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The trilobites of Order Agnostida appeared in the Early where they were diverse and ubiquitous, and declined to become rare in the prior to their complete extinction by the end of the Ordovician. Among trilobites, they are so relatively unusual that some researchers have suggested, based on larval limb structure, that one of the two suborders, Agnostina, should not be included in Class Trilobita.The agnostids have been popularly thought to have been planktonic, often lacked eyes, and mainly exhibited symmetry of and size, so much so that it may be hard to distinguish front from rear. There are two Suborders. Members of Suborder have two segments between the cephalon and pygidium, and lack have sutures on the cephalon. In contrast, members of Suborder may have two or three body segments, and some have small eyes and proparian sutures that have a less rear-sloping angle that the genal angle.
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Of the two suborders, the Eodiscina appear to be "normal" trilobites with only two or three segments in the thorax; some resemble trilobites of the order . As a group, agnostids have (tails) that are similar in size and shape to their (heads). Neither looks much like the corresponding regions of other trilobites. There has been more than one argument about which end is the "head".
Agnostids were probably benthic (bottom-dwelling) creatures, rather than being plankters. This is because of the fact that most agnostid species have no eyes but however the sub-order Eodiscina almost always have eyes, whereas the vast majority of planktonic arthropods almost always comparatively huge eyes. 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.
Unfortunately, the appendages are known only for one of agnostid. The legs of that genus look much more like legs than the legs of other trilobites with preserved appendages. This has caused many to question whether the agnostids are truly trilobites. Another view is that the agnostids represent the first line to have diverged from the trilobites. However, four orders of trilobites (, , , ) considerably predate the earliest Agnostids in the record.
Agnostina are generally referred to simply as "agnostids" even though they probably should be called "agnostines".
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Agnostida [Miomera]
| i. s.: Clavagnostidae
| |--Clavagnostus
| `--Aspidagnostus
| Ammagnostidae
| |--Ammagnostus Öpik 1967
| |--Proagnostus Butts 1926
| `--Hadragnostus Öpik 1967
| Peronopsidae
| |--Peronopsis interstrictus
| `--Lisogoragnostus Rozova in Lisogor et al. 1988
| |--*L. kalisae Rozova in Lisogor et al. 1988
| |--L. circularis (Yang & Liu in Yang et al. 1991)
| |--L. coreanicus Hong, Lee & Choi 2003
| |--L. hybus Peng & Robison 2000
| |--L. mictus Peng & Robison 2000
| |--L. minor (Kobayashi 1962) [=Phalacromina minor]
| |--L. rasettii (Pratt 1992)
| `--L. shergoldi (Pratt 1992)
| Ptychagnostidae
| |--
| `--Ptychagnostus
| |--P. nathorsti
| `--P. punctuosus
| Glyptagnostus
| |--G. reticulatus
| `--G. stolidotus
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`--Agnostina
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`--Agnostidae
| i. s.: Homagnostus Howell 1935
| |--H. comptus Palmer 1962
| `--H. obesus
| Hypagnostus Jaekel 1909 [incl. Cyclopagnostus Howell 1937]
|--
`--Quadrahomagnostinae
|--Phalacroma Hawle & Corda 1847
`--Peratagnostus Öpik 1967
|--*P. nobilis Öpik 1967
|--‘Cyclopagnostus’ asper Lazarenko 1966
|--‘Leiopyge’ controversa Kryskov in Borovikov & Kryskov 1963
|--P. hillardensis Palmer 1968
|--‘Agnostus (Leiopyge)’ obsoletus Kobayashi 1935 (see below for synonymy)
|--‘Cyclopagnostus’ orientalis Lazarenko 1966
`--‘Phalacroma’ sinica Hsiang in Jegorova et al. 1963
‘Agnostus (Leiopyge)’ obsoletus Kobayashi 1935 [=Phoidagnostus obsoletus]
* Type species of generic name indicated
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Cooper, R. A. 1968. Lower and Middle Paleozoic fossil localities of north-west Nelson. Transactions of the Royal Society of New Zealand, Geology 6 (7): 75-89.
Fensome, R. A., F. J. R. Taylor, G. Norris, W. A. S. Sarjeant, D. I. Wharton & G. L. Williams. 1993. A classification of living and fossil dinoflagellates. Micropaleontology Special Publication 7: i-viii, 1-351.
Fortey, R. A. 2001. Trilobite systematics: The last 75 years. Journal of Paleontology 75 (6): 1141-1151.
Hong, P. S., J. G. Lee & D. K. Choi. 2003. Trilobites from the Lejopyge armata zone (upper Middle Cambrian) of the Machari Formation, Yongwol Group, Korea. Journal of Paleontology 77 (5): 895-907.
Lauridsen, B. W., & A. T. Nielsen. 2005. The Upper Cambrian trilobite Olenus at Andrarum, Sweden: A case of iterative evolution? Palaeontology 48 (5): 1041-1056.
Prothero, D. R. 1998. Bringing Fossils to Life: An introduction to paleobiology. WCB McGraw-Hill: Boston.
Shergold, J. H. 1980. Late Cambrian trilobites from the Chatsworth Limestone, western Queensland. Bureau of Mineral Resources, Geology and Geophysics – Bulletin 186: 1-111.
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Introduction: Roger Perkins; taxonomy and references 05:08, 3 October 2008 (PDT); Remainder of text: DanielCD