Soft Body Parts

The unmineralized or soft parts of the trilobite body are very rarely preserved in the fossil record. Allison and Briggs (1993) made a listing of sites of exceptional fossil preservation, called by the German name Konservat-Lagerstatten. They recognized 19 marine sites worldwide in the Paleozoic, where soft body fossils are preserved. Nine of these sites are in the United States, and six of them yield trilobites. Only one site in the United States in their listing has significant trilobite appendage and other soft body information: Beecher's Trilobite Bed in New York. In fact, most of what we know about the soft parts of trilobites comes from only five localities worldwide, including the really remarkable new Cambrian sites in China (Shu et al. 1995). Two of these five sites are in New York and one of them, the Walcott-Rust Quarry, was not included in Briggs and Allison's list. Information on soft or weakly skeletonized body parts is very rare, and because of this these data are generalized to a wide range of trilobites. The rarity of this soft-bodied information is exemplified by the remarkably preserved biota of the Burgess Shale in British Columbia. Most of what is known of soft-bodied animals in the Cambrian initially came from these beds, yet of the 22 species of trilobites known from the Burgess Shale beds, apparently only 4, so far, have yielded appendage information and in one of these it is from a single specimen. When you are reading through the following descriptions, remember that all the soft tissue data come from a very few sites and only a bare handful of trilobite species.

An indirect relationship of trilobites to the annelid worms was introduced earlier. This evolutionary trail is supported by the multisegmented body of the trilobite and the observation that each recognizable segment bears a pair of appendages. This observation includes the cephalon and pygidium, in which the segments are fused. Careful studies, by C. D. Walcott (1876, 1881, 1918, 1921), of the specimens he had available established that the appendages are biramous (Figure 2.10A). In other words, each individual appendage is divided into two parts, one part for walking, the endopod (Figure 2.10D), and one part, the exopod (Figure 2.1 OB), possibly an apparatus similar to the gill of fishes, for breathing.

The only trilobite appendages that are not biramous are the most anterior, which are modified into antennae (Figure 2.11A, B, C). C. E. Beecher (1893c, 1894a, 1894b, 1896), after years of study on meticulously prepared Triarthrus eatoni from Beecher's Trilobite Bed, published the most famous, and most often reproduced, illustration of the trilobite ventral anatomy (Figure 2.11A). Beecher, possibly following Walcott's lead, gave T. eatoni an extra set of appendages under the cephalon, but this does not take away from his remarkable achievement.

The works of Walcott and of Beecher have been modified and augmented by a number of later workers, particularly Raymond (1920a), Stormer (1939, 1951), Sturmer (1970), Bergstrom (1969, 1972, 1990), Bergstrom and Brassel (1984), Cisne (1975, 1981), Whittington (1980, 1992), and Whittington and Almond (1987).

Biramous appendages are also the rule in extant crustaceans. In the trilobites all the appendages, with the exception of the antennae, are very similar, differing primarily in size. All but one of the trilobites recently studied have three pairs of biramous appendages in the cephalon, a pair for each thoracic segment, and multiple pairs in the pygidial section. Four pairs of biram-ous cephalic appendages reportedly were found in one trilobite (Bergstrom and Brassel 1984), but some workers think this finding is questionable. In the pygidium the segmentation has to be inferred. For example, the number of appendage pairs under

FIGURE 2.9. Trilobite exoskeletons with attached fauna or injury. A. Arctinurus boltoni (USNM 449453). This trilobite has brachiopods of different sizes (arrows) attached, indicating the length of time between molts of mature specimens. Arctinurus specimens with attached brachiopods are not rare in the Rochester Shale beds, where these came from. It is unlikely that they settled on the exoskeleton after death because the exoskele-ton would have to have been buried to remain articulated. B. The same species of trilobite as in A with healed injuries to the exoskeleton. Three areas (arrows) have been damaged and been through at least one molt (PRI 42095). C. Dalmanites limulurus (F. Barber collection, whitened). This trilobite has attached brachiopods (arrows). This brachiopod attachment is very unlikely to have occurred postmortem for the same reasons. D. The same trilobite as in C, with the eye area enlarged to better show the brachiopods (arrow). E. The same trilobite, with the thoracic area enlarged to show the brachiopods (arrows).

FIGURE 2.9. Trilobite exoskeletons with attached fauna or injury. A. Arctinurus boltoni (USNM 449453). This trilobite has brachiopods of different sizes (arrows) attached, indicating the length of time between molts of mature specimens. Arctinurus specimens with attached brachiopods are not rare in the Rochester Shale beds, where these came from. It is unlikely that they settled on the exoskeleton after death because the exoskele-ton would have to have been buried to remain articulated. B. The same species of trilobite as in A with healed injuries to the exoskeleton. Three areas (arrows) have been damaged and been through at least one molt (PRI 42095). C. Dalmanites limulurus (F. Barber collection, whitened). This trilobite has attached brachiopods (arrows). This brachiopod attachment is very unlikely to have occurred postmortem for the same reasons. D. The same trilobite as in C, with the eye area enlarged to better show the brachiopods (arrow). E. The same trilobite, with the thoracic area enlarged to show the brachiopods (arrows).

