[dorsum (anatomy)|dorsal]] [[exoskeleton|exoskeletal]] features, that Eodiscina and Agnostida are closely united, and the Eodiscina descended from the trilobite order [[Ptychopariida]].<ref>{{cite book | vauthors = Cotton TJ, Fortey RA | author-link2 = Richard Fortey | date = 2005 | chapter = Comparative morphology and relationships of the Agnostida | veditors = Koenemann S, Jenner RA | title = Crustacea and Arthropod Relationships | volume = 16 | pages = 95–136 | publisher = Taylor and Francis Group | isbn = 978-1-4200-3754-8 }}</ref> A 2019 study of adult specimens with preserved soft tissue from the [[Burgess Shale]] found that agnostidans shared morphological similarities to trilobites and other related [[artiopodans]] like [[Nektaspida|nektaspids]], and their placement as stem-crustaceans was unsupported. The study recovered agnostidans as the sister group to other trilobites within the Artiopoda.<ref name=":0" />

==Ecology==
Scientists have long debated whether the agnostids lived a [[pelagic]] or a [[benthic]] lifestyle. Their lack of eyes, a morphology not well-suited for swimming, and their fossils found in association with other benthic trilobites suggest a benthic (bottom-dwelling) mode of life. They are likely to have lived on areas of the ocean floor which received little or no light and fed on [[detritus]] which descended from upper layers of the sea to the bottom. Their wide geographic dispersion in the [[fossil record]] is uncharacteristic of benthic animals, suggesting a pelagic existence. The thoracic segment appears to form a hinge between the head and pygidium allowing for a bivalved [[ostracod]]an-type lifestyle. The orientation of the thoracic appendages appears ill-suited for benthic living. Recent work suggests that some agnostids were benthic predators, engaging in cannibalism and possibly pack-hunting behavior.<ref>{{cite journal | vauthors = McMenamin MA | title = Cambrian cannibals: agnostid trilobite ethology and the earliest known case of arthropod cannibalism. | journal = Geological Society of America Abstracts with Programs | date = October 2010 | volume = 42 | issue = 5 | page = 320 }}</ref>

They are sometimes preserved within the voids of other organisms, for instance within empty [[hyolith]] conchs,<ref>{{cite journal | vauthors = Fatka O, Vokáč V, Moravec J, Šinágl M, Valent M | title = Agnostids entombed in hyolith conchs. | journal = Memoirs of the Association of Australasian Palaeontologists | date = January 2009 | volume = 37 | pages = 481–489 }}</ref> within [[sponge]]s, [[Selkirkia|worm tubes]] and under the carapaces of [[Sidneyia|bivalved arthropods]],<ref name=Chatterton_2003>{{Chatterton 2003|name-list-style=vanc}}</ref> presumably in order to hide from predators or strong storm currents; or maybe whilst scavenging for food.<ref name=Chatterton_2003/> In the case of the tapering worm tubes ''[[Selkirkia]]'', trilobites are always found with their heads directed towards the opening of the tube, suggesting that they reversed in; the absence of any moulted carapaces suggests that moulting was not their primary reason for seeking shelter.<ref name=Chatterton_2003/>

== References ==
{{Reflist}}

== External links ==
* [http://www.trilobites.info/ordagnostida.htm Order Agnostida] by Sam Gon III.
* [http://www.fossilmuseum.net/Fossil_Galleries/Trilobites-Agnostida.htm The Virtual Fossil Museum – Trilobite Order Agnostida]
* [https://web.archive.org/web/20040407030239/http://www.aloha.net/~smgon/ordagnostida.htm Agnostida fact sheet] by Sam Gon III.
* [http://news.discovery.com/animals/early-animals-cannibals.html "Earth's Early Cannibals Caught in the Act"], by Larry O'Hanlon, news.discovery.com.

{{Trilobites}}
{{Taxonbar|from=Q134109}}

[[Category:Agnostida| ]]
[[Category:Trilobite orders]]
[[Category:Cambrian trilobites]]
[[Category:Ordovician trilobites]]
[[Category:Cambrian first appearances]]
[[Category:Late Ordovician extinctions]]
[[Category:Taxa named by John William Salter]]