Exoskeleton

The word trilobite, freely translated from Latin, means "having the nature of three lobes." The name refers to the three lengthwise, lateral parts or lobes of the trilobite body (Figure 2.1 A), not the three parts making up the body —the cephalon or head (Figure 2.1 D), thorax (Figure 2.1E), and pygidium or tail (Figure 2.IF). The central of the three lobes is referred to as the axial lobe (Figure 2.IB) and the side lobes of the thorax and pygidium, as the pleural lobes (Figure 2.2C).

Trilobites and other arthropods probably evolved from annelid wormlike ancestors. This is reflected in their segmented body and the observation that each segment, whether fused together or jointed, carries a pair of appendages. The segments in the front of the body are fused to form the cephalon, and those in the rear of the body are fused to form the pygidium. The central segments forming the thorax or trunk were joined by flexible tissue that enabled many trilobites to flex inward to the point of enrolling (Plate 109), but probably did not allow for much, if any, sideway flexing. Some trilobites are found arched or flexed dorsally, which indicates the flexibility of the trilobite articulation. For a more detailed account of the elements of trilo-bite articulation, see the publications by Whittington (1992), Bergstrom (1973), Levi-Setti (1975, 1993), Moore (1959), and Kaesler (1997). The work by Moore (1959), Treatise on Invertebrate Paleontology, Part O, Arthropoda 1, often is referred to simply as the Treatise. Kaesler's publication (1997) is the first of a three-part revision in progress but only the first part has been released.

Four terms are used repeatedly in describing trilobites. These are dorsal, meaning the uppermost surface of the body in the animal's life position; ventral, the lower surface of the body;

Trilobite Structure

FIGURE 2.1. Trilobite structure using Eldredgeops rana (PRI 49656, whitened). A. The three lobes from which the name trilobite was derived. B. The axial lobe. The axial lobe is subdivided into the thoracic axis and the pygidial axis. C. The pleural lobes. These may be further defined as thoracic pleurae and pygidial pleurae. D. The cephalon or head of the trilobite. E. The thorax. F. The pygid-ium or tail. G. The glabella. H. The eyes. I. The palpebral lobes on top of the eyes.

FIGURE 2.1. Trilobite structure using Eldredgeops rana (PRI 49656, whitened). A. The three lobes from which the name trilobite was derived. B. The axial lobe. The axial lobe is subdivided into the thoracic axis and the pygidial axis. C. The pleural lobes. These may be further defined as thoracic pleurae and pygidial pleurae. D. The cephalon or head of the trilobite. E. The thorax. F. The pygid-ium or tail. G. The glabella. H. The eyes. I. The palpebral lobes on top of the eyes.

Trilobite Enrolling

FIGURE 2.2. Trilobite structure using Ketlneraspis tuberculata (GJK collection, whitened). A. The exoskeleton without the right free cheek. B. The genal spine extending off the free cheek (arrow). C. Lateral thoracic pleural spines (arrows). D. Pygidial spines (arrow). E. Anterior cephalic spines (arrow). F. Axial nodes, raised areas on the thoracic axis (arrows). G. Axial occipital node (arrow). H. Pustules, small, randomly scattered raised areas (arrow).

FIGURE 2.2. Trilobite structure using Ketlneraspis tuberculata (GJK collection, whitened). A. The exoskeleton without the right free cheek. B. The genal spine extending off the free cheek (arrow). C. Lateral thoracic pleural spines (arrows). D. Pygidial spines (arrow). E. Anterior cephalic spines (arrow). F. Axial nodes, raised areas on the thoracic axis (arrows). G. Axial occipital node (arrow). H. Pustules, small, randomly scattered raised areas (arrow).

anterior, toward the front; and posterior, toward or at the rear of the body part being described. Dorsal and ventral are absolute terms in the sense that the dorsal and ventral surfaces are the same regardless of the part being described or its orientation. Anterior and posterior relate to the particular part or parts being described. Thus, a suture, or inflexible joining, might be anterior to one body part and posterior to another.

The hard mineral exoskeleton, also called the cuticle, is a complex structure. The external surface is variously ornamented with ridges, terrace lines, nodes (Figure 2.2F, G), pustules (Figure 2.2H), tubercles, and spines (Figure 2.2B, C, D). Terrace lines are common on the trilobite exoskeleton and are described in some detail by Miller (1975). Nodes are discrete, rounded, raised areas on the dorsal exoskeleton, which are usually bilaterally symmetrical (Plates 45 and 134). Pustules, on the other hand, are raised areas that are more frequent on the surface and also tend to be more randomly distributed (Plates 24 and 25). Large pustules are often called tubercles (Plate 27). There commonly are channels from the surface of these structures to the interior, and it is believed that these channels served for sensory capability, enabling the trilobite to sense currents and chemical changes to the environment (Figure 2.6). Some of these perforations also may have been follicles for sensory hairs. Dorsal spines are the physically extended equivalents to nodes (Plates 46 and 128). Spines, which extend or radiate from the edges of body parts, will be discussed later.

