{{Taxobox
| color = pink
| name = Agnostida
| fossil_range = [[Early Cambrian]] - [[Late Ordovician]]
| image = Peronopsis.jpg
| image_width = 250px
| image_caption = ''Peronopsis interstrictus''
| regnum = [[Animal]]ia
| phylum = [[Arthropod]]a
| classis = [[Trilobita]]
| ordo = '''Agnostida'''
| ordo_authority = [[John William Salter|Salter]], 1864
| subdivision_ranks = Families
| subdivision =
'''Suborder [[Agnostina]]'''<br />
*'''Superfamily [[Agnostoidea]]'''
**[[Agnostidae]]
**[[Ammagnostidae]]
**[[Clavagnostidae]]
**[[Diplagnostidae]]
**[[Doryagnostidae]]
**[[Glyptagnostidae]]
**[[Metagnostidae]]
**[[Peronopsidae]]
**[[Ptychagnostidae]]
**[[Spinagnostidae]]
*'''Superfamily&nbsp;[[Condylopygoidea]]'''
**[[Condylopygidae]]
'''Suborder [[Eodiscina]]'''<br />
*'''Superfamily&nbsp;[[Eodiscoidea]]'''
**[[Calodiscidae]]
**[[Eodiscidae]]
**[[Hebediscidae]]
**[[Tsunyidiscidae]]
**[[Weymouthiidae]]
**[[Yukoniidae]]
}}

'''Agnostida''' (the '''agnostids''') is an [[order (biology)|order]] of [[trilobite]]. These small trilobites first appeared toward the end of the Early [[Cambrian]] and thrived in the Middle Cambrian. They are present in the lower Cambrian fossil record along with trilobites from Orders [[Redlichiida]], [[Corynexochida]], and [[Ptychopariida]]. The last agnostids went [[extinct]] in the Late [[Ordovician]].

The Agnostida are divided into two suborders -- [[Agnostina]] and [[Eodiscina]] -- that are then divided into a number of [[family (biology)|families]]. As a group, agnostids have ''[[Pygidium|pygidia]]'' (tails) that are so similar in size and shape to their ''[[cephalon]]s'' (heads) that it is difficult to distinguish which end is which. Most agnostid species were eyeless.

The systematic position of Order Agnostida within 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 the Agnostina partly because juveniles of one genus have been found with legs greatly different from those of adult trilobites, suggesting they are separately descended from [[Crustaceans]]. Other researchers have suggested, based on [[cladistic]] analyses, that Eodiscina and Agnostida are closely united, and that the Eodiscina descended from the trilobite Order [[Ptychopariida]].

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 all suggest a benthic (bottom-dwelling) mode of life. They likely lived on areas of the ocean floor that received little or no light and fed on [[detritus]] that descended from upper layers of the sea to the bottom. In contrast, 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|ostracodan]]-type lifestyle. Furthermore, the orientation of the thoracic appendages appears ill suited for benthic living.

Agnostina are generally referred to simply as "agnostids" even though they probably should be called "agnostines".

==References==

*Cotton, TJ and Fortey. RA. (2005) Comparative morphology and relationships of the Agnostida. In: Koenemann, S. & Jenner, R. (eds.). Crustacean Issues 16, Crustacea and Arthropod Relationships (CRC Press: Boca Raton).
*Fortey RA. (2001) Trilobite systematics: The last 75 years. Journal of Paleontology Fortey, RA. and Theron J. (1994) A new Ordovician arthropod Soomaspis and the agnostid problem. Palaeontology 37:841-61.
*Jell, P A (2003) Phylogeny of Early Cambrian trilobites. Special Papers in Palaeontology 70:45-57. 5:1141–1151.
* [http://www.trilobites.info/ordagnostida.htm A Guide to the Orders of Trilobites - ORDER AGNOSTIDA]
* [http://www.fossilmuseum.net/Fossil_Galleries/Trilobites-Agnostida.htm The Virtual Fossil Museum - Trilobite Order Agnostida]

==External links==
* [http://www.aloha.net/~smgon/ordagnostida.htm Agnostida fact sheet]

[[Category:Trilobites]]
[[Category:Cambrian animals]]
[[Category:Ordovician animals]]

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