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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 Order, ORDER or Orders may refer to: * Categorization, the process in which ideas and objects are recognized, differentiated, and understood * Heterarchy, a system of organization wherein the elements have the potential to be ranked a number of d ...
of trilobites that lived from the Late
Cambrian The Cambrian Period ( ; sometimes symbolized 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 years ag ...
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, w ...
. 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, 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 ...
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 The Cambrian Period ( ; sometimes symbolized 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 years ag ...
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 ''Eldredgeops rana'' (formerly ''Phacops rana'') is a species of trilobite from the middle Devonian period. Their fossils are found chiefly in the northeastern United States, and southwestern Ontario. Because of its abundance and popularity wit ...
'' ( Phacopidae) and '' Dalmanites limulurus'' (
Dalmanitidae Dalmanitidae is a family of trilobites in the order Phacopida that lived from the Floian (Ordovician) to the Devonian The Devonian ( ) is a geologic period and system of the Paleozoic era, spanning 60.3 million years from the end of ...
) are two of the best-known members of this order. Other known phacopids include '' Cheirurus'' ( Cheiruridae), '' Deiphon'' (Cheiruridae), '' Calymene'' ( Calymenidae), '' Flexicalymene'' (Calymenidae) and ''
Ceraurinella ''Ceraurinella'' is an extinct genus 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 trilo ...
'' (Cheiruridae).


Evolution

The origin of the order Phacopida is uncertain. It comprises three suborders ( Phacopina, Calymenina, and
Cheirurina Cheirurina is a suborder of the trilobite order Phacopida. Known representatives range from the uppermost Cambrian (upper Furongian) to the end of the Middle Devonian (Givetian The Givetian is one of two faunal stages in the Middle Devonian Pe ...
) which share a distinctive protaspis (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, though it is not included in the subclass
Librostoma Librostoma is a subclass of trilobites. References Arthropod subclasses Prehistoric arthropod taxa Fossil taxa described in 1990 {{Librostoma-stub ...
. Phacopida was one of only two trilobite orders (along with the Proetida) to survive the Kellwasser event (
Late Devonian extinction The Late Devonian extinction consisted of several extinction events in the Late Devonian Epoch (geology), Epoch, which collectively represent one of the five largest extinction event, mass extinction events in the history of life on Earth. The t ...
) at the Frasnian- Famennian boundary. However, they would die out soon afterwards at the Hangenberg event (
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 usual ...
) at the end of the Famennian.


Classification

* Suborder Calymenina ** Family
Bathycheilidae Bathycheilidae is a family of trilobites comprising the genera ''Bathycheilus ''Bathycheilus'' is a genus of trilobites in the order Phacopida. It was described by Holub in 1908. Species * ''Bathycheilus castilianus'' Hammann, 1983 * ''Bath ...
** Family Bavarillidae ** Family Calymenidae ** Family Homalonotidae ** Family Pharostomatidae * Suborder
Cheirurina Cheirurina is a suborder of the trilobite order Phacopida. Known representatives range from the uppermost Cambrian (upper Furongian) to the end of the Middle Devonian (Givetian The Givetian is one of two faunal stages in the Middle Devonian Pe ...
** Family Cheiruridae ** Family Encrinuridae ** Family Pilekiidae ** Family
Pliomeridae Pliomeridae is a family of phacopide trilobites, containing the following genera: *'' Anapliomera'' *'' Benedettia'' *'' Canningella'' *'' Colobinion'' *'' Coplacoparia'' *'' Cybelopsis'' *'' Ectenonotus'' *'' Encrinurella'' *'' Evropeites'' ...
* Suborder Phacopina ** Superfamily
Acastoidea Acastoidea is a superfamily of trilobites from the order Phacopida, suborder Phacopina. This superfamily is divided into two families, Acastidae and Calmoniidae. This superfamily is distinguishable from the Phacopidae in that eyes are closer to th ...
*** Family
Acastidae Acastidae is a family of trilobites in the order Phacopida, suborder Phacopina, superfamily Acastoidea Acastoidea is a superfamily of trilobites from the order Phacopida, suborder Phacopina. This superfamily is divided into two families, Acast ...
*** Family Calmoniidae ** Superfamily
Dalmanitoidea Dalmanitoidea is a superfamily of trilobites in the order Phacopida, containing the three families Dalmanitidae, Diaphanometopidae Diaphanometopidae is a family of trilobites. Its representatives lived during the Arenig and Llanvirn stages of ...
*** Family
Dalmanitidae Dalmanitidae is a family of trilobites in the order Phacopida that lived from the Floian (Ordovician) to the Devonian The Devonian ( ) is a geologic period and system of the Paleozoic era, spanning 60.3 million years from the end of ...
*** Family
Diaphanometopidae Diaphanometopidae is a family of trilobites. Its representatives lived during the Arenig and Llanvirn stages of the Ordovician Period, approximately 479 to 463 million years ago.Moore, R. C. (ed.). Treatise on Invertebrate Paleontology. Part O ...
*** Family Prosopiscidae ** Superfamily Phacopoidea *** Family Phacopidae *** Family
Pterygometopidae The Pterygometopidae are a family of trilobites, that is known from the Floian to the Katian (Ordovician), and reappears from the Telychian to the Sheinwoodian (Silurian). As part of the Phacopina suborder, its members have schizochroal eyes. D ...


References


External links


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