# Agnostida > Mediated Wiki article. Canonical URL: https://mediated.wiki/source/Agnostida > Markdown URL: https://mediated.wiki/source/Agnostida.md > Source: https://en.wikipedia.org/wiki/Agnostida > Source revision: 1353133504 > License: Creative Commons Attribution-ShareAlike 4.0 International (https://creativecommons.org/licenses/by-sa/4.0/) {{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 = [Salter](/source/John_William_Salter), 1864 | synonyms = Isopygia Gürich, 1907
Miomera Jækel, 1909 | subdivision_ranks = Families | subdivision = '''Suborder [Agnostina](/source/Agnostina)''' * '''Superfamily Agnostoidea''' ** [Agnostidae](/source/Agnostidae) ** [Ammagnostidae](/source/Ammagnostidae) ** [Clavagnostidae](/source/Clavagnostidae) ** [Diplagnostidae](/source/Diplagnostidae) ** [Doryagnostidae](/source/Doryagnostidae) ** [Glyptagnostidae](/source/Glyptagnostidae) ** [Metagnostidae](/source/Metagnostidae) ** [Peronopsidae](/source/Peronopsidae) ** [Ptychagnostidae](/source/Ptychagnostidae) ** [Spinagnostidae](/source/Spinagnostidae) * '''Superfamily [Condylopygoidea](/source/Condylopygoidea)''' ** [Condylopygidae](/source/Condylopygidae) '''Suborder [Eodiscina](/source/Eodiscina)''' * '''Superfamily [Eodiscoidea](/source/Eodiscoidea)''' ** [Calodiscidae](/source/Calodiscidae) ** [Eodiscidae](/source/Eodiscidae) ** [Hebediscidae](/source/Hebediscidae) ** [Tsunyidiscidae](/source/Tsunyidiscidae) ** [Weymouthiidae](/source/Weymouthiidae) ** [Yukoniidae](/source/Yukoniidae) }} '''Agnostida''' are an [order](/source/order_(biology)) of extinct [arthropods](/source/arthropod) which have classically been seen as a group of highly modified [trilobites](/source/Trilobite), though some recent research has doubted this placement. Regardless, they appear to be close relatives as part of the [Artiopoda](/source/Artiopoda).{{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 | article-number = 20182314 | date = January 2019 | pmid = 30963877 | pmc = 6367181 | doi = 10.1098/rspb.2018.2314 }} They are present in the [Lower Cambrian](/source/Lower_Cambrian) fossil record along with [trilobites](/source/trilobites) from the [Redlichiida](/source/Redlichiida), [Corynexochida](/source/Corynexochida), and [Ptychopariida](/source/Ptychopariida) orders, and were highly diverse throughout the [Cambrian](/source/Cambrian). Agnostidan diversity severely declined during the Cambrian-[Ordovician](/source/Ordovician) transition, and the last agnostidans went [extinct](/source/extinct) in the Late Ordovician.{{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 | article-number = e2019208118 | pmid = 33827921 | doi = 10.1073/pnas.2019208118 | pmc = 8053996 | s2cid = 233184869 | doi-access = free }} ==Systematics== The Agnostida are divided into two suborders — [Agnostina](/source/Agnostina) and [Eodiscina](/source/Eodiscina) — which are then subdivided into a number of [families](/source/Family_(taxonomy)). As a group, agnostids are isopygous, meaning their [pygidium](/source/pygidium) is similar in size and shape to their [cephalon](/source/cephalon_(arthropod_anatomy)). 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](/source/stem_group). The challenge to the status has focused on Agnostina partly due to the [juveniles](/source/juvenile_(organism)) of one genus have been found with legs differing dramatically from those of adult trilobites,{{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 = 82-00-07511-7 }} suggesting they are not members of the [lamellipedia](/source/lamellipedia)n [clade](/source/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](/source/proximal_endite), which defines that group. The study suggested that they were likely the [sister taxon](/source/sister_taxon) to the crustacean stem lineage, and, as such, part of the clade, [Crustaceomorpha](/source/Crustaceomorpha).{{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 }} Other researchers have suggested, based on a [cladistic](/source/cladistic) analyses of [dorsal](/source/dorsum_(anatomy)) [exoskeletal](/source/exoskeleton) features, that Eodiscina and Agnostida are closely united, and the Eodiscina descended from the trilobite order [Ptychopariida](/source/Ptychopariida).{{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 }} A 2019 study of adult specimens with preserved soft tissue from the [Burgess Shale](/source/Burgess_Shale) found that agnostidans shared morphological similarities to trilobites and other related [artiopodans](/source/artiopodans) like [nektaspids](/source/Nektaspida), and their placement as stem-crustaceans was unsupported. The study recovered agnostidans as the sister group to other trilobites within the Artiopoda. ==Ecology== Scientists have long debated whether the agnostids lived a [pelagic](/source/pelagic) or a [benthic](/source/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](/source/detritus) which descended from upper layers of the sea to the bottom. Their wide geographic dispersion in the [fossil record](/source/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](/source/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.{{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 }} They are sometimes preserved within the voids of other organisms, for instance within empty [hyolith](/source/hyolith) conchs,{{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 }} within [sponge](/source/sponge)s, [worm tubes](/source/Selkirkia) and under the carapaces of [bivalved arthropods](/source/Sidneyia),{{Chatterton 2003|name-list-style=vanc}} presumably in order to hide from predators or strong storm currents; or maybe whilst scavenging for food. In the case of the tapering worm tubes ''[Selkirkia](/source/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. == 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"] {{Webarchive|url=https://web.archive.org/web/20111119210609/http://news.discovery.com/animals/early-animals-cannibals.html |date=19 November 2011 }}, 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 --- Adapted from the Wikipedia article [Agnostida](https://en.wikipedia.org/wiki/Agnostida) by Wikipedia contributors ([contributor history](https://en.wikipedia.org/wiki/Agnostida?action=history)). Available under [Creative Commons Attribution-ShareAlike 4.0 International](https://creativecommons.org/licenses/by-sa/4.0/). Changes may have been made.