Isotelus (Trilobita) "Hunting Burrow" from Upper Ordovician Strata ...

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Isotelus (Trilobita) "Hunting Burrow" from Upper Ordovician Strata, Ohio Danita S. Brandt; David L. Meyer; Peter B. Lask Journal of Paleontology, Vol. 69, No. 6. (Nov., 1995), pp. 1079-1083. Stable URL: http://links.jstor.org/sici?sici=0022-3360%28199511%2969%3A6%3C1079%3AI%28%22BFU%3E2.0.CO%3B2-D Journal of Paleontology is currently published by Paleontological Society.

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SCHIAPPA ET AL.- PERMIAN AMMONOID RITTER,S. M. 1986. Taxonomic revision and phylogeny of post-Early Permian crisis bisselli-whitei zone conodonts with comments on Late Paleozoic diversity. Geologica et Palaeontologica, 20: 139-1 65. RUMENCEV, V. E. 1940. On the family Adrianitidae Schindewolf. Comptes Rendus (Doklady) de l'Academie des Sciences de l'URSS. 26:837-840. . 1950. Verkhnekamennougol nye Ammonity Urals (Upper Carboniferous Ammonoidea Urals). Akademiya Nauk SSSR, Paleontologichskago Instituta Trudy? 29: 1-223. [In Russian] 0.H. 1931. Uber den Ammoniten-Sipho. Preuss GeoSCHINDEWOLF, logischen Landesanst Sitzungsber, 6: 197-209. SCHIAPPA, T. A. 1993. Selected Early Permian ammonoids from Portuguese Springs, White Pine County, Nevada. Unpubl. M.S. thesis, Boise State University, 107 p. SCHWARZ, D. L. 1987. Geology of the Lower Permian Dry Mountain trough, Buck Mountain, Limestone Peak, and Secret Canyon areas, east-central Nevada. Unpubl. M.S. thesis, Idaho State University and Boise State University, 149 p. AND D. M. GWEGOS. 1991. PennsylSNYDER, W. S., C. SPINOSA, vanian-Permian tectonism along the western U.S. continental margin: recognition of a new tectonic event, p. 5-20. In G. L. Raines, R. E. Lisle, R. W. Schafer, and W. H. Wilkinson (eds.), Geology and Ore Deposits of the Great Basin, Symposium Proceedings. Geological Society of Nevada. 1986. The Lower Permian Dry Mountain trough, , AND -. eastern Nevada: possible flexural response to a reactivated Antler Orogenic Belt. Geological Society of America Abstracts with Programs, 18:414-415. 1994. Tectonic sequence stratigraphy of the Late , AND -. Pennsylvanian Strathearn basin and Lower Permian Dry Mountain trough, eastern Nevada. Geological Society of America abstracts with Programs, 26:94. SPINOSA, C., W. W. NASSICHUK, W. S. SNYDER,AND

D. M. GALLEGOS. 1991. Paleoecologicimplications of high latitude and middle latitude

affinities of the ammonoid Lrraloceras, p. 839-846. In J. D. Cooper

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and C. H. Stevens (eds.), Paleozoic Paleogeography of the Western United States-11: Pacific Section SEPM, v. 67. , AND W. S. SNYDER.1993. Ammonoid and conodont biostratigraphy: implications for Late Carboniferous-Early Permian protracted tectonism at a non-collisional plate margin, eastern Nevada. Carboniferous to Jurassic Pangea Conference, Canadian Society of Petroleum Geologists, Program and Abstracts, p. 295. STEELE, G. 1959. Stratigraphic interpretation of the PennsylvanianPermian systems of the eastern Great Basin. Unpubl. Ph.D. dissertation, University of Washington, 294 p. -. 1960. Pennsylvanian-Permian stratigraphy of east-central Nevada and adjacent Utah, p. 91-1 13. In J. W. Boettcher and W. W. Sloan (eds.), Geology of East-Central Nevada. Intermountain Association of Petroleum Geologists and Eastern Nevada Geological Society, l lth Annual Field Conference Guidebook. STEVENS, C. H. 1977. Permian depositional provinces and tectonics, western United States, p. 113-135. In J. H. Stewart, C. H. Stevens, and A. E. Fritche (eds.), Paleozoic Paleogeography of the Western United States, Pacific Coast Paleogeographic Symposium 1. Los Angeles, Pacific Section, Society of Economic Paleontologists and Mineralogists. . 1979. Lower Permian of the Central Cordilleran miogeosyncline.

