Cyclopygidae is a family of asaphid

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 from the

Ordovician The Ordovician ( ) is a geologic period and system, the second of six periods of the Paleozoic Era. The Ordovician spans 41.6 million years from the end of the Cambrian Period million years ago (Mya) to the start of the Silurian Period Mya. ...

. Cyclopygids had a
extratropical

distribution Distribution may refer to: Mathematics *Distribution (mathematics), generalized functions used to formulate solutions of partial differential equations *Probability distribution, the probability of a particular value or value range of a varia ...

, and there is evidence that they lived in darker parts of the water column (around 175m deep). Cyclopygids are characterized by enlarged eyes, with a wide angle of view, both horizontal and vertical, reminiscent of the eyes of

dragonflies A dragonfly is a flying insect belonging to the infraorder Anisoptera below the order Odonata. About 3,000 extant species of true dragonfly are known. Most are tropical, with fewer species in temperate regions. Loss of wetland habitat threaten ...

. These typically touch the glabella directly on the side. Cyclopygids all lack genal spines, but '' Symphysops'' carries a forward directed frontal spine on the glabella. It is presumed that at least the members of the genus '' Pricyclopyge'' swam upside down and had

bioluminescent Bioluminescence is the production and emission of light by living organisms. It is a form of chemiluminescence. Bioluminescence occurs widely in marine vertebrates and invertebrates, as well as in some fungi, microorganisms including some b ...

organs on the third thorax segment. Cyclopygids had between 7 and 5 thorax segments, a wide and stout axis, and short side lobes (or pleurae).

Taxonomy

The genera of Cyclopygidae are grouped into the following subfamilies: *Cyclopyginae ** '' Cyclopyge'' Hawle & Corda, 1847 ** '' Delgamella'' Marek, 1961 ** '' Gastropolus'' Whittard, 1966 ** '' Heterocyclopyge'' Marek, 1961 ** '' Microparia'' Hawle & Corda, 1847 ** '' Novakella'' Whittard, 1952? ** '' Prospectatrix'' Fortey, 1981 ** '' Sagavia'' Koroleva, 1967 *Ellipsotaphrinae ** '' EIlipsotaphrus'' Whittard, 1952 ** '' Psilacella'' Whittard, 1952 *Pricyclopyginae ** '' Pricyclopyge'' Richter & Richter, 1954 ** '' Circulocrania'' Fortey, 1987 ** '' Emmrichops'' Marek, 1961 ** '' Symphysops'' Raymond, 1925 *Assignment unclear ** '' Aspidaeglina'' Holub, 1911 ** '' Phylacops'' Cooper & Kindle, 1936 ** '' Xenocyclopyge'' Lu, 1962 *Genera previously assigned to Cyclopygidae ** '' Girvanopyge'' (Remopleurididae)

Extinction

The extinction ending the Ordovician was one of the most radical for life to have experienced, and the trilobites were heavily affected. Those with

pelagic The pelagic zone consists of the water column of the open ocean, and can be further divided into regions by depth (as illustrated on the right). The word ''pelagic'' is derived . The pelagic zone can be thought of as an imaginary cylinder or w ...

or deep water

benthic The benthic zone is the ecological region at the lowest level of a body of water such as an ocean, lake, or stream, including the sediment surface and some sub-surface layers. The name comes from ancient Greek, βένθος (bénthos), meaning "t ...

life styles (such as species in Olenidae and Agnostida) died out. Also those trilobites having

plankton Plankton are the diverse collection of organisms found in water (or air) that are unable to propel themselves against a current (or wind). The individual organisms constituting plankton are called plankters. In the ocean, they provide a cr ...

larva A larva (; plural larvae ) is a distinct juvenile form many animals undergo before metamorphosis into adults. Animals with indirect development such as insects, amphibians, or cnidarians typically have a larval phase of their life cycle. Th ...

e became extinct, and these include most of the superfamilies in the order

Asaphida Asaphida is a large, morphologically diverse order of trilobites found in marine strata dated from the Middle Cambrian until their extinction during the Silurian. Asaphida contains six superfamilies (Anomocaroidea, Asaphoidea, Cyclopygoidea, D ...

, save for Trinucleoidea. A reduction in diversity already occurred before this major extinction, but many families persisted into the

Hirnantian The Hirnantian is the final internationally recognized stage of the Ordovician Period of the Paleozoic Era. It was of short duration, lasting about 1.4 million years, from to Ma (million years ago). The early part of the Hirnantian was char ...

