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. 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 was proposed to be raised to an order ( Trinucleida) and removed from Asaphida.

Morphology

The Asaphids generally have

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(head) and pygidium (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'', a trilobite with long eyestalks. This line is found in the Middle Ordovician Asery Level deposits of the Volkhov River region near

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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

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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 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 ** Ceratopygidae *Superfamily Cyclopygoidea ** Cyclopygidae ** Nileidae(?) ** Taihungshaniidae *Superfamily Dikelocephaloidea ** Dikelocephalidae ** Eurekiidae ** Ptychaspididae ** Saukiidae *Superfamily Remopleuridioidea ** Auritamiidae ** Bohemillidae ** Hungaiidae ** Idahoiidae ** Remopleurididae *Superfamily Trinucleioidea ? ** Alsataspididae ** Dionididae ** Liostracinidae ** Raphiophoridae ** Trinucleidae *''

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