Agnostida
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Suborder Agnostina
Suborder Eodiscina |
Agnostida (the agnostids) is an order of trilobite. These small trilobites 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; some resemble trilobites of the order Ptychopariida. As a group, agnostids have pygidia (tails) that are similar in size and shape to their cephalons (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. Most agnostid species have no 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 genus of agnostid. The legs of that genus look much more like crustacean legs than the legs of other trilobites with preserved appendages. This has caused many taxonomists 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 (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".
External link
- Agnostida fact sheet (http://www.aloha.net/~smgon/ordagnostida.htm)
- A Guide to the 8 Orders of Trilobites (http://www.trilobites.info/ordagnostida.htm) By Sam Gon IIIde:Agnostida