''Pricyclopyge'' is a genus 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 assigned to the family

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 ...

that occurs throughout 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 ...

. ''Pricyclopyge'' had a
extratropical
distribution, and there is evidence that it lived in darker parts of the water column (around 175m deep). ''Pricyclopyge'' has huge eyes, an inverted pear-shaped glabella, six thorax segments, with on the 3rd two small discs.Whittington, H. B. et al. (1997) Treatise on Invertebrate Paleontology. Part O, Revised, Volume 1 – Trilobita – Introduction, Order Agnostida, Order Redlichiida. ''Pricyclopyge'' is known from what are today China, the Czech Republic, France, and the United Kingdom.Paleobiology Database

Description

Like other cyclopygids, ''Pricyclopyge'' lacks genal spines. ''Pricyclopyge'' has six thorax segments of which the third from the front carries two very characteristic round discs that are presumed to have been bioluminescent organs, and if true, this suggests that ''Pricyclopyge'' swam upside down. It has a wide and stout axis, that tapers gradually backwards until it ends in a semicircle close to the border of the pygidium. The 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 ...

) of the thorax segments gradually become wider further back so that the thorax is widest at its back, with the tips of the pleurae of the 6th segment sometimes enlarged, bent backwards and so bordering the

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 ...

. The pygidium is more than twice as wide as long, the axis showing 3 or 4 clearly defined rings, while segmentation of the pleural area is barely visible. The pygidial border furrow is clear.

Eyes

Like all cyclopygids, ''Pricyclopyge'' has huge eyes, that largely encircle the inverted pear-shaped

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 ...

. These eyes have 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 ...

. The individual lenses in the eyes are hexagonally arranged, with their centers approximately 0.25 mm apart. Merger of both eyes, 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 ...

, has appeared in several cyclopigid genera, but 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 ...

. 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 ''Cystisoma '' is a genus of amphipod. It is the only member of the family Cystisomatidae within the Hyperiidea. The genus is noted for its nearly completely transparent body, adapted for life in low light waters. Description Gallery ''Cy ...

'' also has such fused eyes. 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''. 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.

Ecology

It is presumed that ''Pricyclopyge'' swam upside down in dimly lit oceanic waters outside the tropic belt. It occurs together with other cyclopygids, blind or nearly blind deepwater benthic trilobites, and free-floating oceanic

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 ...

. It probably hunted the

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 ...

and may have migrated in the evening towards the surface and in the morning to greater depths, following prey and possibly avoiding some of its potential predators.

References

{{Taxonbar, from=Q16988487 Cyclopygidae Asaphida genera Ordovician trilobites Fossils of China Fossils of the Czech Republic Fossils of France Fossils of Great Britain Fossil taxa described in 1954