Phacops is a type of trilobite that belongs to the order Phacopida and the family Phacopidae. It lived in Europe, northwestern Africa, North and South America, and China during the Late Ordovician period and continued living until the end of the Devonian period. Fossils from the Late Ordovician show a longer time range. Phacops had a round body, a round head, and large eyes. It likely ate small pieces of dead plants and animals. Phacops is often found rolled up, a way to protect itself. This defense method is common among smaller trilobites but is more developed in this genus.

Description

The eyes of Phacops are made up of many large, separate lenses that do not share a common cornea, a feature called schizochroal eyes. Like most other Phacopina, the middle part of Phacops' body (called the thorax) has 11 segments.

The central raised part of the head (called the glabella) is somewhat to very rounded near the front, flat on top, and drops down to or slightly hangs over the border furrow. Up to three side furrows may be visible behind the large frontal lobe. From the back, there is a clear ring called the occipital ring, and in front of it, a ring called the preoccipital ring, which is weakly divided into a strongly curved central part and weaker side parts. The eyes are large to medium in size, shaped like a crescent, and are high above the cheeks. The visual surface is kidney-shaped. The back corners of the head (genal angles) are sharply to bluntly rounded, but adult Phacops do not have spines on these corners. On the bottom surface of the head, there is a continuous groove in the front half, bordered by ridges and with notches on the sides. This groove, called the vindicular furrow, helps lock the tail shield to the head shield when the trilobite curls up.

The segments of the thorax (axial rings) do not have rounded bumps on their outer surface. The tail shield (pygidium) is well segmented. The central part of the tail shield has 9 to 11 segments, and the sides have 5 to 8 pairs of visible ribs. The grooves between the ribs are deep, while the grooves that divide each rib into front and back parts are very shallow. The front parts of the ribs are the widest. The outer shell is covered in small bumps called tubercles.

Some Phacops rana specimens have many irregular black spots. Similar spots in Greenops boothi from the same location are arranged in rows, suggesting the spots may have been original and not caused by the fossilization process. The spots are irregular and have small branches extending outward, similar to melanophores in many living animals. In one specimen, the black spots are larger than in another. This variation in spot size may have helped Phacops rana blend into different environments.

Taxonomy

Scientists have made changes to how they classify many fossil species, including Phacops. Because of these changes, Boeckops, Chotecops, Paciphacops, Prokops, and Viaphacops were once considered subgroups of Phacops but are now recognized as separate genera. Recently, scientists have found that some species from North America and North Africa are different enough from European species to be placed in separate genera. Since the type species of Phacops is the European P. latifrons, the North American species are now called Eldredgeops. However, older names like Phacops rana are still commonly used by fossil collectors. Eldredgeops has a raised ridge along the bottom edge of the head, a more rounded head area, rectangular shapes instead of rounded parts near the back of the head, larger eye areas, and lacks certain features found in P. latifrons. Not all of these traits may clearly separate Eldredgeops from other Phacops species.

During the Eifelian period in what is now the Belgian Ardennes, several Phacops species evolved one after another. The oldest was P. imitator, followed by P. fragosus, then P. latifrons, and finally P. sartenaeri. These species show a trend of fewer eye lenses, a pattern also seen in many other Phacopinae species.

Fossils of Phacops salteri have been found in the trilobite-rich Late Emsian to Early Givetian Floresta Formation of the Altiplano Cundiboyacense, Colombia.

Phacops currently includes the following species:

A number of species previously assigned to the genus Phacops have since been transferred to other genera.