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family Peninitidae. Journal of Paleontology, 58:804-833. 0.G. 1937. 0 predstavitelyakh novogo roda Crimites TOUMANSKAYA, v permskikh otlozheniyakh (Representatives of the new genus Crimites in Permian sediments). Ezhegodnik Vserossiiskogo Paleontologicheskogo Obshchestva, 11 :146-147. [In Russian] . 1963. Permian ammonoids of the central Pamir Mountains and their stratigraphical importance. Izdatelstvo Akademiya Nauk SSSR, 119 p. [In Russian]

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J . Paleonl., 69(6), 1995, pp. 1079-1083 Copyright O 1995, The Paleontological Society 0022-3360/95/0069-1079$03.00

ISOTEL US (TRILOBITA) "HUNTING BURROW FROM

UPPER ORDOVICIAN STRATA, OHIO

D A N I T A S. BRANDT,' DAVID L. MEYER,2 AND PETER B. LASK2 'Department of Geological Sciences, Michigan State University, East Lansing 48824 and ZDepartmentof Geology, University of Cincinnati, Cincinnati, Ohio 45221

ABSTRA~-Recuningassociations of the trilobite ichnogenus Rusophycus with various "worm" burrows suggest an interaction between the two tracemakers, specifically, capture of the worm by the trilobite. An exceptional ichnofossil from the Upper Ordovician of southwestern Ohio shows characters consistent with previously described "trilobite hunting burrows" from Cambrian and Silurian strata. Preserved in convex hyporelief is R. carleyi, attributable to the trilobite Isotelus, on which is superimposed the case of a worm burrow of the ichnogenus Palaeophycus. The cast of the worm burrow appears to have been truncated by the digging activities of the trilobite, suggesting its predation of the worm. The exquisite preservation of ventral axial morphology of the trilobite distinguishes this Rusophycus from simpler bilobate forms attributable to filter-feeding behavior. Congruency in the preservation of worm and trilobite trace supports the conclusion that both were created at the same time, as the trilobite exited the intrastratal burrow. This is the first report of a trilobite hunting burrow from the Ordovician, and the first evidence for predatory behavior for the trilobite genus Isotelus.

INTRODUCTION CURRING ASSOCIATIONS of the trilobite ichnogenera

Ru-

sophycus a n d Cruziana with various "worm burrows" suggest a n interaction between the two tracemakers, specifically, capture of the "worm" by the trilobite. Putative trilobite hunting burrows are reported from the Lower Cambrian of Sweden (Bergstrom, 1973; Jensen, 1990), Middle Cambrian of Sweden

(Martinsson, 1965), and Silurian of New York (Osgood and Drennen, 1975, after Hall, 1852). A recently discovered ichnofossil from the Upper Ordovician of southwestern Ohio shows characters consistent with previously described trilobite hunting burrows. If this specimen truly represents a predator-prey relationship, it is the first such trilobite hunting burrow reported from Ordovician strata, and the first evidence for predatory

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FIGUREI -Rusophycus carleyi (J. F. James); cast of trilobite trace intersecting Palaeophycus-liketrace.Anterior direction is toward the left; casts of genal spines, pygidial and cephalic doublures, and thoracic pleurae are visible along the perimeter of the trace; the medial opening reveals eight pairs of coxae casts; UCGM 464 1 1, x 0.9.

