phacopids

Phacopids are an extinct order of trilobites, a group of marine arthropods that lived from the Cambrian to the Permian periods. They are characterized by their distinctive compound eyes, which are composed of numerous small lenses arranged in a honeycomb pattern, giving them excellent vision. Phacopids also have a characteristic three-lobed body structure, with a central lobe (the glabella) and two side lobes. Their exoskeleton was typically thick and robust, providing protection from predators.

Fossil evidence indicates that phacopids were a diverse and widespread group, found in marine environments across the globe. They occupied various ecological niches, from bottom-dwellers to free-swimming predators. The phacopid lineage flourished during the Devonian period, often referred to as the "Age of Fishes," where they were a common component of marine faunas. Their fossils are valuable for paleontologists in understanding the evolution of arthropods and ancient marine ecosystems.

The taxonomic classification of phacopids has been subject to revision, but they are generally recognized as a distinct order within the class Trilobita. Key features used for identification include the number of segments in the pygidium (the posterior part of the body), the arrangement of cephalic sutures (lines on the head shield), and the structure of the glabella. Their extinction, like that of all trilobites, occurred at the end of the Permian period during the largest mass extinction event in Earth's history.