The cephalon is the head section of an arthropod. It is a tagma, i.e., a specialized grouping of arthropod segments
The word cephalon derives from the Greek κεφαλή (cephale), meaning "head". It is rather used in Trilobite terminology. In insects, head is a preferred term. In arachnids and crustaceans, the cephalothorax is derived from the fusion of the cephalon and the thorax, and is usually covered by a single unsegmented carapace. In relation with the arthropod head problem, phylogeny studies show that members of the Malacostraca class of crustaceans have five segments in the cephalon, when not fused with the thorax to form a cephalothorax.
In the Late Precambrian or Lower Cambrian proarticulatan species Praecambridium sigillum, that resembles a segmented trilobite-like arthropod, the term is also used to describe the anterior part of the animal. The Agnostida, an order of arthropod which first developed near the end of the Early Cambrian period and thrived during the Middle Cambrian, also have a cephalon.
The Thylacocephala, a unique group of extinct arthropods, with possible crustacean affinities, thought to date from the lower Cambrian, while the class has a definite presence in Lower Silurian marine communities and survived to the Upper Cretaceous, has also a cephalon.
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Morphology of the Trilobite cephalon
The cephalon of trilobites is highly variable with a lot of morphological complexity. The glabella, the expression of the axial lobe in the cephalon, forms a dome underneath which sat the "crop" or "stomach". Generally the exoskeleton has few distinguishing ventral features, but the cephalon often preserves muscle attachment scars and occasionally the hypostome, a small rigid plate comparable to the ventral plate in other arthropods. A toothless mouth and stomach sat upon the hypostome with the mouth facing backwards at the rear edge of the hypostome.
Hypostome morphology is highly variable; sometimes supported by an un-mineralised membrane (natant), sometimes fused onto the anterior doublure with an outline very similar to the glabella above (conterminant) or fused to the anterior doublure with an outline significantly different from the glabella (impendent). Many variations in shape and placement of the hypostome have been described.[1] The size of the glabella and the lateral fringe of the cephalon, together with hypostome variation, have been linked to different lifestyles, diets and specific ecological niches.[2]
The lateral fringe of the cephalon is greatly exaggerated in the Harpetida, in other species a bulge in the pre-glabellar area is preserved that suggests a brood pouch.[3] Highly complex compound eyes are another obvious feature of the cephalon.
When trilobites moulted, the librigenae ("free cheeks") separated along the facial suture to assist moulting, leaving the cranidium (glabella + fixigenae) exposed. Trilobite facial sutures can be roughly divided into three main types (proparian, gonatoparian, and opisthoparian) according to where the sutures end relative to the genal angle (the edges where the side and rear margins of the cephalon converge). Early Cambrian trilobites belonging to the suborder Olenellina (like Fallotaspis) lacked facial sutures. Other later trilobites also lost facial sutures secondarily.[4]