The cephalon is the most complex and the most important part of the trilobite to be understood by the student or collector because it is often the most easily recognized evidence for trilo-bites in the rocks. The center of the cephalon is the glabella (Figure 2.1G, 2.3E), a raised portion with characteristics often important to the recognition of trilobite species. Although the glabella is treated here as separate from the occipital ring (Figure 2.3F, the lobe labeled LO), the terminology used in the Treatise, most trilobite workers today include the occipital ring as part of the glabella (this latter protocol is used in the Kaesler (1997). Laterally outward from the glabella are the cheeks or genae (singular, gena). These areas usually have sutures (Figure 2.5) that separate them into free cheeks (Figure 2.3C) and fixed cheeks (Figure 2.3D), called librigenae and fixigenae, respectively. The librigenae usually separate from the cephalic area on molting, and the fixigenae remain permanently attached. The glabella, together with the fixagenae, is called the cranidium (Figure 2.3B). Many trilobite species, particularly in the Cambrian, are differentiated on the basis of details in their cranidia.

The posterior part of the glabella, the occipital ring, is a raised portion almost always separated from the main body of the glabella by a groove. Some occipital rings have a central prominently raised area, node, or spine that is very characteristic for particular genera or species. Spines are often overlooked because of the ease with which they are broken off and lost during the process of removing the trilobite from the rock.

The glabella is the dorsal covering of the stomach of the trilo-bite. It usually has lateral grooves in its surface known as lateral glabellar furrows (Figure 2.3G). The furrows rarely cross the surface completely but can be deep, forming prominent areas on the glabella called lateral glabellar lobes (Figure 2.3F) or simply, glabellar lobes. Because of the use of furrows and lobes in trilo-bite identification, they are designated starting from the occipital ring and occipital furrow. The furrow or sulcus separating the occipital ring from the glabella is labeled SO; the next most anterior glabellar furrow is SI, and so on. (The O in SO and LO refers to "occipital" and is not a zero.) The lobes are similarly designated, with the occipital ring called LO, the lobe directly anterior to it LI, and so forth. It is not always easy to distinguish the most anterior lateral lobes because the furrows can be very faint; thus, the most anterior portion of the glabella is called La, for anterior glabellar lobe. The furrow separating the glabella from the cheek area (Figure 2.3H) is the glabellar furrow. On many trilobites the edge of the cephalon is distinctive (Figure 2.31) and is called the border.

The cheeks generally bear the eyes of the trilobite (Figure 2.1H). (There are eyeless, presumably blind, trilobites but these are an exception and will be noted in the specific descriptions.) The eye is often prominently raised, actually being on a stalk, in a few species. The area between the eye and the glabella is called the palpebral area (Figure 2.11) and can have its own furrows and lobes.

The surface of the eye has multiple lenses to form images, and this surface can be very distinctive. Eyes with a closely packed optical structure and a smooth, continuous outer surface (cornea) are called holochroal (Figure 2.4A). Although they have a smooth appearance, these are compound eyes, similar to those in some modern insects. Eyes with discrete individual lenses, each with its own corneal surface, are called schizochroal (Figure 2.4B). In addition, the lenses in schizochroal eyes are separated by a thick interlensal sclera. There is a third type of eye, resembling the schizochroal eye, found in the family Pagetidae, called abathochroal (Jell 1975). Abathochroal eyes lack the deep inter-lensar scleral projection and the intrascleral membrane of schizo-chroal eyes.

Holochroal eyes arc found in the majority of trilobites, while the schizochroal eyes are found only in the suborder Phacopina, which arose in the Ordovician and disappeared by the end of the Devonian. Phacopins are well represented in New York, and this type of eye structure is often observed. The number of optical elements in schizochroal eyes is usually less, often far less, than that in holochroal eyes.

