Asaphida is a large, morphologically diverse

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 ...

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 fossil record defines the base of the ...

s found in marine

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

dated from the Middle

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 ago ...

until their extinction during the

Silurian The Silurian ( ) is a geologic period and system spanning 24.6 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 shortest period of the Paleoz ...

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

trilobites. In 2020, the superfamily Trinucleoidea was proposed to be raised to an order ( Trinucleida) 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., neuroscientist Michael Lewis, and organic chemist James C. Kauer—all three former scientists with the DuPont Company. Baldino s ...

(head) and

pygidium The pygidium (plural 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 compo ...

(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. Trilobite2

One Asaphida line, the Superfamily Asaphoidea, shows a continuous

evolution Evolution is change in the heritable characteristics of biological populations over successive generations. These characteristics are the expressions of genes, which are passed on from parent to offspring during reproduction. Variation ...

of eyestalks, from individuals with stubby eyes to '' Asaphus kowalewskii'', 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 fic ...

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

Volkhov River The Volkhov (russian: Во́лхов) is a river in Novgorodsky and Chudovsky Districts of Novgorod Oblast and Kirishsky and Volkhovsky Districts of Leningrad Oblast in northwestern Russia. It connects Lake Ilmen and Lake Ladoga and form ...

region near

Saint Petersburg Saint Petersburg ( rus, links=no, Санкт-Петербург, a=Ru-Sankt Peterburg Leningrad Petrograd Piter.ogg, r=Sankt-Peterburg, p=ˈsankt pʲɪtʲɪrˈburk), formerly known as Petrograd (1914–1924) and later Leningrad (1924–1991), i ...

Russia Russia (, , ), or the Russian Federation, is a transcontinental country spanning Eastern Europe and Northern Asia. It is the largest country in the world, with its internationally recognised territory covering , and encompassing one-ei ...

. During the Ordovician, the region that is now Eastern

Europe Europe is a large peninsula conventionally considered a continent in its own right because of its great physical size and the weight of its history and traditions. Europe is also considered a Continent#Subcontinents, subcontinent of Eurasia ...

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 naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of wind, water, or ice or by the force of gravity acting on the particles. For example, sand ...

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 Trinucleioidea, and they too disappeared before the end of the

Period.

Taxonomy

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

Cyclopygidae Cyclopygidae is a family of asaphid trilobites from the Ordovician. Cyclopygids had aextratropicaldistribution, and there is evidence that they lived in darker parts of the water column (around 175m deep). Cyclopygids are characterized by enl ...

** Isocolidae? ** Nileidae ** Taihungshaniidae *Superfamily Dikelokephaloidea ** Dikelocephalidae ** Eurekiidae ** Loganellidae ** Ptychaspididae **

Saukiidae ''Saukiidae'' is a family of trilobites of the Asaphida order. These fast-moving low-level epifaunal deposit feeders lived in the Cambrian and Ordovician The Ordovician ( ) is a geologic period and system, the second of six periods of the Pal ...

*Superfamily Remopleuridioidea ** Auritamiidae ** Bohemillidae ** Hungaiidae ** Idahoiidae ** Remopleurididae *Superfamily Trinucleioidea ? ** 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 cephalons) tha ...

** Trinucleidae *''

incertae sedis ' () or ''problematica'' is a term used for a 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, uncertain ...

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

References

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