behavior in the trilobite genus Zsotelus. Despite exquisitely preserved anatomical detail of the trilobite, the exact relationship between the two traces is ambiguous, and two alternative explanations, coincidenceand exploitation, are explored to explain the relatively rare but recurrent association of trilobite and worm traces in the Cincinnatian. Evidencefor predation. -Martinsson (1965), Bergstrijm (1973), and Jensen (1990) described associated trilobite ichnofossils (Cruziana dispar) and other burrows from Cambrian strata of Sweden and Poland. In most of these specimens, the worm burrow cast is superposed on the trilobite cast, and extends twothirds or more of the length of the trilobite cast (see Jensen, 1990, figures 2-5) before "abruptly ending" (Bergstrom, 1973, p. 54). Bergstrom (1973) and Jensen (1990) interpreted the superimposition of traces as the result of the trilobite locating the worm from above and behind and digging to a depth "considerably above the lowest part of the [worm] burrow" (Jensen, 1990, p. 39). According to Jensen (1990), this configuration would be expected if the trilobite were a predator, as it would "dig no deeper than down to the prey" (Jensen, 1990, p. 39). In a few specimens (Bergstrom, 1973, plate 5, figure 10; Jensen, 1990, figure 2B) trilobite scratch marks are superimposed on the worm burrow cast, recording the "irregular manipulation of the arthropod legs" around the prey (Bergstrijm, 1973, p. 54). In several specimens the digging trilobite changed direction, consistent, in Jensen's (1990) view, with the behavior of a pred-

ator actively pursuing prey rather than microphagous feeding by the trilobite. Bergstrom (1973) postulated that the best criterion for recognizing a predator-prey relationship is the common position of the Cruziana burrow exactly over the other burrow, but Jensen (1990) countered that termination of a worm burrow beneath a Rusophycus is insufficient evidence of capture. Alternatively, Jensen (1990, p. 40) noted as "perhaps the strongest evidence" of predation, a positive size correlation between the trilobite and worm traces (R = 0.80, see Jensen, 1990, figure 7). Jensen concluded that the trilobites were size selective in their choice of prey, and that these data mitigate against a mere chance encounter. To summarize, the recumng association of superimposed trilobite and worm traces is suggestive, but not in itself adequate diagnosis of, a predator-prey relationship. The most compelling specimens are those that show unambiguous evidence of interaction between the two traces. The Cincinnatian ichnofossil is from Corryville (Maysvillian; =Grant Lake Formation) strata and was collected by Meyer from talus along Stonelick Creek, Clermont County, Ohio. The specimen is housed in the University of Cincinnati Geology Museum (UCGM). UCGM 46411. -The trilobite trace is preserved as convex

BRANDT E T AL. - ORDO VICIAN TRILOBITE BEHA VIOR

FIGURE 2-Outline sketch of Figure 1 with major features labeled; outline is dashed where inferred: C k p h a l i c doublure, S-genal spines, Tthoracic pleurae, Pwpygidial doublure, C-coxae casts, P-Paleophycus cast, G-telopodite (?) scratch mark, H-approximate outlineof hypostome.

hyporelief in a three-cm-thick calcisiltite (Figure 1). The trace is readily identified as Rusophycus carleyi (J. F. James), one of three species of Rusophycus known from the Cincinnati area. Osgood (1970) attributed R. carleyi to the trilobite Isotelus. Distinguishing features of this ichnospecies include its size (it is the largest of the Cincinnatian Rusophycus), elliptical outline, and a median furrow enclosing paired, transverse ridges. This specimen is remarkable for its preservation of anatomical detail of the trilobite. It clearly shows casts of the genal spines, cephalic and pygidial doublures, proximal portion (coxae) of eight pairs of legs, and ten thoracic pleurae (Figures 1, 2). The pleural casts are best shown on the right side of the Rusophycus (the trilobite's left side). The specimen is 17.5 cm long. The left (relative to the trilobite) pygidial doublure is truncated by the edge of the slab. The trace is 11 cm at the widest transverse axis. The medial opening is 5 cm wide and reveals eight paired transverse ridges. The casts of the genal spines are 9.5 cm long from anterior border of the cephalic doublure to distal tip of the spine. This Rusophycus carleyi is distinguished from previously described specimens (Osgood, 1970) by the presence of a separate, elongate trace that bisects the anterior doublure of the Rusophycus and disappears just left (relative to the trilobite) of the midline. This second trace is 7 cm long; it is terminated distally by the broken edge of the slab. It is 6 m m wide, and is structureless except for weakly developed transverse annulations. The proximal end of the trace is obscured by casts made by digging action of the trilobite's legs. The trace terminates proximally at a point 5.5 cm from the cephalic margin and just to

the left (relative to the trilobite) of the sagittal line. This second trace is attributable to the ichnogenus Palaeophycus Hall (see Osgood, 1970, for description of Cincinnatian specimens of this ichnogenus). DISCUSSION

