The cephalon (orange in figure below) is a head shield composed of fused segments containing a trilobite's main sensory organs (eyes, antennae), as well as the mouth and a special ventral plate called the hypostome, thought to function as a mouthpart. The first three or four pairs of legs of a trilobite are also associated with the cephalon, behind the antennae, with a pair of legs associated with each of the main glabellar lobes. As a major trilobite feature with many parts that vary greatly between species and higher taxa (from genera to families, superfamilies, suborders, and orders), the cephalon provides details particularly important in trilobite classification. Pages dedicated to cephalic features are listed below.
Right: cephalon of Greenops boothi
©1999-2007 by S.M. Gon III
The trilobite cephalon (orange) is the most anterior of the three trilobite tagmata (major body sections). It bears eyes, mouthparts and antennae, and articulates with a thorax (yellow) of multiple articulated segments (that in some species allowed enrollment), and a pygidium, (green) or tail section of fused segments.
The underside of the cephalon reveals the hypostome (H), a mouthpart that underlies the glabella (G). The function of the hypostome and its role in classification is discussed on a page dealing with hypostome terms.
The axial portion of the cephalon is called the glabella (green), and is largely occupied by the anterior digestive system. The cheeks (genae) are the pleural lobes on each side of the glabella. When trilobites molt or die, the librigenae (purple) or "free cheeks" often separate along the facial sutures, leaving only the cranidium --that is, the combined glabella and fixigenae (blue) or "fixed cheeks"
Terms for genal spines: while the typical cephalic spine placement is at the genal angle (the lateral posterior corner of the cephalon), where it is called simply a genal spine, other spine locations may be anterior (pro) or adaxial (meta) of the genal angle, and are further defined by their placement on either the fixigena (shown in yellow here) or the librigena (in purple). Note that a prolibrigenal spine might occur close to the genal angle, or be placed far forward along the anterior margin of the cephalon. A profixigenal spine is usually on the anterior margin. In some trilobites, a series of small spines might be present along most of the genal margin (e.g., see specimens in the family Odontopleuridae)
When describing differences between different taxa of trilobites, the presence, size, shape, and proportions of the cephalic features are often involved. The diagram above indicates via colors some of the important features use to distinguish taxa. For example, if the glabella is large, it reduces the size of the preglabellar area, in some groups displacing it altogether. To the right are cephalic (cranidial) features described when the librigenae are missing.
Cephalic features are covered in more detail on these pages:
Dorsal cephalic features
Ventral cephalic features
Cephalic internal anatomy
all figures above ©1999-2008 by S.M. Gon III created with Macromedia