In a series of elegant experiments, Towe (1973) demonstrated that the eyes in at least two trilobites were single crystal calcite oriented to give high-quality imaging. Towe mounted the eye surface of Eldredgeops rana and an Isotelus species and demonstrated that each facet of a holochroal eye (Isotelus species) and each lens of a schizochroal eye (E. rana) gave an

Fossil Trilobit

FIGURE 2.3. The structure of the trilobite cephalon using Calymene species (S. Insalaco collection, whitened) (cephalon only). A. Cephalon lacking the right free cheek. B. The cranidium (arrow). C. Free cheek (arrow). D. Fixed cheeks (arrows). E. The glabella (arrow). F. The glabella, with the glabellar lobes numbered (arrows). G. The glabella, with the lateral glabellar furrows numbered (arrows). H. The glabellar furrows (outlined, with arrows). I. The cephalic border (arrows).

FIGURE 2.3. The structure of the trilobite cephalon using Calymene species (S. Insalaco collection, whitened) (cephalon only). A. Cephalon lacking the right free cheek. B. The cranidium (arrow). C. Free cheek (arrow). D. Fixed cheeks (arrows). E. The glabella (arrow). F. The glabella, with the glabellar lobes numbered (arrows). G. The glabella, with the lateral glabellar furrows numbered (arrows). H. The glabellar furrows (outlined, with arrows). I. The cephalic border (arrows).

FIGURE 2.4. Trilobite eyes. A. Monodechenella macrocephala (G. Jennings collection, whitened) with holochroal eyes (arrow). B. Viaphacops bombifrons (GJK collection, whitened) with schizochroal eyes (arrow).

FIGURE 2.5. Cephalic sutures. A. Ceraurus pleurexanthemus (GJK collection, whitened) showing proparian cephalic sutures (arrows), both ends emerging anterior to the genal angle. B. Calymene niagarensis (S. Insalaco collection) with gonatoparian cephalic sutures (arrows), the posterior end emerging at the genal angle. One free cheek has been lost on the right side. C. Isotelus maximus (PRI 49651) with opisthoparian cephalic sutures (arrows), where the posterior end emerges along the rear of the cephalon. D. Calyptaulax callicephalus (GJK collection, whitened), a trilobite in which the facial sutures are fused and do not separate upon molting. This is common in the order Phacopida.

FIGURE 2.5. Cephalic sutures. A. Ceraurus pleurexanthemus (GJK collection, whitened) showing proparian cephalic sutures (arrows), both ends emerging anterior to the genal angle. B. Calymene niagarensis (S. Insalaco collection) with gonatoparian cephalic sutures (arrows), the posterior end emerging at the genal angle. One free cheek has been lost on the right side. C. Isotelus maximus (PRI 49651) with opisthoparian cephalic sutures (arrows), where the posterior end emerges along the rear of the cephalon. D. Calyptaulax callicephalus (GJK collection, whitened), a trilobite in which the facial sutures are fused and do not separate upon molting. This is common in the order Phacopida.

inverted image when viewed from the rear, just as a simple glass lens does.

Stunner and Bergstrom (1973) studied the internal structure of the eyes of Phacops specimens from the Hunsriick slates in

Germany. Some of the soft tissue of the trilobites was replaced by the mineral pyrite (i.e., it was pyritized), and X-ray photographs showed clear evidence of optical fibers extending from the lens into the central cephalon. One asteropyge in their study also had similar fibers extending from the lens. These structures were considered comparable to similar structures in extant arthropods. Structures of this kind were reported previously in asteropyges but not confirmed until this study.

On many trilobites there is a medial, glabellar node on the meraspid (juvenile) that generally disappears by the holaspid (adult) phase or early in the holaspid growth. This node is interpreted as having a visual or light-sensing capability (Ruedemann 1916b, Jell 1975). CryptoJithus is a common Ordovician trilobite genus in New York that lacked normal eyes but retained this median glabellar node into the mature holaspis (Plates 163 to 167). A review of the evolution of trilobite eyes with references for detailed reading is given by Clarkson (1975). More will be said about eye function later in this chapter when the possible modes of life of trilobites are discussed.

In post-Cambrian and most Cambrian trilobites there is a suture running across the upper edge of the eye, separating the lens surface from the palpebral area. This suture or separation continues to the edge of the cephalon in both directions and results in a portion of the cheek area (librigena) that can separate from the cranidium. This facial or cephalic suture separates the free cheek (librigena) from the fixed cheek (fixigena).

In most Cambrian trilobites an additional suture runs below the eye and joins the facial suture, forming the circumocular suture so that the visual surface separates on molting and is lost. It is also common in articulated calymenids for the visual surface to be missing. This absence suggests that a suture surrounded the visual surface or that it was weakly mineralized, if at all.

