Trilobites were divided across the body into a head (or cephalon), thorax, and tail (or pygidium), and along the body into three lobes - a central axial lobe covering the main body cavity, and two pleural lobes covering the legs and gills on either side (Fig. 8.3). The cuticle from which the trilobite exoskeleton was constructed was layered, with a thin outer layer and a thicker inner one. Both were made from calcite, arranged in an organic matrix which has not yet been characterized.
The head, or cephalon, was generally large relative to the rest of the body, and was usually divided into a series of sections separated by sutures that facilitated molting. These sutures form a small number of distinctive patterns across the cephalon. The section of the head outside the facial sutures is known as the free cheek. The part inside, adjacent to the glabella, is called the fixed cheek. The eyes of the trilobite could attach to either the free or fixed parts of the cephalon.
The eyes of trilobites were typically large and compound, made from multiple lenses like the eyes of flies. Eyes in arthropods have evolved several times, and those in trilobites are unique to the group. They had very clear vision, often using the physical characteristics of calcite to enhance light capture and focus. The visual range of trilobites was very variable, and appears strongly related to their mode of life. Free-swimming forms had forward-facing eyes, sometimes organized in a single, continuous band around the front of the head. Burrowing forms often had raised eyes close to the top of their cephalon.
Running down the middle of the head was the glabella, a raised region that protected the stomach. The size of the glabella correlated to the size of the stomach, and furrows that partly ran across the glabella may be the external expression of internal ridges, to which ligaments supporting the stomach were attached. The mouth was situated underneath the cephalon, near the back of the head, and is associated with a plate of cuticle called the hypostome. This could be attached to the front of the cephalon, or could be free lying within the soft cuticle of the underside ofthe trilobite (Fig. 8.3c).
The thorax of a typical trilobite was made up of a series of nearly identical segments, usually between two and 20 in number. Below each of these was a pair of legs and gills. In Ordovician and later trilobites these segments were usually jointed in such a way that the trilobite could roll up for defense (Fig. 8.3b). Sometimes structures on the head and tail locked together to make enrollment even more effective.
The trilobite tail, or pygidium, was usually small and made up of a series of fused segments that look similar to those on the thorax. It is likely that the leg/gill pairs occurred under some segments of the tail in most trilobites, though in some species these segments may have lacked limbs.
Three unusual evolutionary directions were taken by tri-lobites, taking them away from this rather conservative body plan. One involved them becoming extremely spiny. A second involved secondary loss of the eyes, and sometimes the addition of highly pitted fringes around the front of the head. The third involved a great reduction in size, and in the number of thoracic segments to only one or two (Fig. 8.3d). Each of these adaptations can be related to changes in lifestyle (see p. 58).
Head or cephalon
Glabella: a raised central area of the head under which was the stomach
Facial suture: a natural break in the exoskeleton to facilitate molting
Segments: below each segment was a limb/gill pair
Axial Pleural lobe lobe
Hypostome: mouth located at rear. In Calymene the hypostome was fixed to the front of the cephalon
Central groove between legs: food was probably manipulated by the legs into this groove and then moved to the mouth. The first leg segments are serrated to provide gripping and tearing functions
Leg/gill pairs: one pair for each segment in the thorax, probably three under the cephalon and several vestigial ones under the pygidium
Fig. 8.3 The main elements of trilobite morphology. (a) Calymene, a Silurian predatory trilobite in dorsal view, showing the main, well-calcified, elements of the carapace. (b) Two views of the same animal enrolled, showing the tight fit that was made between the front of the cephalon and the back of the pygidium. (c) Calymene as it might have looked from below, showing the lightly calcified or organic skeleton including the hypostome, legs, and gills. (d) The three main adaptive strategies of trilobites away from a highly conserved body plan: (i) Trinucleus, a blind trilobite with a large frontal pitted region, which probably had a sensory function; (ii) Agnostus, a tiny trilobite with a much reduced thorax; and (iii) Selenopeltis, a representative of the extremely spiny adaptation of trilobites.
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