Phacopida


Phacopida is an order of trilobites that lived from the Late Cambrian to the Late Devonian. It is made up of a morphologically diverse assemblage of taxa in three related suborders.

Characteristics

Image:Phacops and Walliserops.jpg|left|thumb|Reconstruction of Phacops and Walliserops
Phacopida had 8 to 19 thoracic segments and are distinguishable by the expanded glabella, short or absent preglabellar area, and schizochroal or holochroal eyes. Schizochroal eyes are compound eyes with up to around 700 separate lenses. Each lens has an individual cornea which extended into a rather large sclera.
The development of schizochroal eyes in phacopid trilobites is an example of post-displacement paedomorphosis. The eyes of immature holochroal Cambrian trilobites were basically miniature schizochroal eyes. In Phacopida, these were retained, via delayed growth of these immature structures, into the adult form.
Eldredgeops rana and Dalmanites limulurus are two of the best-known members of this order. Other known phacopids include Cheirurus, Deiphon, Calymene, Flexicalymene and Ceraurinella.

Evolution

The origin of the order Phacopida is uncertain. It comprises three suborders which share a distinctive protaspis with three pairs of spines on its body. The Cheirurina and Calymenina retain a rostral plate but in virtually all Phacopina the free cheeks are yoked as a single piece. This sort of similarity in development suggests phylogenetic unity. The suborder Calymenina is the most primitive of the Phacopida order and shares some characteristics with the order Ptychopariida, though it is not included in the subclass Librostoma.
Phacopida was one of only two trilobite orders to survive the Kellwasser event at the Frasnian-Famennian boundary. However, they would die out soon afterwards at the Hangenberg event at the end of the Famennian.

Classification