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 ...

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 ...

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 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 ...

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 human eye containing mainly collagen and some crucial elastic fiber. In humans, and s ...

. 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 ''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 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 ''Dalmanites'' is a genus of trilobite in the order Phacopida. They lived from the Late Ordovician to Middle Devonian. The trilobites of this genus have slightly convex exoskeletons with an average length of . The cephalon is semicircular or par ...

'' (

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 ''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, Europ ...

'' ( Cheiruridae), '' 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 trilobites, containing the following genera: *'' Alcymene'' *'' Apocalymene'' *'' Arcticalymene'' *'' Calymene'' *'' Calymenella'' *'' Calymenesum'' *'' Colpocoryphe'' *'' Dekalymene'' *'' Diacalymene'' *'' Flexicalymen ...

), '' 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 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 Pale ...

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 relate ...

, 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 P ...

) 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, in snakes and scaled reptiles Other uses * Rostral col ...

plate (an

apomorphy In phylogenetics, an apomorphy (or derived trait) is a novel character or character state that has evolved from its ancestral form (or plesiomorphy). A synapomorphy is an apomorphy shared by two or more taxa and is therefore hypothesized to have ...

) 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–Siluria ...

, 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 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 ext ...

(

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 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 t ...

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 ...

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 usu ...

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

Classification

* Suborder

** 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

** Family Homalonotidae ** Family Pharostomatidae * Suborder

** 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

** Superfamily Acastoidea *** 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

*** 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