The head shield or cephalon resulted from the fusion of a number of tergites (five or seven) and often carries telltale memory of the original segmentation. It is the most significant and characteristic part of the trilobite morphology.
The outline of the cephalon may be semicircular to ogival in its anterior portion, straight or gently curved at the posterior margin, where articulation with the first thoracic segment occurs. The lateral and anterior margin of the cephalon are inflected into the doublure, a narrow strip of the ventral side of the exoskeleton that is mineralized like the dorsal side. Attached to the anterior part of the doublure is a small shield called hypostoma, one of the few hard parts to be found on the underside of the trilobite. The angle between the backward-sloping lateral margin of the cephalon and the posterior margin is called the genal angle. This termination can be rounded or, as in our example in plate 1, prolonged into long genal spines. The axial lobe extends into the cephalon, where it takes the name of glabella. This can be a very convex bulging region, sometimes extending all the way to the anterior margin, or it can terminate earlier, defining a flat preglabellar field. The furrows may delineate an anterior lobe and several pairs of lateral glabellar lobes. This particular structure together with the occipital ring provides suggestive evidence of the original segmentation of the cephalic region and is apparent in the trilobite chosen for our reconstruction. The furrows which surround the glabella take the name of the particular area where they are located, as indicated in figure 2. (This is a general rule for furrows occurring elsewhere in the trilobite exoskeleton.) On the sides of the glabella we find the cheeks, which are split into two regions, the free cheeks and the fixed cheeks, by the facial sutures. The assembly of the two fixed cheeks together with the glabella constitutes the cranidium. About the middle of the exterior margin of the fixed cheeks is a kidney-shaped elevation, the palpebral lobe. The visual surface of the eye is usually located on the outer slope of the palpebral lobe. Much will be said about the eyes of trilobites in section 3.3, so we shall not go into details here.
There are several junctions, or sutures, between the various parts of the cephalon. The facial sutures mentioned above have been considered meaningful for classification until recently, when other overriding criteria have been developed. Based on the position of the facial sutures, the
Description of trilobite terminology, (a) Dorsal view of the complete exoskeleton. (b) Ventral view of cephalon. The trilobite represented is Paradoxides gracilis ( B o e c k ).
Dorsal view of a complete specimen of Paradoxides gracilis (Boeck), a Middle Cambrian trilobite from the Jince Formation of Jince (Jinetz), Bohemia. The reconstruction in figure 2 schematizes the characters visible in this adult individual. Distortion due to tectonic shear is often present in the trilobites from this locality. In this photograph, the shear has been partially corrected by tilting the object relative to the camera viewing direction. Specimen whitened with magnesium oxide, (xl.3, RLS coll.).
trilobites are termed opisthoparian if the suture cuts the posterior margin, proparian if the sutures run to the lateral margin. In the former case, the genal angle and spine are carried by the free cheeks, in the latter by the cranidium. Figure 2 shows an opisthoparian suture. The free cheeks often separate easily from the cranidium, and this feature greatly facilitated molting. A third group exists in which the suture terminates at the genal angle, termed gonatoparian (e.g., Calymene), and a fourth group without facial sutures (e.g., the Olenellida). As we shall see, the cephalon of trilobites has differentiated into a variety of forms characteristic of particular groups, and much of trilobite classification is based on cephalic features.
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