Trilobites are an extinct clade of marine arthropods known from the Early Cambrian to the end-Permian. They are defined by a three-lobed dorsal exoskeleton (one axial and two pleural lobes), segmented into cephalon, thorax, and pygidium. Their calcified cuticle and well-developed compound eyes, often with calcite lenses, set them apart from other arthropods. Unlike mandibulates, trilobites lacked differentiated jaws and had biramous limbs (seldom preserved). They were exclusively marine and occupied diverse ecological roles, from benthic crawlers to swimmers, with many capable of enrolling like a ball for defense. Their abundant fossils make them key markers in Paleozoic stratigraphy.