Trilobite Tracks Bell Island Nl

Exuviae of Phacops rana milleri Stewart, from the Silica Shale, Devonian, Sylvania, Ohio (x4.5). (RLS coll.; now at FMNH.)

An example of "soft-shelled trilobite" (x2.7). Due to the unusually thin and translucent carapace, this example of Phacops rana milleri Stewart is interpreted as representing a newly molted animal. Silica Shale, Devonian, Sylvania, Ohio. (RLS coll.; now at FMNH.)

Exuviae of Dalmanites verrucosus (H all), seen from the ventral side (x3.8). Silurian, Waldron Shale, Waldron, Indiana. (RLS coll.; now at FMNH.) Note the displaced hypostoma.

In spite of the predominance of exuviae in the fossil record of trilobites, an animal would occasionally be buried intact, as the majority of the pictures in the atlas section will illustrate.

3.2 Appendages and Internal Anatomy

Although the more commonly preserved part of the trilobite body is the hard, mineralized carapace, there are a few fossil deposits that have yielded completely preserved bodies, from which, by proper techniques, even the soft parts and sometimes details of the internal structure can be recognized. Famous formations of this kind are the Middle Cambrian Burgess shale of British Columbia, where the ventral appendages appear as flattened, extremely detailed impressions; the Middle Ordovician Utica Shale of New York; and the Lower Devonian Hunsriick Slate of Germany. In the fossils from the latter two deposits, the trilobite soft parts are replaced by fine crystals of iron pyrite, and this fortunate feature lends itself to detection by visual observation and, more effectively, by radiography. Trilobites with preserved appendages have occasionally also been found in very finegrained limestones. From the above occurrences, the ventral appendages of not more than twenty species of trilobites have been described in several famous studies by Walcott, Raymond, Stormer, and others. These studies are comprehensively summarized in the Treatise (Moore 1959). A most unusual and complete description of the ventral anatomy of the larvae of agnostids from the Upper Cambrian of Sweden (Miiller and Walossek 1987) has given a glimpse of the extraordinary complexity of these reputedly primitive creatures.

In recent years, improved X-ray techniques have yielded much new information on the anatomy of trilobite's soft parts, in particular from studies by J. Cisne (1973) on the trilobites from the Utica Shale and by Stiirmet and Bergstrom (1973) on the trilobites from the Hunsriick Slate. I had the fortune of working in close collaboration with Dr. Cisne when he was preparing his Ph.D. dissertation on the anatomy of Triarthrus eatoni (Hall), the most abundant of the tri-lobite species in the Utica Shale. The material at the disposal of Dr. Cisne was the exceptional collection of specimens assembled and prepared by C. E. Beecher toward the end of the last century and now belonging to the American Museum of Natural History, Field Museum of Natural History, Harvard Museum of Comparative Zoology, and the Yale Peabody Museum. The trilobites originate from a thin layer, about ten millimiters thick, in the black Utica Shale. Beecher quarried this layer extensively, collecting some seven hundred specimens, and exposed the appendages of some sixty specimens by gently rubbing away the matrix with pencil erasers. Several trilobites were further prepared by Dr. Cisne to make them suitable for soft X-ray examination, which was carried out by taking stereoscopic radiographs. In addition to furthering a better understanding of the already known appendages, Dr. Cisne's study revealed the presence of previously unsuspected details of the internal anatomy of the trilobites. As it turns out, the entire digestive tract of these specimens was often preserved, as well as muscle fibers and other surprising features of the trilobite's body. Several of Dr. Cisne's original radiographs were kindly loaned to the author for printing and are presented in this book (see plates 9, 11, 127a, 128). Furthermore, several specimens were selected and loaned to enable me to experiment with the optical techniques most suitable to enhance the visual contrast of the exposed appendages. Some of the photographs in this section and later in the atlas constitute a unique record of Beecher's trilobites.

