Agnostida Salter 1864
Agnostida is an order of trilobite which first developed near the end of the Early Cambrian period and thrived during the Middle Cambrian. They are present in the lower Cambrian fossil record along with trilobites from the Redlichiida, Corynexochida, and Ptychopariida orders. 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 families. As a group, agnostids have pygidia (tails) that are so similar in size and shape to their cephalons (heads) that it is difficult to distinguish which end is which. 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 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 are likely to have 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 ostracodan-type lifestyle. Furthermore, the orientation of the thoracic appendages appears ill suited for benthic living.
* Cotton, T.J. and Fortey. R.A. (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).
Source: Wikispecies, WIkipedia: All text is available under the terms of the GNU Free Documentation License