most salient and distinguishing trait of the Harpetida (pronounced
Har-pe-tee-da) is a semicircular brim surrounding the cephalon
(head) that is often perforated by small pores. This often very
wide brim is thought to have filtered sediment while feeding.
The brim extends backward from the front of the cephalon, and
typically has a prominent suture along its anterior boundary.
The Harpetida fossils normally show small eye nodules, 12 or more
thoracic segments, and small pygidia. They lived from the Upper
Cambrian and persisted to the Late Devonian period.
became a distinct trilobite order only recently in 2002, after
being split from Order Ptychopariida in which they were suborder
Harpina. The Harpetids are most often considered to be descended
from the Ptychopariids. Moreover, the Harpetida are now often
considered to be a member of the Librostoma, a suborder erected
by Fortey in 1990 to encompass Ptychopariida, Harpetida, Asaphida,
and Proetida based in part on the shared trait of a natant hypostomal
attachment (i.e., a hypostome mouthpart not anchored to the anterior
Ebach, M.C. & K.J. McNamara.
2002. A systematic revision of the family Harpetidae (Trilobita).
Records of the Western Australian Museum 21:135-67.
Fortey, R.A. 1990. Ontogeny, hypostome atachment and trilobite
classification. Palaeontology 33:529-76