Significance of the ichnofossil. -This specimen may provide the first evidence of a possible predatory habit in Isotelus. In the absence of evidence to the contrary, trilobites are generally thought to have been detritus feeders (Bergstrom, 1973; Seilacher, 1985); most trilobites apparently lacked specialized appendages possessed by modem predatory arthropods. However, trilobite appendages are only exceptionally preserved as fossils, and the true nature of most trilobite appendages is not known. Several authors (Stiirmer and Bergstrom, 1973; Whittington, 1975, 1980, 1992) suggested that trilobites with spinose telopodites could grasp and capture prey, "chewing" with the spinose inner parts of the coxae, and passing the food forward to the backward-directed mouth. The association of burrows of other organisms with Rusophycus is relatively uncommon in the Cincinnatian. Of 15 specimens ofR. carleyi in the UCGM, only two specimens are clearly associated with a second, discrete, superimposed cast. Osgood and Drennen (1 975) concluded that the paucity of hunting burrows in Cincinnatian and Devonian strata indicates that prey capture was not a common method of feeding for most trilobites. The paucity of hunting burrows may more likely be a result of the vagaries of preservation. Trace fossil preservation requires specific substrate conditions (e.g., Hallam, 1979, and most trace-

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FIGURE3-Close-up, oblique view of sagittal axis of trilobite trace, showing possible trilobite scratch mark (at m w ; feature " G on Figure 2). P = Paleophycus cast, C = coxae casts, S = genal spine cast, T = thoracic pleurae casts; x 1.5. making behavior leaves no fossil record. A second preservational consideration is the variation in piesewation of detail within any one ichnospecies. Most of the specimens ofR. carleyi in the UCGM are preserved as elliptical bilobed casts, separated by a medial gap. Coxal, thoracic, and genal casts are seldom preserved, and any one specimen may show all, a few, or none of these details. These differences are probably due to a combination of substrate composition and cohesiveness and the depth to which the trilobite excavated the burrow (e.g., Seilacher, 1985, p. 233, figure 3). THE CASE FOR PREDATION

Comparison to other "hunting burrows. "-That the Cincinnatian specimen of R. carleyi represents true biotic interaction rather than chance superimpositionof temporally unrelated traces is supported by its similarity to previously described specimens. Many of the previously described trilobite hunting burrows show alignment of the trilobite and prey traces; i.e., the long axes of the prey trace and trilobite trace are parallel. Bergstrom (1973) and Jensen (1990) interpreted this alignment as evidence for active pursuit of the prey by the trilobite. UCGM 4641 1 shows this alignment. Jensen (1990) noted that the trilobites consistently positioned themselves so that only legs of one side were in contact with the worm burrow. He interpreted this as evidence for the mode of capture by the trilobite: that the trilobite captured the worm by flexing the legs of one side around it. The Cincinnatian specimen shows this same termination of the worm trace at one side (trilobite's left). One notable difference between UCGM 464 11 and previously described trilobite hunting burrows is the position of the "prey" trace relative to the trilobite trace. In the swimens described by Osgood and Drennen (1975), Bergstrbmm(1973),and Jensen (1990), the prey trace overlaps the posterior half of the trilobite trace, suggesting that the trilobite overtook the prey from behind, got slightly ahead of the prey, then dug down to intercept the burrow. These authors presume that trilobite and prey were