The anterior margin of the cephalon is rounded and the posterior margin is less curved laterally, resulting in the formation of a corner at the posterolateral extremity known as the genal angle. I )epending on the species, this structure varies from a blunt well-rounded angle to a genal spine (Figure 2.2B) that extends back along the body. One end of the facial suture crosses the genal area and emerges on the anterior margin of the cephalon, and the other end emerges either in front or behind the genal angle. If both ends of the sutures emerge anterior to the genal angle, it is known as a proparian (Fig 2.5A) suture; if one end emerges on the posterior cephalic margin, the suture is called opisthoparian (Figure 2.5C). As one might expect, there are trilobites where the suture emerges precisely on, or very near, the genal angle, and that condition is called gonatoparian (Figure 2.5B). In some trilobites the cephalic suture (Figure 2.5D) is fused and does not open on molting.

The area immediately in front of the glabella varies from nearly nonexistent to a broad border platform or brim. In some families, particularly the calymenids (Figure 2.31), the shape of this area is important to the identification of genera.

Sometimes this anterior border has a lateral furrow. The area between this preglabellar furrow and the glabellar furrow is the preglabellar field (Plates 139 and 142). The area between the preglabellar groove and the anterior margin is the anterior cephalic border.

The thorax is divided into a number of segments that form a highly flexible part of the exoskeleton. The number of thoracic segments can range from zero in the unusual Cambrian family

Naroidae to 40 or more. Each segment has a central or axial portion and a lateral part on each side of the body called the pleura (plural, pJeurae). The usually grooved pleurae also may extend laterally beyond the body into short rounded extensions called lappets (Plates 50 to 56) or even into more extended and pointed pleural spines (Figure 2.2C). The distinction between lappets and spines is qualitative, and in some cases it is not clear whether the extensions should be termed Jong Jappets or short spines (Plates 46 and 48). Other structures can increase flexibility and sometimes tight enrolling, but these are beyond the scope of this book. These structures are covered in detail in the work by Bergstrom (1973).

The most posterior part of the trilobite is the pygidium (Figure 2.IF). The pygidium, similar to the thorax, has segments with axial and pleural portions. However, the pygidial segments are totally fused so there is no flexibility among the parts. The number of axial segments, the number of pleurae, and the general shape of the pygidium are often diagnostic to species. Pygidia are common in the trilobite fossil record, and these features are very important to the identification of trilobites. The pygidium can have marginal lappets or marginal spines (Figure 2.2D) as extensions of the pleurae, and sometimes has a central posteriorly directed spine called the terminal axial spine (Plate 88). In addition, there is sometimes a narrow featureless area around the margin of the pygidium simply called the border. In some genera of trilobites, the pygidium is almost featureless, and when found separate, it looks like nothing more than a dark thumbnail on or in the rock. This finding should not be overlooked as an indicator of the presence of particular species.

On the surface of many trilobites, there are pitted areas that often penetrate the exoskeleton. The exoskeleton of the large trilobite DipJeura dekayi is literally covered with pits, seen in Figure 2.6A as small white specks. Figure 2.6B shows a broken area of this trilobite, with the pits as complete perforations through the cuticle. IsoteJus gigas is similarly covered with pits (Figure 2.6D). Pits such as on these two trilobites may have served a sensory purpose and may have contained sensory hairs. A different kind of pit is seen on the cephalic border of CryptoJithus species (Figure 2.6C). These are also perforations but go completely through the border area. Their role is unknown, but they may have served to sense water currents or movement.

The exoskeleton covering the entire dorsal surface of the trilo-bite sometimes curls under at the edges of the cephalon and pygidium to form the doublure (Figure 2.7H, I), a fiat terrace around the ventral edges of the cephalon and pygidium. In addition to the doublure, there are two important pieces of the exoskeleton on the ventral side of the cephalon. The rostral plate, found on many but not all trilobites, is a continuation of the doublure but is separated from the rest of the cephalon by sutures; it is located directly under the front central part of the cephalon (Plate 65 shows a displaced rostral plate immediately in front of the cephalon). Under the approximate center of the cephalon is

FIGURE 2.6. Exoskeletal pits or circular perforations. A. Dipleura dekayi(PR\ 49629). White specks over the body are sediment-filled pits. The arrow points to the area shown in B. B. Close-up of Dipleura pits, showing that they extend through the exoskeleton (arrow). C. Cryptolithus lorettensis (PRI 49657, whitened), showing pits on the cephalic border (arrow). These are actually perforations that extend all the way through the border. D. Isotelus gigas (TEW collection, whitened). The exoskeleton' is heavily pitted (arrow), a diagnostic character of this species. E. Greenops grabaui (F. Barber collection, whitened), showing rows of circular perforations characteristic of the New York astropygids. F. Greenops? species (GJK collection, whitened). Unnamed, new?, species of Greenops in which the circular perforations are degenerate.

