o Arthropoda `--o TRILOBITA |?-Agnostina `--+--o Redlichiida | |--Olenellina | `--Redlichiina |--Corynexochida |--Phacopida |--Lichida `--o Librostoma |--Proetida |--Harpetida [Harpoidea] |--Ptychopariida |?-Eodiscina `--Asaphida
The Harpetida are characterized among trilobites by bearing a semicircular brim around the cephalon (head) which is often perforated by small pores. This brim is thought to serve as a filter-feeding apparatus. The brim stretches backward on either side of the cephalon (head) and typically has a pronounced suture along the outside.
The eyespots are typically reduced to small tubercles, though they have strong ridges stretching to the glabella (central region of the cephalon). They also typically have 12 or more thoracic segments. The pygidia are usually small.
The Harpetida were formerly included in the order Ptychopariida, but were recently given their own order (Ebach & McNamara 2002). The subclass Librostoma was created in 1990 by Richard Fortey to cover the orders formally included with the Ptychopariida. The name Harpidae was once used as the name for the trilobite family containing the type genus Harpes. However, this is in conflict with the use of the same name for a family of extant molluscs and that taxon had precedence.
Ascii phylogenetic tree goes here
- Order Harpetida - from A Guide to the Orders of Trilobites by Sam Gon III
Introduction by DanielCD Wikipedia