Trilobita

Trilobita is an extinct class of marine arthropods that inhabited Earth's oceans for most of the Paleozoic Era, from the Early Cambrian (about 521 million years ago) to the end of the Permian (about 252 million years ago). They were among the earliest highly diversified groups of complex arthropods and left a rich fossil record.

Trilobites have a distinctive three-lobed body plan, with a central axial lobe and two pleural lobes. The body is divided into a cephalon (head), a segmented thorax, and a pygidium (tail shield). Most possessed a calcified exoskeleton that they moulted as they grew. The cephalon often bore compound eyes, a glabella (central lobe of the head), and facial sutures that facilitated molting. The thorax could contain numerous articulated segments, and many species could enroll by curling into a ball as a defensive posture.

Ecology and morphology varied widely. Trilobites were primarily benthic, occupying a range of marine environments from shallow shelves to deeper settings. They exhibited substantial variation in size, shape, and ornamentation, with feeding strategies including scavenging, detritivory, grazing, and predation. Eye development ranged from well-lit, visually complex eyes to highly reduced or absent eyes in some deep-water lineages.

The fossil record of trilobites is extensive and widely used in biostratigraphy, helping correlate rock layers across regions. Exceptional preservation in some deposits, such as the Burgess Shale, provides detailed views of soft-tissue anatomy. Trilobites are an important group for understanding early arthropod evolution and Paleozoic marine ecosystems.

Trilobita is the class within the phylum Arthropoda. They declined through the late Paleozoic and became extinct at the end-Permian mass extinction, but their fossils continue to be a cornerstone of paleontological study.