Figure 3 presents a reconstruction of the dorsal and ventral side of Triarthrus eatoni (Hall) based on the observations by J. Cisne. It differs from previous reconstructions in several details, the most prominent being the number of cephalic appendages (three instead of four pairs) and the presence of an abdominal extension carrying the anal tract. The latter protrudes beyond the border of the pygidium, a feature that could also be interpreted as due to compres-sional shift of the soft ventral structures relative to the tergum, caused by sediment load. The basic feature of the ventral anatomy is the presence of a pair of biramous appendages carried by each of the thoracic segments. The first pair is modified into two segmented antennae, which serve an obvious sensory function. The cephalic region also contains three pairs of slightly modified appendages, and the py-gidium five pairs, while about eight pairs are carried by the terminal abdominal tract. In total, approximately thirty-one pairs of limbs can be recognized. The basic structure of the biramous appendages is shown in figure 4. It is, in many respects, similar to that found in certain Crustacea. The base portion of the limb carries two differentiated branches: a featherlike or gill-bearing outer branch, thepre-epipodite (like the crustacean exopod), and an inner branch or walking leg, the telopodite (like the crustacean endopod), composed of seven articulated limb segments. Short bristles or setae appear at various locations. The featherlike structure of the outer branch has been interpreted as representing gill-blades, which performed the respiratory function. This interpretation, however, is still controversial. Nevertheless, there is no doubt that the inner branches were constructed for crawling. The tracks left by the motion of the latter on the soft sea floor are preserved occasionally as fossils trails, called Cruziana. An example of such trails is shown in plate 7. The outer branches could have helped in swimming, their shape and arrangement suggesting a 'Venetian blind" oarlike stroke.

Although the telopodites terminate into diminutive, clawlike processes, it is doubtful that these were of much use in gathering food. Trilobite s like Triarthrus seemed to have fed on detritus or microorganisms. Their food, in the X-ray plates of J. Cisne, can occasionally be observed as clouds of finely particulate material squeezed out of the gut canal following burial compression. The mouth is thought to have been a small opening, posteriorly oriented, just at the tip of the hypostome, devoid of jaws or mandibles. It would be the terminal feature of the long enclosure defined by the limb bases. This suggests a so-called trunk-limb feeding mechanism. It involves the creation of a feeding current by a rhythmical motion of the limbs, in this case making the telopodite bases convey the food particles to the mouth through the ventral food groove. The bases of the head limbs were somewhat differentiated from the thoracic limbs, perhaps serving some kind of weak masticatory function. Locomotion, feeding, and mastication were probably part of a mechanically related 'sequence of events produced by the movement of the limbs. Once ingested, the minute food particles would pass through an esophagus into the stomach, located beneath the frontal glabellar lobe, and then into the intestine, a long tube running through the axial region and terminating in the anal duct. Still considered as part of the digestive system in the agnostids and called genal caeca is a network of ramifications radiating away from the axial region into the genal area of the cephalon. This anatomical feature gives rise to the prosopon (functional ornamentation) observed on the dorsal cephalic surface of many trilobites as a fine mesh of radiating and ramifying ridges. It is now believed to represent part of the vascular or circulatory system (Bergstrom 1973a) and will be repeatedly noticeable in the atlas photographs.

Another unusual feature of Beecher's trilobites is the apparent preservation of the muscle structure. As observed by Dr. Cisne, this consisted of a very efficiently engineered network of longitudinal, dorsoventral, horizontal, and limb muscles, which would ensure articulation, limb movement, and enrollment. Figure 4 shows only a few of such muscles in a transverse cross section of the thoracic region. Figure 5 shows portions of a longitudinal cross section, indicating, on the one hand, the complexity of the trilobite design but, on the other, its rationality.

Many details of other organic functions have been revealed in Dr. Cisne's study. I will limit the description here, however, to a presentation of some of the material that led to so much insight into the anatomy of Triarthrus. In plate 8 we see the ventral side of a specimen of Triarthrus, as originally prepared by C. E. Beecher. The photograph has been taken with the specimen totally immersed in xylene (see Appendix B). The optical contact established by this liquid of high refractive index eliminates surface reflections and enables the achievement of maximum optical resolution. The hypostome, the antennae, and most of the successive appendages can be seen. The pyritized organic material appears white against the black matrix. Plates 9 and 10

The furrowed trails left behind by trilobites crawling over muddy sea floor, are sometimes preserved as trace fossils. This example of Cruziana semiplicata

Salter, was collected by Jan Bergstrom from Lower Ordovician sandstones found on Bell Island, Conception Bay, Newfoundland, (xl.4, RLS coll.)