FIGURE 2.6. Exoskeletal pits or circular perforations. A. Dipleura dekayi(PR\ 49629). White specks over the body are sediment-filled pits. The arrow points to the area shown in B. B. Close-up of Dipleura pits, showing that they extend through the exoskeleton (arrow). C. Cryptolithus lorettensis (PRI 49657, whitened), showing pits on the cephalic border (arrow). These are actually perforations that extend all the way through the border. D. Isotelus gigas (TEW collection, whitened). The exoskeleton' is heavily pitted (arrow), a diagnostic character of this species. E. Greenops grabaui (F. Barber collection, whitened), showing rows of circular perforations characteristic of the New York astropygids. F. Greenops? species (GJK collection, whitened). Unnamed, new?, species of Greenops in which the circular perforations are degenerate.

FIGURE 2.7. Ventral anatomy of the exoskeleton. A-D. Ceraurus pleurexanthemus (PRI 49658, whitened), prepared to show the ventral exoskeleton. B. The hypostome (arrow). C. Two of the apodemes (arrows). The apodemes are arranged along the ventral surface under the axial furrow. There is a pair of apodemes for each pair of cephalic appendages, two for each pair of thoracic appendages (which also correspond to the number of thoracic segments), and apodemes associated with the pygidial appendages. The often large numbers of pygidial appendages are not as well reflected in prominent apodemes. D. The arrows point out the entrance into the hollow exoskeletal genal spines, pleural spines, and pygidial spines. E-l. Isotelus species (Kevin Brett collection), ventral exoskeletal anatomy. A specimen from Canada. F. The hypostome (arrow), the anterior margin and "wings" of which are under the doublure. G. The apodemes (arrows) of Isotelus, which are far less prominent than those of Ceraurus. Given that these represent muscle attachment, the shape must represent their use and consequently the life-mode of the trilobite. H. The cephalic doublure (arrow). I. The pygidial doublure (arrow), which is incomplete in this specimen.

a rounded plate called the hypostome (plural, hypostoma) (Figure 2.7B, F). The mouth was at the rear of the hypostome.

The hypostome is a more important feature. For some species it is very robust, distinctive, and a not uncommon part of the fossil record. (In at least one rare Ordovician trilobite, Hypodi-cranotus, the presence of its hypostome is a very good indicator, and one of the only indicators, of its stratigraphic range.) The significance of the hypostome will be pointed out for the individual species. The hypostome is directly under the glabella, and together they form an envelope covering and protecting the stomach. The mouth of the trilobite was at the posterior central notch of the hypostome (Plates 37, 47, 77, 117, 153, and 157).

The attachment of the hypostome is proposed to have significance for the high-order classification of trilobites (Fortey [1990a] using observations made by Fortey and Chatterton [ 1988]). Natent hypostoma are those separated from the cephalic doublure, or rostral plate when present, by a gap and are displaced or absent in most trilobite specimens. Conterminant hypostoma are closely joined to the cephalic doublure or rostral plate and consequently are more likely to be present on specimens (Figure 2.7B, F). In both of the above cases, the anterior margin of the glabella is directly above the anterior margin of the hypostome. The third type of hypostome, impendent, is when the anterior margin of the hypostome is not close to the anterior margin of the glabella and the anterior margin of the glabella coincides with the cephalic margin. Examples of trilobites with the different types of hypostome attachment are as follows: natent — some proetids; conterminant —/. gigas (Plates 153 and 155) and CerauruspJeurexanthemus (Plate 77); and impendent — £. rana.

Under the lateral sides of the ventral axial region of the thoracic segments, glabella, and pygidium are thickened areas called apodemes (Figure 2.7C, G) or sometimes appendifers. The apo-demes are believed to have been points of muscle attachment and may provide evidence for the lifestyle of the trilobite. On some trilobites, such as Ceraurus pJeurexanthemus (Plate 77), the apodemes extend down from the ventral surfaces of the segments to form prominent ridges or posts, yet in others, as illaenids, the apodemal area is fairly smooth. The long apodemes suggest good leverage for the attached muscles and a high level of limb mobility, perhaps for swimming, rapid crawling, or digging.

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    How many lobes did trilobites exoskeletons?
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