, and it is possible that they would quickly have been restored to their former diversity. The crisis that started 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 Paleozoi ...

must have exceptionally severe, and was associated with low oxygen levels in the oceans after an ice age.Whittington, H. B. et al. Part O, Treatise on Invertebrate Paleontology. Revised, Volume 1 – Trilobita – Introduction, Order Agnostida, Order Redlichiida. 1997

Description

Cyclopygids have particularly large eyes with a wide angle view, also vertically, that occupy most of the free cheeks, and the fixed cheeks absent or reduced to a very narrow strip at the sides of the glabella, and a zone between the both eyes. In the earliest cyclopygids ('' Prospectatrix'') the eyes are less enlarged, but in some later

taxa In biology, a taxon ( back-formation from '' taxonomy''; plural taxa) is a group of one or more populations of an organism or organisms seen by taxonomists to form a unit. Although neither is required, a taxon is usually known by a particular n ...

, eyes are so big they have even fused. The most backward lobe of the glabella (the occipital ring) cannot be identified, except in the Ellipsotaphrinae subfamily. Further furrows crossing the glabella may be absent or are reduced to pairs of slight depressions. Genal spines are lacking. Cyclopygids have between 5 and 7 thorax segments. The pleurae become successively wider further back, making the thorax widest across the last segment.

Eyes

trilobites, such as the species in the proetid family Telephinidae, and in Cyclopygidae, as in many

extant Extant is the opposite of the word extinct. It may refer to: * Extant hereditary titles * Extant literature, surviving literature, such as ''Beowulf'', the oldest extant manuscript written in English * Extant taxon, a taxon which is not extinct, ...

pelagic

crustacea Crustaceans (Crustacea, ) form a large, diverse arthropod taxon which includes such animals as decapods, seed shrimp, branchiopods, fish lice, krill, remipedes, isopods, barnacles, copepods, amphipods and mantis shrimp. The crustacean gro ...

ns, the eyes are particularly large and have very wide angles of view, both horizontal and vertical. This is in stark contrast to contemporary

trilobites, that may have an extensive horizontal angle of view, but always have a limited vertical angle of view. In a few species of the genera ''Cyclopyge'', ''Microparia'', ''Ellipsotaphrus'', ''Pricyclopyge'' and in ''Symphysops'' the eyes are merged in front of the head creating a

visor A visor (also spelled vizor) is a surface that protects the eyes, such as shading them from the sun or other bright light or protecting them from objects. Nowadays many visors are transparent, but before strong transparent substances such a ...

. This development improves the sensitivity of the eye for objects that move relative to the eye, which might have been particularly useful under low-light conditions and when rapidly moving. The extant hyperiid amphipod '' Cystisoma'' also has such fused eyes. Monocular trilobites are always younger than closely related species with normal paired eyes, and is an example of a trend that occurred several times in parallel. Only in ''Pricyclopyge binodosa'' several stages in this development can be seen as a consecutive series of subspecies collected from successive zones in the late

Arenig In geology, the Arenig (or Arenigian) is a time interval during the Ordovician period and also the suite of rocks which were deposited during this interval. History The term was first used by Adam Sedgwick in 1847 with reference to the "Areni ...

to the Llanvirn. Although the distance between the eyes varies within any one population of the earlier subspecies, the eyes only touch and merge in ''P. binodosa synophthalma''.

Ecology

Cyclopygids are absent from shallow water strata, such as alluvial and calcareous deposits. They are not found together with well-sighted

trilobite species or corals. They do occur with blind or nearly blind benthic trilobites, a typical adaptation to a lightless environment, and oceanic free-floating graptolites. Hence, cyclopygids are considered to have been confined to deeper water, swimming at the lower limit of the photic zone (or

mesopelagic The mesopelagic zone (Greek μέσον, middle), also known as the middle pelagic or twilight zone, is the part of the pelagic zone that lies between the photic epipelagic and the aphotic bathypelagic zones. It is defined by light, and begins a ...

), but still high above the benthic species they were deposited with. This is also evidenced by the presumed present of bioluminescent organs on the third thorax segment of ''Pricyclopyge'', which also occur on the functional underside of extant mesopelagic species. This is why it is assumed ''Pricyclopyge'' may have swum upside down. Very large, convex eyes and a narrow zone of thoracic pleurae are typical for all Cyclopygidae, and are indications of a pelagic lifestyle. The stout

exoskeleton An exoskeleton (from Greek ''éxō'' "outer" and ''skeletós'' "skeleton") is an external skeleton that supports and protects an animal's body, in contrast to an internal skeleton ( endoskeleton) in for example, a human. In usage, some of the ...

is consistent with rapid swimming and it is likely cyclopygids actively hunted

zooplankton Zooplankton are the animal component of the planktonic community ("zoo" comes from the Greek word for ''animal''). Plankton are aquatic organisms that are unable to swim effectively against currents, and consequently drift or are carried along by ...

References

{{Taxonbar, from=Q899756 Cyclopygoidea Trilobite families Ordovician trilobites Fossils of China Fossils of the Czech Republic Fossils of France Fossils of Great Britain Ordovician first appearances Ordovician extinctions