Editing Agnostida (section)

==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" />