Trilobites are extinct marine arthropods that flourished from the Cambrian to the Permian (between 520 and 250 million years ago). Over 5000 genera have been recognized, distributed between five orders, and they reached their greatest diversity during the Ordovician Period. They suffered significant extinctions at the end of the Ordovician, and again late in the Devonian, when many higher taxonomic groups were lost. Only one Order, the Proetida, survived into the Carboniferous, and with its extinction at the end of the Permian the trilobites disappeared forever.
Trilobites occupied many different marine habitats and this is reflected in a wide variety of morphological types. Most were benthic, living on the sea bed, but some burrowed into soft sediments, whilst others lived in open seas (pelagic). The first trilobites in the geological record have well-developed eyes, but some forms lost them altogether; in some pelagic forms they were enormously expanded, affording vision in all directions. Trilobite eyes were made of calcite crystals, orientated so that the c-axis runs along the length of each lens, and in this way behave as if made by glass. Trilobite eyes are the earliest sophisticated visual organ known.
Like brachiopods, many trilobites that occupied epicontinental seas have been used for the discrimination of ancient continents and terranes. They are also important as zonal fossils for use in biostratigraphy, especially in rocks of Cambrian age.