Asaphida is a large, morphologically diverse

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 ...

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

s found in marine

strata In geology and related fields, a stratum (: strata) is a layer of Rock (geology), rock or sediment characterized by certain Lithology, lithologic properties or attributes that distinguish it from adjacent layers from which it is separated by v ...

dated from the Middle

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 ...

until their extinction during the

Silurian The Silurian ( ) is a geologic period and system spanning 23.5 million years from the end of the Ordovician Period, at million years ago ( Mya), to the beginning of the Devonian Period, Mya. The Silurian is the third and shortest period of t ...

. Asaphida contains six superfamilies (Anomocaroidea, Asaphoidea, Cyclopygoidea, Dikelocephaloidea, Remopleuridoidea and Trinucleioidea), but no

suborders Order () is one of the eight major hierarchical taxonomic ranks in Linnaean taxonomy. It is classified between family and class. In biological classification, the order is a taxonomic rank used in the classification of organisms and recognized ...

. Asaphids comprise some 20% of described

fossil A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserve ...

trilobites. In 2020, the superfamily

Trinucleioidea Trinucleioidea is a Superfamily (taxonomy), superfamily of trilobites. Traditionally placed within the Asaphida, it is now sometimes considered its own order, Trinucleida. Taxonomy There are five families in the super family; they are the: Ala ...

was proposed to be raised to an order (

Trinucleida Trinucleioidea is a superfamily of trilobites. Traditionally placed within the Asaphida, it is now sometimes considered its own order, Trinucleida. Taxonomy There are five families in the super family; they are the: Alataspididae, Dionididae ...

) and removed from Asaphida.

Morphology

The Asaphids generally have

cephalon Cephalon, Inc. was an American biopharmaceutical company co-founded in 1987 by pharmacologist Frank Baldino Jr., Frank Baldino, Jr., neuroscientist Michael Lewis, and organic chemist James C. Kauer—all three former scientists with the DuPont ...

(head) and

pygidium The pygidium (: pygidia) is the posterior body part or shield of crustaceans and some other arthropods, such as insects and the extinct trilobites. In groups other than insects, it contains the anus and, in females, the ovipositor. It is compos ...

(tail) parts similar in size, and most species have a prominent median ventral suture. Heads are often flat, and carapace furrows in the head area are often faint or not visible. Thoracic segments typically number 5 - 12, though some species have as few as two and some as many as 30. They also generally have a wide doublure, or rim, that surrounds the cephalon. This causes some specimens to be described as having a characteristic "snowplow" shaped cephalon. When present, eyes are typically large. One Asaphida line, the superfamily Asaphoidea, shows a continuous

evolution Evolution is the change in the heritable Phenotypic trait, characteristics of biological populations over successive generations. It occurs when evolutionary processes such as natural selection and genetic drift act on genetic variation, re ...

of eyestalks, from individuals with stubby eyes to ''

Asaphus kowalewskii ''Asaphus kowalewskii'' () is one of the 35 species of trilobites of the genus ''Asaphus'' (this particular species is sometimes placed in its own genus, ''Neoasaphus''). Fossils of this species are popular among collectors because of their prom ...

'', a trilobite with long

eyestalk In anatomy, an eyestalk (sometimes spelled eye stalk and also known as an ommatophore) is a protrusion that extends an eye away from the body, giving the eye a better field of view. It is a common feature in nature and frequently appears in fict ...

s. This line is found in the Middle Ordovician Asery Level deposits of the

Volkhov River The Volkhov ( ; ; ) is a river in Novgorodsky District, Novgorodsky and Chudovsky Districts of Novgorod Oblast and Kirishsky District, Kirishsky and Volkhovsky Districts of Leningrad Oblast in northwestern Russia. The Volkhov River, Volkhov, whi ...

region near

Saint Petersburg Saint Petersburg, formerly known as Petrograd and later Leningrad, is the List of cities and towns in Russia by population, second-largest city in Russia after Moscow. It is situated on the Neva, River Neva, at the head of the Gulf of Finland ...

Russia Russia, or the Russian Federation, is a country spanning Eastern Europe and North Asia. It is the list of countries and dependencies by area, largest country in the world, and extends across Time in Russia, eleven time zones, sharing Borders ...

