cephalons

A cephalon (plural cephalons) is a term in arthropod anatomy referring to the head region or head shield. The word derives from Greek kefalos, meaning “head.” In paleontology and arthropod anatomy, cephalon commonly designates the anterior body region that bears the eyes, mouthparts, and sensory structures, and in segmented species may include projections such as the frontal margin or glabella. In trilobites, a well-known group of extinct marine arthropods, the cephalon is the cephalic shield that covers the anterior segments and is often distinct from the thorax and pygidium. The cephalon may display variations such as raised glabella, facial sutures, eye ridges, and free cheeks called librigina; its morphology is a key feature in trilobite taxonomy and functional analyses of feeding mechanics.

In living arthropods, the term cephalon is used less commonly, with anatomy usually described in terms of fused head regions or the entire cephalosomal complex or prosoma in chelicerates and crustaceans. However, in some group-specific descriptions, cephalon-like structures are identified as recognizable head capsules or shields.

Functionally, the cephalon serves to protect vital sensory organs and feeding appendages while providing attachment points for muscles that operate the mouthparts and antennae. Its shape and ornamentation can reflect ecological adaptation, including feeding strategy and habitat.

Taxonomic usage: The cephalon is a central character in identifying trilobite species and genera, and comparative studies across groups help researchers understand arthropod head evolution.