Phacopida

Phacopida is an order of trilobites that existed from the Early Ordovician to the Late Devonian period. They are characterized by their schizochroal eyes, a type of compound eye with individual lenses separated by sclera. These lenses were typically large and arranged in a hexagonal pattern.

Phacopida trilobites are further distinguished by their glabellar lobation, which often shows prominent furrows and lobes on the central part of their cephalon (head). The exoskeleton (carapace) is generally well-calcified. Many phacopids were capable of enrolling, a defensive behavior where they curled up their bodies into a tight ball.

The order is subdivided into several suborders, including Phacopina and Calymenina (though the inclusion of Calymenina is sometimes debated). Phacopida represents a significant group within the trilobites, displaying diverse forms and adaptations during their long geological history. Their fossil record provides valuable information about the paleontology, paleoecology, and biostratigraphy of the Paleozoic Era. The decline of the Phacopida during the Devonian period is associated with major extinction events.