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 or deep waterbenthic life styles died out. Also those trilobites having planktonic larvae 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, 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.
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 the eyes are less enlarged, but in some later taxa, eyes are so big they have even fused. The most backward lobe of the glabella 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
In pelagic trilobites, such as the species in the proetid familyTelephinidae, 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. 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 hyperiidamphipodCystisoma 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 to the Llanvirn. Although the distancebetween 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. Hence, cyclopygids are considered to have been confined to deeper water, swimming at the lower limit of the photic zone, 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 is consistent with rapid swimming and it is likely cyclopygids actively huntedzooplankton.
