Asaphida is a large, morphologically diverse
order 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 At ...
s found in marine
strata dated from the Middle
Cambrian
The Cambrian Period ( ; sometimes symbolized C with bar, 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 ...
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 Paleozo ...
. 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 (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](https://upload.wikimedia.org/wikipedia/commons/0/0c/Asaphus_kowalewskii_3.jpg)
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
''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 promi ...
'', 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 forms pa ...
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 List of transcontinental countries, transcontinental country spanning Eastern Europe and North Asia, Northern Asia. It is the List of countries and dependencies by area, largest country in the ...
. 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 an ...
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
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 Paleozo ...
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
**
Tsinaniidae
*Superfamily
Cyclopygoidea
**
Cyclopygidae
Cyclopygidae is a family of asaphid trilobites from the Ordovician. Cyclopygids had aextratropical distribution, and there is evidence that they lived in darker parts of the water column (around 175m deep). Cyclopygids are characterized by en ...
**
Isocolidae?
**
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 1 ...
**
Taihungshaniidae
*Superfamily
Dikelokephaloidea
**
Dikelocephalidae
**
Eurekiidae
**
Loganellidae
**
Ptychaspididae
**
Saukiidae
*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
Trinucleidae is a family of small to average size asaphida, asaphid trilobites that first occurred at the start of the Ordovician and became extinct at the end of that Period (geology), period. It contains approximately 227 species divided over 5 ...
*''
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