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Agnostida
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!bgcolor="pink"|Families
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Suborder Agnostina
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- Superfamily Agnostoidea
- Agnostidae
- Ammagnostidae
- Clavagnostidae
- Diplagnostidae
- Doryagnostidae
- Glyptagnostidae
- Metagnostidae
- Peronopsidae
- Ptychagnostidae
- Superfamily Condylopygoidea
Suborder Eodiscina
- Superfamily Eodiscoidea
- Calodiscidae
- Eodiscidae
- Hebediscidae
- Tsunyidiscidae
- Weymouthiidae
- Yukoniidae
| } | Agnostida (the agnostids) is an order of trilobite. They first appeared toward the end of the lower Cambrian, and thrived in the middle Cambrian. The last agnostids held out until the late Ordovician. The Agnostida are divided into two suborders -- Agnostina and Eodiscina -- that are then divided into a number of families. The Eodiscina appear to be "normal" trilobites with only two or three segments in the thorax. They have pygidia (tails) that look much like their cephalons (heads), and neither looks much like the corresponding regions of other trilobites. There has been more than one argument about which end is the "head".
Unfortunately, the appendages are known only for one genus of Agnostid. The legs of that one genus look much more like crustacean legs than the legs on other trilobites with preserved appendages. This has caused many taxonomists to question whether the Agnostids truly are trilobites. Another view is that the agnostids represent the first line to have diverged from the trilobites. However, four orders of trilobites (Redlichiida, Corynexochida, Naraoidia, Ptychopariida) considerably predate the earliest Agnostids in the fossil record.
Agnostina are generally referred to simply as "agnostids" even though they probably should be called "agnostines".
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To view or edit this article at Wikipedia go to http://www.wikipedia.org/wiki/Agnostida
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