headed in the same direction. In UCGM 464 11, the prey trace overlaps the anterior portion of the trilobite trace. It is not clear which way the prey was headed when the trilobite encountered it, although faint meniscate annulations along the Paleophycus trace suggest by analogy with modem annelid traces that the prey was headed toward the trilobite. Origin ofRusophycus.-Recent workers (e.g., Seilacher, 1985; Goldring, 1985) agreed that Rusophycus and the related Cruziana were intrastratal in origin, formed by the burrowing of the trilobite at a buried sediment interface (usually mud-silt or mud-sand). This mode of origin is consistent with the predominant mode of preservation of Rusophycus as convex hyporelief. Birkenmajer and Bruton (197 1) and Seilacher (1985) attributed the characteristic bilobate form of most Rusophycus to filterfeeding behavior. According to this model, the trilobite created a ventral feeding chamber, scraping sediment towards the median line where it was washed out by a respiratory current (Seilacher, 1985). This scenario accounts for the characteristic bilobed morphology of Rusophycus and the fact that ventral axial morphology of the trilobite is rarely preserved in these traces; the trilobite "floats" above the feeding chamber and only its telopodites scrape the substrate. Rusophycus typically is ornamented with transverse scratch marks along its length, an observation compatible with the feeding chamber scenario. Morphological details of the trilobite's margin (e.g., genal spines, pygidial or doublure imprints) are uncommon in Rusophycus ichnospecies (see examples in Osgood, 1970), and details of the ventral axial region are even less commonly preserved. Seilacher (1985) noted that the semi-infaunal filter-feeding model does not apply to all trilobites, and thus this origin does not apply to all Rusophycus. This model satisfactoriallyexplains the origin of the transversely striated, bilobed forms but does not account for the nearly perfect body cast of the specimen described here. UCGM 4641 1 was probably not created by sediment processing behavior, which would have obscured morphologic details of the trilobite's axial region, but through some activity that brought the trilobite briefly to the mud-silt interface. Topology of the traces.-The relative position of the two traces and their cross-cutting relationships offer further evidence of potential interaction of the two organisms. A predator-prey relationship is circumstantially supported by the termination of the prey trace in the casts made by digging action of the trilobite's appendages; the prey trace does not emerge from the trilobite trace. Further, the worm trace terminates in a position posterior to the hypostome, in front of the trilobite's backward-directed mouth. The prey trace disappears on the right side of the Rusophycus (left side of the trilobite as it excavated the impression). The thoracic pleurae of the triliobite's left side are more clearly impressed than those on the right, indicating that the trilobite was listing to port (tilted downward slightly to its left). This coincidence suggests interaction between predator and prey. The worm burrow is not cross cut by the imprint of the trilobite's doublure because the anterior of the cephalon was tilted up. This position is confirmed by the deeply impressed trace of the genal spine. Such a posture is consistent with trilobite feeding behavior (Seilacher, 1985). Casts of trilobite hunting burrows figured by Jensen (1990) show trilobite scratch marks superimposed on the worm cast. The Cincinnatian specimen does not show as clear an interaction between the trilobite and the prey. A latex mold made to clarify the primary relationship of the two traces appears to show that the worm trace cuts across the Rusophycus unblemished by any aspect of the trilobite trace except for one area where a small (about 2 mm wide) groove cast cuts diagonally across the worm cast about 4 cm from the trilobite's anterior border (Figure 3). The diameter and position of the groove cast is consistent with

BRANDT E T AL. -ORDOVICIAN TRILOBITE BEHA VIOR the possibility that it could be a scratch mark created by one of the trilobite's telopodites. To summarize, the genesis envisioned for the UCGM specimen is one of a trace produced at an interface between organicrich mud and less-rich overlying silt. The worm may have been feeding at this interface, was sensed by the Isotelus from above, and was seized by the trilobite, which burrowed and intercepted the worm at this interface. The evidence for a predator-prey relationship in the Cincinnatian specimen is suggestive but not entirely unambiguous; more distinct cross-cutting relationships between trilobite appendages and prey would make the case more compelling. Therefore, other possible scenarios to explain the recurrent relationship between these traces are considered below. ALTERNATIVES TO THE PREDATION SCENARIO

Coincidence.-The base of the slab containing UCGM 464 11 is irregular, pocked with casts of small, sharply incised, parallel aligned current structures and larger, irregular, nonaligned casts that are probably biotic in origin. The presence of numerous traces on the base of the bed that do not intersect the Rusophycus suggests a high degree of biological activity at this sediment interface. If the trilobite and other trace-makers were mining the same organic-rich horizon for food, superimposition of traces could be coincidental. However, the exquisite preservation of delicate ventral structures of the trilobite indicates that the burrow was excavated in soft sediment, overlain by mobile sediment that filled in the trace immediately upon the trilobite's evacuation of the burrow. There is no obvious textural difference between the filling of the trilobite trace and the worm burrow. The congruent preservation points to both burrows being filled at one time. Therefore, it is not likely that the superposition was the result of chance. Exploitation. -Detritus-feeding worms may have been attracted to, and exploited, either 1) the efforts of an actively feeding trilobite, which may have created feeding currents and stirred up organic-rich detritus (e.g., Bergstrom, 1969; Miller, 1975; Seilacher, 1985), or 2) the residue left by the trilobite, especially if it left edible material exposed after feeding or left its own organic-rich wastes behind. The first scenario is rebutted by the conclusion above that this Rusophycus was not formed as the result of sediment processing. The congruent preservation of the two traces, and thus, presumed contemporaneity of the traces, argues against the second scenario. CONCLUSIONS

