Phacopida is an order

Order (biology)

In scientific classification used in biology, the order is# a taxonomic rank used in the classification of organisms. Other well-known ranks are life, domain, kingdom, phylum, class, family, genus, and species, with order fitting in between class and family...

 of trilobite

Trilobite

Trilobites are a well-known fossil group of extinct marine arthropods that form the class Trilobita. The first appearance of trilobites in the fossil record defines the base of the Atdabanian stage of the Early Cambrian period , and they flourished throughout the lower Paleozoic era before...

 that lived from the Ordovician

Ordovician

The Ordovician is a geologic period and system, the second of six of the Paleozoic Era, and covers the time between 488.3±1.7 to 443.7±1.5 million years ago . It follows the Cambrian Period and is followed by the Silurian Period...

 to the Devonian

Devonian

The Devonian is a geologic period and system of the Paleozoic Era spanning from the end of the Silurian Period, about 416.0 ± 2.8 Mya , to the beginning of the Carboniferous Period, about 359.2 ± 2.5 Mya...

. It is made up of a morphologically diverse group of related suborders.

The origin of the Phacopida order is uncertain. It comprises three suborders (Phacopina

Phacopina

Phacopina is a suborder including many trilobites.-External links:* - various photos, some of Phacopina...

, Calymenina

Calymenina

Calymenina is a suborder of the trilobite order Phacopida....

, and Cheirurina) which share a distinctive protaspis (developmental stage lacking segments) type. The Cheirurina and Calymenina retain a rostral plate (an apomorphy) 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

Ptychopariida

Ptychopariida is a large, heterogeneous order of trilobite containing some of the most primitive species known. Many date to the Early Cambrian Period, but the order was extant through the Late Ordovician...

, though it is not included in the subclass Libristoma.

Phacopida had 8 to 19 thoracic segments and are distinguishable by the expanded glabella, short or absent preglabellar area, and schizochroal (Phacopina) or holochroal (Cheirurina and Calymenina) eyes. Schizochroal eyes are compound eyes with up to around 700 separate lenses. Each lens has an individual cornea

Cornea

The cornea is the transparent front part of the eye that covers the iris, pupil, and anterior chamber. Together with the lens, the cornea refracts light, with the cornea accounting for approximately two-thirds of the eye's total optical power. In humans, the refractive power of the cornea is...

 which extended into a rather large sclera

Sclera

The sclera , also known as the white or white of the eye, is the opaque , fibrous, protective, outer layer of the eye containing collagen and elastic fiber. In the development of the embryo, the sclera is derived from the neural crest...

.

The development of schizochroal eyes in phacopid trilobites is an example of post-displacement paedomorphosis. The eyes of immature holochroal Cambrian

Cambrian

The Cambrian is the first geological period of the Paleozoic Era, lasting from Mya ; it is succeeded by the Ordovician. Its subdivisions, and indeed its base, are somewhat in flux. The period was established by Adam Sedgwick, who named it after Cambria, the Latin name for Wales, where Britain's...

 trilobites were basically miniature schizochroal eyes. In Phacopida, these were retained, via delayed growth of these immature structures (post-displacement), into the adult form.

Phacops rana

Phacops rana

Phacops rana is a species of trilobite from the middle Devonian period. Their fossils are found chiefly in the northeastern United States, southwestern Ontario, and in Morocco.-Physiology:...

and Dalmanites limulurus are two of the most well-known members of this order. Other known phacopids include Cheirurus

Cheirurus

Cheirurus is a genus of trilobite that lived from the Late Cambrian to the Middle Devonian. Its remains have been found in Africa, Asia, Australia, Europe, and North America.-External links:** in the Paleobiology Database...

, Deiphon

Deiphon

Deiphon was a distinctive genus of Silurian phacopid trilobites of the family Cheiruridae found in Western and Central Europe, and in Central and Eastern United States. The type species, D...

, Calymene

Calymene

Calymene is a genus of trilobites in the order Phacopida that were found throughout North America, North Africa, and Europe in primarily Silurian outcrops. Calymene are easily confused with Gravicalymene and Flexicalymene, also part of the Calymenidae family...

, Flexicalymene

Flexicalymene

Flexicalymene is a genus of trilobites of the order Phacopida, suborder Calymenina. They are found abundantly in North America. Flexicalymene specimens can be mistaken for Calymene, Gravicalymene, Diacalymene and a few other Calymenina genera. They are used as an index fossil in the Ordovician...

and Ceraurinella.