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. T ...

. Cyclopygids had a
extratropical
distribution, 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 threa ...

. These typically touch the

glabella The glabella, in humans, is the area of skin between the eyebrows and above the nose. The term also refers to the underlying bone that is slightly depressed, and joins the two brow ridges. It is a cephalometric landmark that is just superior to ...

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 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 The pulmonary pleurae (''sing.'' pleura) are the two opposing layers of serous membrane overlying the lungs and the inside of the surrounding chest walls. The inner pleura, called the visceral pleura, covers the surface of each lung and dips be ...

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 life styles (such as species in

Olenidae Olenidae is a family of ptychopariid trilobites. Some genera, '' Balnibarbi'' and '' Cloacaspis'', are thought to have evolved a symbiotic relationship with sulfur-eating bacteria from which they derived nutrition. Genera * Acerocare * Acero ...

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 crucia ...

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. ...

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

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

, 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 Telephinidae is a family of pelagic trilobites with large wide-angle eyes, occupying most of the free cheeks, downward directed facial spines and 9-10 thorax segments. The family is known during the entire Ordovician and occurred in deep water ar ...

, and in Cyclopygidae, as in many extant pelagic crustaceans, the eyes are particularly large and have very wide angles of view, both horizontal and vertical. This is in stark contrast to contemporary benthic 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 The Hyperiidea are a suborder of amphipods, small aquatic crustaceans. Unlike the other suborders of Amphipoda, hyperiids are exclusively marine and do not occur in fresh water. Hyperiids are distinguished by their large eyes and planktonic habi ...

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 "Arenig ...

to the

Llanvirn 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. Th ...

. 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 benthic 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 Graptolites are a group of colonial animals, members of the subclass Graptolithina within the class Pterobranchia. These filter-feeding organisms are known chiefly from fossils found from the Middle Cambrian ( Miaolingian, Wuliuan) through th ...

. 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 at ...

), 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