. During the Ordovician, the region that is now Eastern

Europe Europe is a continent located entirely in the Northern Hemisphere and mostly in the Eastern Hemisphere. It is bordered by the Arctic Ocean to the north, the Atlantic Ocean to the west, the Mediterranean Sea to the south, and Asia to the east ...

was a shallow inland sea. This eyestalk development is believed to be an adaptation to changes in turbidity during this time, with eye-stalked trilobites like ''A. kowalewski'' presumably arising in a time of increased turbidity. One thought is that this trilobite may have lain in wait for prey buried in the bottom

sediment Sediment is a solid material that is transported to a new location where it is deposited. It occurs naturally and, through the processes of weathering and erosion, is broken down and subsequently sediment transport, transported by the action of ...

with only its periscope eyestalks protruding. The major extinction event marking the end of the Ordovician Period reduced the diversity of all trilobite orders with most asaphid families disappearing. The only surviving asaphids were members of superfamily

, and they too disappeared before the end of the

Period.

Taxonomy

The following superfamilies and families are included: *Superfamily Anomocaroidea ** Andrarinidae ** Anomocarellidae **

Anomocaridae Anomocaridae is a family of trilobites, containing the following genera: *'' Abharella'' *'' Afghanocare'' *'' Amginia'' *'' Anomocare'' *'' Anomocarina'' *'' Anomocarioides'' *'' Anomocariopsis'' *'' Callaspis'' *'' Chondranomocare'' *'' Dilata ...

** Aphelaspididae ** Parabolinoididae ** Pterocephaliidae *Superfamily Asaphoidea **

Asaphidae Asaphidae is a family of asaphid trilobites. Although the first genera originate in Upper Cambrian marine strata, the family becomes the most widely distributed and most species-rich trilobite family during the Ordovician. 754 species assigne ...

** Ceratopygidae *Superfamily

Cyclopygoidea Cyclopygoidea is a superfamily of trilobites in the order Asaphida, containing the families Cyclopygidae, Nileidae, Symphysurinidae, and Taihungshaniidae. Some subgroups such as the Cyclopygidae are thought to have lived in the water column in ...

Cyclopygidae Cyclopygidae is a family of asaphida, asaphid trilobites from the Ordovician. Cyclopygids had an extratropical Range (biology), distribution, and there is evidence that they lived in mesopelagic, darker parts of the water column (around 175m dee ...

Nileidae Nileidae is a family of trilobites of the Asaphida order. These fast-moving nektonic carnivores lived in the Ordovician and Cambrian periods. Genera *''Aocaspis'' Dolambi and Gond 1992 *''Barrandia'' McCoy 1849 *''Borthaspidella'' Rasetti 1954 ...

(?) ** Taihungshaniidae *Superfamily Dikelocephaloidea **

Dikelocephalidae Dikelocephalidae is a family (biology), family of trilobites, that lived during the Upper Cambrian. Relatively large eyes close to the Trilobite#Cephalon, glabella, is a shared characteristic. The following genera have been assigned to this famil ...

** Eurekiidae ** Ptychaspididae ** Saukiidae *Superfamily Remopleuridioidea ** Auritamiidae ** Bohemillidae ** Hungaiidae ** Idahoiidae ** Remopleurididae *Superfamily

? ** Alsataspididae ** Dionididae ** Liostracinidae **

Raphiophoridae Raphiophoridae is a family of small to average-sized trilobites that first occurred at the start of the Ordovician and became extinct at the end of the Middle Silurian. Anatomy All raphiophorids are blind, with headshields (or Trilobite#Ceph ...

Trinucleidae Trinucleidae is a family of small to average size asaphid trilobites that first occurred at the start of the Ordovician and became extinct at the end of that period. It contains approximately 227 species divided over 51 genera in 5 subfamilies. ...

*''

incertae sedis or is a term used for a taxonomy (biology), taxonomic group where its broader relationships are unknown or undefined. Alternatively, such groups are frequently referred to as "enigmatic taxa". In the system of open nomenclature, uncertainty ...

'' ** Rhyssometopidae (includes Mapaniidae, Plectriferidae) ** Monkaspididae (Chelidonocephalidae ?)

References

Cambrian trilobites Ordovician trilobites Silurian trilobites Trilobite orders Cambrian first appearances Silurian extinctions Librostoma {{Asaphida-stub