FIGURE 2.10. Trilobite appendage reconstruction and nomenclature. A-l. Triarthrus eatoni biramous appendage after Starmer (1939). B. The exite or brachial appendage (arrow) with the comblike structures. C. The basis (arrow), also called the coxite in early literature. D. The walking leg or telepodite (arrow). This leg has seven segments, including the foot, in all trilobites where the appendages have been studied. E. A podomere or individual segment (arrow) of the walking leg. F. Endites (arrows), small triangular inwarding-facing projections on the podomere. These were possibly used to help transport food along to the mouth at the rear of the hypostome. G. Setae (arrow), hairlike projections on the exites. H. Gnathobases (arrow) are sharp projections on the basis that may have been used to masticate, to reduce the size of food particles. I. The foot (arrow) with its setae. J. A reconstruction of the filaments of the exopod of Ceraurus pleurexanthemus by Starmer (1939). K. A partial axial view, looking toward the rear, of Triarthrus as reconstructed by Whittington and Almond (1987, p. 42, Fig. 43). Reproduced with permission.

FIGURE 2.10. Trilobite appendage reconstruction and nomenclature. A-l. Triarthrus eatoni biramous appendage after Starmer (1939). B. The exite or brachial appendage (arrow) with the comblike structures. C. The basis (arrow), also called the coxite in early literature. D. The walking leg or telepodite (arrow). This leg has seven segments, including the foot, in all trilobites where the appendages have been studied. E. A podomere or individual segment (arrow) of the walking leg. F. Endites (arrows), small triangular inwarding-facing projections on the podomere. These were possibly used to help transport food along to the mouth at the rear of the hypostome. G. Setae (arrow), hairlike projections on the exites. H. Gnathobases (arrow) are sharp projections on the basis that may have been used to masticate, to reduce the size of food particles. I. The foot (arrow) with its setae. J. A reconstruction of the filaments of the exopod of Ceraurus pleurexanthemus by Starmer (1939). K. A partial axial view, looking toward the rear, of Triarthrus as reconstructed by Whittington and Almond (1987, p. 42, Fig. 43). Reproduced with permission.

FIGURE 2.11. Ventral anatomy and appendages. A. Triarthrus eatoni, ventral anatomy. The first essentially correct reconstruction of a trilobite's ventral surface and appendages. After Beecher (1896). B. Ceraurus pleurexanthemus, ventral anatomy. This reconstruction from Raymond (1920a) was from cross sections made by Walcott in the late 1900s. C. Ceraurus pleurexanthemus, ventral anatomy. This reconstruction by Stormer (1951) was from specimens collected by Walcott and uniquely prepared. D. Triarthrus eatoni, appendage structure developed by Cisne (1975, p. 49, Fig. 3) from high-resolution radiographs of pyritized specimens. Reproduced from Fossils and Strata, www.tandf.no/fossils, by J. L. Cisne, 1975, vol. 4, 45-63, by permission of Taylor and Francis AS. E. Triarthrus eatoni, appendage structure developed by Whittington and Almond (1987, p. 31 , Fig. 41), from direct observation of very carefully prepared specimens. Reproduced with permission. F. Ceraurus pleurexanthemus, appendage structure drawn by Bergstrom (1972) from information developed by Stormer (1939, 1951). Reproduced with permission. G. Cryptolithus bellulus, appendage structure developed by Bergstrom (1972, 1973) from pyritized specimens prepared by Beecher. Reproduced with permission. H. Phacops cf. P. ferdinandi, appendage structure developed by Bergstrom (1969) using the radiographs of pyritized specimens from the Hunsruck Shale in Germany. It is assumed that the phacopid trilobites of New York will have similar structures. Reproduced with permission.

the pygidium in T. eatoni is significantly more than the number of axial furrows on the dorsal surface of the pygidium, showing a weak relationship between segments and axial furrows.

