Asaphida Fortey & Chatterton 1988
Middle Cambrian --> Upper Silurian
Asaphida is a large, morphologically diverse order of trilobite found in strata dated from the Middle Cambrian to the Silurian. The order contains six superfamilies, but no suborders. Asaphids comprise some 20% of described fossil trilobites.
The Asaphida generally have cephalon (head) and pygidium (tail) parts similar in size, and most species have a prominent median ventral suture. Heads are often flat, and carapace furrows in the head area are often faint or not visible. Thoracic segments typically number 5 - 12, though some species have as few as two and some as many as 30. They also generally have a wide doublure, or rim, that surrounds the cephalon. This causes some specimens to be described as having a characteristic "snowplow" shaped cephalon. Eyes are typically large.
The major extinction event marking the end of the Ordovician Period reduced the diversity of all trilobite orders with most Asaphid families disappearing. The only surviving Asaphids were members of Superfamily Trinucleioidea, and they too disappeared before the end of the Silurian Period.
Source: Wikispecies, Wikipedia: All text is available under the terms of the GNU Free Documentation License