A modern reconstruction of

Triarthrus eatoni (Hall), completed from partial drawings by Cisne (1973). Details of the ventral side are occasionally omitted to show underlying structure.

figure 4

Structure of the biramous appendages and cross-sectional view of the thoracic region in Triarthrus eatoni (Hall). Combined assembly from separate reconstructions by Cisne (1973).

figure 4

Structure of the biramous appendages and cross-sectional view of the thoracic region in Triarthrus eatoni (Hall). Combined assembly from separate reconstructions by Cisne (1973).

figure 5

Longitudinal section through parts of the body of Triarthrus eatoni (Hall). This drawing, adapted from reconstructions by Cisne (1973), shows details of the muscle structure and of the intestinal duct.

figure 5

Longitudinal section through parts of the body of Triarthrus eatoni (Hall). This drawing, adapted from reconstructions by Cisne (1973), shows details of the muscle structure and of the intestinal duct.

r represent another specimen of the same trilobite as seen in, respectively, a radiograph by Dr. Cisne and a photograph by the author. It must be remembered that the radiographs were taken in stereo pairs. The stereoscopic observations of such pairs enables the distinguishing of features occurring throughout the depth of the specimen. Such features overlap in a single projection and make their interpretation more difficult. The appendages are clearly visible, however, and can be seen even when underlying the dorsal shield. Although the radiograph has very high resolution, the featherlike gill-bearing branches are not as visible as in plate 10, which was obtained by immersing the specimen in a bath of xylene, as for plate 8. The photographic technique employed here is showing its advantages. The gill-branch setae are now clearly discernible together with the segmented structure of the preepipodite axis, which carries them.

Plate 11 is a radiograph of another specimen. Here the digestive tract is visible, in particular in the vicinity of the anal termination. Many of the anatomical details mentioned previously can be made out from these examples, in particular with the aid of a magnifier. It should be remembered, however, that no individual specimen will show all the features exhibited in the reconstructions illustrated in figure 3. It is only through the examination of a large number of specimens that the overall structure can be visualized.

Other examples of Triarthrus IP'WX be shown in the atlas section, where the taxonomic information concerning this trilobite will also be given. Much more is contained in Dr. Cisne's dissertation, in particular very important observations relating trilobites to the other groups of arthropods.

The other important study referred to—that of Sturmer and Bergstrom (1973), was preceded by the spectacular observations by W. Sturmer (1970). The late Professor Sturmer was a physicist colleague and, as myself, also a trilobite enthusiast. In his work at Siemens AG, he had access to advanced X-ray and image processing equipment. After retirement, he owned his own X-ray station. On several occasions, in his jovial and exuberant vein, my friend gave colorful descriptions of how he searched for fossils in the field, by X-raying freshly quarried slabs brought into his X-

ray van parked at the bottom of a famous slate quarry. This slate quarry in Hunsruck, West Germany, is the depository of a paleontological treasure. The slate is of Lower Devonian age, 1,000 meters thick, containing a pyritized fossil fauna retaining the record of the soft parts of a variety of marine animals. Complete trilobites are found in the shale, with all appendages preserved, much as in the Utica Shale of Rome, New York; sudden burial in mud, in a favorable chemical environment, may have been responsible for the exceptional preservation in both occurrences. By applying contrast enhancement techniques to his radiographs, Sturmer was able to give a new description of old fossils, uncovering surprising details that had escaped previous investigations.

A classic example of these observations is provided by the comparison between an ordinary photograph of a specimen of Phacops (plate 12a) and its radiograph (plate 12b). The latter made news immediately (Sturmer 1970): gracefully floating with the eerie appearance of a luminescent deep-sea creature, Phacops sp. WS 295 acquired a new dimension in contrast with its previous image in stone. The structure of the appendages, the exitic spines, the digestive tract are sharply delineated; it should be noticed that were the radiograph of plate 12b that of a living trilobite, the contrast would not be as good as for the pyritized fossil. The mineralization of the soft parts in the latter is actually acting as a heavy element staining of the specimen, which enhances radiographic contrast, much as is done in angiography and other medical X-ray procedures. Another of Stunner's radiographs will be shown in the next section.

From a small quarry in Vastergotland, Sweden, cut into Upper Cambrian rocks, came one of the most surprising paleontological discoveries of recent years. Among the myriad of shields of the diminutive agnostid Agnostus pisiformis (Wahlenberg), which populate limestone nodules found in the quarry, Miiller and Walossek (1987) were able to isolate larval stages, mostly enrolled, enclosing in full three-dimensional relief all the ventral soft parts of the animal, including integument, limbs, setae and hairs. Due to phosphatic replacement, these delicate organs could be cleaned of the embedding matrix by etching with weak acids.

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