A more detailed look at an individual trilobite appendage illustrates that it is a complex structure. The attachment to the ventral surface of the trilobite body is through a part called the basis (Figure 2.IOC). (For a current view on appendage nomenclature, see the article by RamskoTd and Edgecombe (19%).) The exopod or outer branch is attached to the basis, which in turn is attached to the ventral membrane of the trilobite. Beneath the exopod, and also attached to the basis, is the walking leg, or endopod. Until the work of Cisne (1975, 1981), all reconstructions of the appendages included a precoxa from which the exopod extended (Stormer 1939, Figure 1, p. 155). This view is incorrect, based on all the material recently examined; both the exopod and the walking leg are attached to the same apparatus, the basis. The exopod has a series of thin, flattened filaments extending from it, giving it a feather- or comb-like appearance, and it is carried up under the pleurae. Only in 7' eatoni are the exopods known to be long enough to extend well out from the lateral edge of the dorsal exoskeleton. The large surface area of the small filaments led most authors to believe that they have served for breathing, as an external gill. Bergstrom (1969) argued that the filaments are too small to support an effective circulatory system, and instead they may have served as a filter of food, as a means to circulate water over gill membranes on the ventral surface, or possibly as a swimming function. In all the studies the exopods are drawn with the filaments lateral and posterolateral. Stormer (1939) actually found the filaments of Ceraurus pleurex-anthemus pointed forward, but he rotated them in his reconstruction. Bergstrom (1969) believed that they were pointed forward in life (Figure 2.1 IF). Stormer (1939) reconstructed the filaments of the exopod of C. pleurexanthemus (Figure 2.10J), illustrating the high surface area that supports their use as a brachial organ.

The endopod is multiply jointed with distinct sections called podomeres (Figure 2.10E). There is general agreement that there are seven podomeres on all trilobites examined. On some of the podomeres, starting with the ones closest to the basis, are projections, endites (Figure 2.1 OF), with hairlike setae (Figure 2.10G) near or on their tip. On Triarthrus, and probably most other genera, the last podomere is a footlike tip to the walking leg (Figure 2.101).

The most thoroughly examined trilobite appendages are those of T. eatoni and C. pleurexanthemus, both from the Ordovician of New York. There are more specimens available of these trilo-bites with preserved appendages than there are of any others. Figure 2.11A is the Beecher reconstruction of T. eatoni and parts B and C of Figure 2.11 are reconstructions of C. pleurexanthemus by Raymond (1920a) and Stormer (1951). Stormer's figure is modified to show the dorsal anatomy on the right and the ventral on the left. Parts D and E of Figure 2.11 are reconstructions of the legs of T. eatoni by Cisne (1981) and by Whittington and Almond (1987), respectively. Parts F, G, and H are drawings of the legs of Ceraurus, Cryptolithus hellulus, and Phacops cf. P. fer-dinandi, all as reconstructed by Bergstrom (1969).

Figure 2.10K shows T. eatoni as reconstructed by Whittington and Almond (1987). The view is from about the midline of the trilobite looking to the rear and illustrates the orientation of the walking legs and their parts. The bases are shown with toothlike adaxial projections or gnathobases (Figure 2.10H). Food was probably collected, masticated, or pulled apart, and passed along forward to the mouth at the posterior of the hypostome by use of the endopods and bases. Trilobites so equipped would be effective bottom feeders, both as predators and as scavengers. Since this kind of information on appendages is known for so few trilo-bites, it is difficult to extrapolate too far as to the life-mode of the others.

Pyritized trilobites from New York and Germany that were studied by high-definition X-ray photography provide much of what we know about the internal anatomy (Stormer 1939; Sturmer and Bergstrom 1973; Cisne 1975, 1981). Their stomach (Figure 2.12A, label c), crop, or foregut is located in the glabella. Along with the stomach in the cephalon are organs called hepatopancreatic organs (Figure 2.12A, label h), probably the equivalent of a liver, under the genal areas.

The gut (Figure 2.12A, B, C, label g) passes from the stomach through the axial region of the thorax and terminates at the anus just under the posterior area of the pygidium (Plate 78). The position of the gut is known from several specimens of trilo-bites because the ingested sediment often survives in place (Figure 2.12D, label g) and has a different texture or color than the surrounding stone (Raymond 1920a; Cisne 1975; Whittington 1993; Whiteley et al. 1993; Brett et al. 1999). It is assumed that some form of circulatory and nervous systems also occupied the axial interior region.

Small rodlike structures seen in radiographs (Cisne 1981) and in unusually well-preserved specimens (Whittington 1993) are believed to represent muscles, and reconstruction of the musculature related to the appendages has been proposed (Figure 2.12A, B, C, label m). Some specimens of E. rana show symmetrical dark spots on the exoskeleton (Babcock 1982). These are interpreted as muscle attachment areas.

Two soft-bodied arthropods, Naraoia compacta and N. spinifer, originally discovered from the Burgess Shale, are now regarded as trilobites and assigned to the family Naraoiidae. The Naraoiidae now includes five genera (Fortey and Theron 1995). Two of these are Ordovician and one survived to Late Ordovi-cian. If, as some authors believe, the heavier exoskeleton of post-Cambrian trilobites is an evolutionary response to more advanced predators, then it is unlikely that soft-bodied trilobites survived past the Ordovician. None of these genera are known from New York, but given the special conditions necessary for their preservation, this does not prove that they were not present.

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