Phacopida ("

lens A lens is a transmissive optical device which 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 of

trilobites Trilobites (; meaning "three lobes") are extinct marine arthropods that form the class Trilobita. Trilobites form one of the earliest-known groups of arthropods. The first appearance of trilobites in the fossil record defines the base of the At ...

that lived from the Late

Cambrian The Cambrian Period ( ; sometimes symbolized C with bar, Ꞓ) was the first geological period of the Paleozoic Era, and of the Phanerozoic Eon. The Cambrian lasted 53.4 million years from the end of the preceding Ediacaran Period 538.8 million ...

to the Late

Devonian The Devonian ( ) is a geologic period and system of the Paleozoic era, spanning 60.3 million years from the end of the Silurian, million years ago (Mya), to the beginning of the Carboniferous, Mya. It is named after Devon, England, whe ...

. 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 to ...

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

which extended into a rather large sclera. 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 physiological, or somatic, development of an organism, typically an animal. Neoteny is found in modern humans compare ...

. 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 of phacopid trilobites that ranges from the Lower Ordovician to the Upper Devonian, with representatives in all paleocontinents. Description As in all Phacopina, the eyes (if present) consist of very large (0.5 mm in ' ...

) and '' Dalmanites limulurus'' (

Dalmanitidae Dalmanitidae is a family of trilobites in the order Phacopida that lived from the Floian The Floian is the second stage of the Ordovician Period. It succeeds the Tremadocian with which it forms the Lower Ordovician epoch. It precedes the Dap ...

) are two of the best-known members of this order. Other known phacopids include ''

Cheirurus ''Cheirurus'' (from Greek ''χείρ, cheir'' meaning "hand" and ''ουρά, oura'' meaning "tail") is a genus of phacopid trilobites that lived from the Ordovician to the Devonian. Its remains have been found in Africa, Asia, Australia, Euro ...

'' (

Cheiruridae Cheiruridae is a family of phacopid trilobites of the suborder Cheirurina. Its members, as with other members of the suborder, had distinctive pygidia modified into finger-like spines. They first appeared in the uppermost Cambrian (upper Furongi ...

), '' Deiphon'' (Cheiruridae), ''

Calymene ''Calymene'' Brongniart, 1822, is a genus of trilobites in the order Phacopida, suborder Calymenina, that are found throughout North America, North Africa, and Europe in primarily Silurian outcrops. ''Calymene'' is closely related to ''Flexicaly ...

'' (

Calymenidae Calymenidae is a family of trilobite Trilobites (; meaning "three lobes") are extinct marine arthropods that form the class Trilobita. Trilobites form one of the earliest-known groups of arthropods. The first appearance of trilobites in the ...

), ''

Flexicalymene ''Flexicalymene'' Shirley, 1936. is a genus of trilobites belonging to the order Phacopida, suborder Calymenina and Family Calymenidae. ''Flexicalymene'' specimens can be mistaken for '' Calymene'', ''Gravicalymene'', '' Diacalymene'' and a fe ...

'' (Calymenidae) and '' Ceraurinella'' (Cheiruridae).

Evolution

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

Phacopina The Phacopina comprise a suborder of the trilobite order Phacopida. Species belonging to the Phacopina lived from the Lower Ordovician ( Tremadocian) through the end of the Upper Devonian (Famennian).Moore, R.C. (ed.). Treatise on Invertebrate Pal ...

Calymenina Calymenina is a suborder of the trilobite order Phacopida Phacopida ("lens-face") 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 ...

, and Cheirurina) which share a distinctive

protaspis ''Protaspis'' is an extinct genus of pteraspidid heterostracan agnathan which lived during the Early Devonian of the United States, with fossils found in marine strata in what is now Utah, Wyoming and Idaho Idaho ( ) is a state in the Pa ...

(unsegmented larva ) with three pairs of spines on its body. 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 is a large, heterogeneous order of trilobite containing some of the most primitive species known. The earliest species occurred in the second half of the Lower Cambrian, and the last species did not survive the Ordovician–Silurian ...

, 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 order of trilobite to go extinct, finally dying out in the Permian-Triassic extinction event. Description These typically small trilobites resem ...

) to survive the

Kellwasser event The Late Devonian extinction consisted of several extinction events in the Late Devonian Epoch, which collectively represent one of the five largest mass extinction events in the history of life on Earth. The term primarily refers to a major ex ...

(

Late Devonian extinction The Late Devonian extinction consisted of several extinction events in the Late Devonian Epoch, which collectively represent one of the five largest mass extinction events in the history of life on Earth. The term primarily refers to a major ex ...

) at the

Frasnian The Frasnian is one of two faunal stages in the Late Devonian Period. It lasted from million years ago to million years ago. It was preceded by the Givetian Stage and followed by the Famennian Stage. Major reef-building was under way during th ...

Famennian The Famennian is the latter of two faunal stages in the Late Devonian Epoch. The most recent estimate for its duration estimates that it lasted from around 371.1 million years ago to 359.3 million years ago. An earlier 2012 estimate, still used b ...

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 in the Devonian Period (roughly 358.9 ± 0.4 million years ago). It is usuall ...

( end-Devonian extinction) at the end of the Famennian.

Classification

* Suborder

** Family Bathycheilidae ** Family Bavarillidae ** Family

** Family

Homalonotidae Homalonotidae is a family of trilobites that lived from the Ordovician to the Devonian. They are characterised by a shovel-like cephalon (head), and are closely related to the family Calymenidae. It contains the following genera: *'' Arduenne ...

** Family Pharostomatidae * Suborder Cheirurina ** Family

** Family

Encrinuridae Encrinuridae is a family of trilobite within the order Phacopida that lived in what would be Africa, Asia, Australia, Europe, North America, and South America from the middle Ordovician to the early Devonian from , existing for approximately . T ...

** Family

Pilekiidae Pilekiidae is a family of trilobites in the order Phacopida, specifically within Suborder Cheirurina Cheirurina is a suborder of the trilobite order Phacopida. Known representatives range from the uppermost Cambrian (upper Furongian) to the en ...

** Family Pliomeridae * Suborder

** Superfamily Acastoidea *** Family

Acastidae Acastidae is a family of trilobites in the order Phacopida, suborder Phacopina The Phacopina comprise a suborder of the trilobite order Phacopida. Species belonging to the Phacopina lived from the Lower Ordovician ( Tremadocian) through the end ...

*** Family

Calmoniidae Calmoniidae is a family of trilobites from the order Phacopida, suborder Phacopina, superfamily Acastoidea. It contains the following genera: *'' Anchiopella'' *'' Andinacaste'' *'' Australocaste'' *'' Australops'' *'' Awaria'' *'' Bainella'' * ...

** Superfamily Dalmanitoidea *** Family

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

Phacopoidea Phacopoidea is a superfamily of trilobite Trilobites (; meaning "three lobes") are extinct marine arthropods that form the class Trilobita. Trilobites form one of the earliest-known groups of arthropods. The first appearance of trilobites ...

*** Family

*** Family Pterygometopidae

References

External links

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