UCGM 4641 1 is significant as an exceptional example of Rusophycus carleyi showing unusually complete detail of Isotelus ventral morphology. It shares characteristics with previously described trilobite hunting burrows. If it truly represents a predator-prey relationship, it is the first evidence for a predatory habit in Isotelus, and it is also the first report of trilobite hunting burrows from Ordovician strata. The predation scenario is supported by several different lines of evidence: the relative position and relationships between the two traces (including the termination of the worm trace at the position of the hypostome, superposed trilobite scratch mark, etc.), the presumed behavior

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of the trilobite that produced the trace (non-filter-feeding), and the probable contemporaneity of the two traces. The evidence for predation, while compelling, may not be conclusive. However, possible alternative explanations (chance and exploitation) are also not entirely satisfactory. Coincidence (chance) as an explanation for this association may be refuted as additional specimens come to light; repetition in pattern is a compelling argument for non-randomness. If the trace association is the product of non-randomness (predation, exploitation or some other unexamined alternative) this specimen offers new information on either the behavior of Isotelus or the ecology of its associated detritus-feeding, soft-bodied fauna. ACKNOWLEDGMENTS

We thank J. Bergstrom, A. Seilacher, W. Noms, and M. A. Velbel for helpful discussions and review. The original version of this manuscript was prepared during the senior author's tenure as National Science Foundation Visiting Professor at Cincinnati. She gratefully acknowledges the financial support of NSFVPW #R11-9002885. REFERENCES

BERGSTROM, J. 1969. Remarks on the appendages of trilobites. Lethaia, 2:395414. -. 1973. Organization, life and systematics of trilobites. Fossils and Strata, 2: 1-69. BIRKENMAJER, K., AND D. BRUTON.1971. Some trilobite resting and crawling traces. Lethaia, 4:303-319. R. 1985. The formation of the trace fossil Cruziana. GeoGOLDRING, logical Magazine, 122:65-72. HALL,J. 1852. Palaeontology. Natural History of New York, Volume 2. C. Van Benthuysen, Albany, 362 p. HALLAM, A. 1975. Preservation of trace fossils, p. 55-63. In R. W. Frey (ed.), The Study of Trace Fossils. Springer-Verlag, New York. JENSEN, S. 1990. Predation by early Cambrian trilobites on infaunal worms-evidence from the Swedish Mickwitzia Sandstone. Lethaia, 23:2942. MARTMSSON, A. 1965. Aspects of a middle Cambrian thanatotope on Oland. Geologiska Foreningens i Stockholm Forhandlingar, 87: 181230. J. 1975. Structure and function of trilobite terrace lines. FosMILLER, sils and Strata, 4: 155438. OSGOOD, R. G. 1970. Trace fossils of the Cincinnati area. Palaeontographica Americana, 6:28 1444. -, AND W. T. DRENNEN. 1975. Trilobite trace fossils from the Clinton Group (Silurian)of east-central New York State. Bulletins of American Paleontology, 67:30C-348. SEILACHER, A. 1985. Trilobite paleobiology and substrate relationships. Transactions of the Royal Society of Edinburgh, 76:23 1-237. W., AND J. BERGSTROM. ST~~RMER, 1973. New discoveries on trilobites by X-ray. Palaontologische Zeitschrift, 47: 104-1 4 1. H. B. 1975. Trilobites with appendages from the MidWHIITINGTON, dle Cambrian Burgess Shale, British Columbia. Fossils and Strata, 4:97-136. -. 1980. Exoskeleton, moult stage, appendage morphology and habits of the Middle Cambrian trilobite Olenoides serratus. Palaeontology, 23: 171-204. -. 1992. Trilobites. Fossils Illustrated, Volume 2. Boydell Press, Woodbridge, Suffolk, 280 p.