Phacopida ("

lens A lens is a transmissive optical device that focuses or disperses a light beam by means of refraction. A simple lens consists of a single piece of transparent material, while a compound lens consists of several simple lenses (''elements'') ...

-face") is an

order Order, ORDER or Orders may refer to: * A socio-political or established or existing order, e.g. World order, Ancien Regime, Pax Britannica * Categorization, the process in which ideas and objects are recognized, differentiated, and understood ...

trilobites Trilobites (; meaning "three-lobed entities") are extinct marine arthropods that form the class Trilobita. One of the earliest groups of arthropods to appear in the fossil record, trilobites were among the most successful of all early animals, ...

that lived from the Late

Cambrian The Cambrian ( ) is the first geological period of the Paleozoic Era, and the Phanerozoic Eon. The Cambrian lasted 51.95 million years from the end of the preceding Ediacaran period 538.8 Ma (million years ago) to the beginning of the Ordov ...

to the Late

Devonian The Devonian ( ) is a period (geology), geologic period and system (stratigraphy), system of the Paleozoic era (geology), era during the Phanerozoic eon (geology), eon, spanning 60.3 million years from the end of the preceding Silurian per ...

. It is made up of a morphologically diverse assemblage of taxa in three related suborders.

Characteristics

Phacopida had 8 to 19 thoracic segments and are distinguishable by the expanded

glabella The glabella, in humans, is the area of skin between the eyebrows and above the nose. The term also refers to the underlying bone that is slightly depressed, and joins the two brow ridges. It is a cephalometric landmark that is just superior ...

, 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 The cornea is the transparency (optics), transparent front part of the eyeball which covers the Iris (anatomy), iris, pupil, and Anterior chamber of eyeball, anterior chamber. Along with the anterior chamber and Lens (anatomy), lens, the cornea ...

which extended into a rather large

sclera The sclera, also known as the white of the eye or, in older literature, as the tunica albuginea oculi, is the opaque, fibrous, protective outer layer of the eye containing mainly collagen and some crucial elastic fiber. In the development of t ...

. The development of schizochroal eyes in phacopid trilobites is an example of post-displacement

paedomorphosis Neoteny (), also called juvenilization,Montagu, A. (1989). Growing Young. Bergin & Garvey: CT. is the delaying or slowing of the Physiology, physiological, or Somatic (biology), somatic, development of an organism, typically an animal. Neoteny i ...

. The eyes of immature holochroal

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

Phacopidae Phacopidae is a family (biology), family of Phacopida, phacopid trilobites that ranges from the Lower Ordovician to the Upper Devonian, with representatives in all Supercontinents, paleocontinents. Description As in all Phacopina, the eyes (if ...

) and '' Dalmanites limulurus'' (

Dalmanitidae Dalmanitidae is a family of trilobites in the order Phacopida that lived from the Floian (Ordovician) to the Devonian and includes 33 genera. References

Dalmanitidae, Dalmanitoidea Trilobite families Early Ordovician first appearances ...

) are two of the best-known members of this order. Other known phacopids include '' Cheirurus'' ( Cheiruridae), '' Deiphon'' (Cheiruridae), '' Calymene'' ( Calymenidae), '' Flexicalymene'' (Calymenidae) and '' Ceraurinella'' (Cheiruridae).

Evolution

The origin of the order Phacopida is uncertain. It comprises three suborders ( Phacopina, Calymenina, and Cheirurina) which share a distinctive protaspis (unsegmented larva ) with three pairs of spines on its body. The Cheirurina and Calymenina retain a

rostral Rostral may refer to: Anatomy * Rostral (anatomical term), situated toward the oral or nasal region * Rostral bone, in ceratopsian dinosaurs * Rostral organ, of certain fish * Rostral scale The rostral scale, or rostral, in snakes and other sca ...

plate (an

apomorphy In phylogenetics, an apomorphy (or derived trait) is a novel Phenotypic trait, character or character state that has evolution, evolved from its ancestral form (or Plesiomorphy and symplesiomorphy, plesiomorphy). A synapomorphy is an apomorphy sh ...

) 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 (along with the

Proetida Proetida is an order of trilobite that lived from the Ordovician to the Permian. It was the last surviving order of trilobite, dying out in the Permian-Triassic extinction event. Description These typically small trilobites resemble those of ...

) to survive the Kellwasser event (

Late Devonian extinction The Late Devonian mass extinction, also known as the Kellwasser event, was a mass extinction event which occurred around 372 million years ago, at the boundary between the Frasnian and Famennian ages of the Late Devonian period.Racki, 2005McGh ...

) at the Frasnian-

Famennian The Famennian is the later of two faunal stages in the Late Devonian epoch. The most recent estimate for its duration is that it lasted from around 371.1 to 359.3 million years ago. An earlier 2012 estimate, still used by the International Commis ...

boundary. However, they would die out soon afterwards at the

Hangenberg event The Hangenberg event, also known as the Hangenberg crisis or end-Devonian extinction, is a mass extinction that occurred at the end of the Famennian stage, the last Stage (stratigraphy), stage in the Devonian Period (roughly 358.9 ± 0.4 million y ...

(

end-Devonian extinction The Hangenberg event, also known as the Hangenberg crisis or end-Devonian extinction, is a mass extinction that occurred at the end of the Famennian stage, the last stage in the Devonian Period (roughly 358.9 ± 0.4 million years ago). It is usu ...

) at the end of the Famennian.

Classification

* Suborder Calymenina ** Family Bathycheilidae ** Family Bavarillidae ** Family Calymenidae ** Family Homalonotidae ** Family Pharostomatidae * Suborder Cheirurina ** Family Cheiruridae ** Family Encrinuridae ** Family Pilekiidae ** Family Pliomeridae * Suborder Phacopina ** Superfamily Acastoidea *** Family Acastidae *** Family Calmoniidae ** Superfamily Dalmanitoidea *** Family

Dalmanitidae Dalmanitidae is a family of trilobites in the order Phacopida that lived from the Floian (Ordovician) to the Devonian and includes 33 genera. References

Dalmanitidae, Dalmanitoidea Trilobite families Early Ordovician first appearances ...

*** Family Diaphanometopidae *** Family Prosopiscidae ** Superfamily Phacopoidea *** Family

*** Family Pterygometopidae

References

External links

Order Phacopida
Trilobite orders Early Ordovician first appearances Famennian extinctions Taxa named by John William Salter {{Phacopida-stub