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Illaenidae (Trilobita): Morphology of thorax, classification, and mode of life

Published online by Cambridge University Press:  20 May 2016

Harry B. Whittington
Sedgwick Museum, Department of Earth Sciences, University of Cambridge, Cambridge CB2 3EQ, U.K.


The axial rings of the thorax of seven Ordovician species of Illaenidae, and of the type species of the Silurian Cybantyx, all have the ring curved downward and forward, lacking the articulating furrow but traversed by a stout ventral ridge; the doublure similarly curves downward and forward. This type of axial ring is apparently unique among post-Cambrian trilobites. The axial ring in Scutelluidae is like that occurring in most post-Cambrian trilobites, having a convex ring, deep articulating furrow, and horizontal half ring and doublure. These and many other characters distinguish the Styginidae-Scutelluidae group from Illaenidae. Cybantyx is not an effaced scutelluid, but an illaenid. Upper Cambrian illaenid-like trilobites, which have the illaenid type of thoracic axial ring, deserve consideration as possible ancestors of post-Cambrian illaenids. The thorax is given considerable flexibility by the illaenid axial ring, enabling the body to have been flexed unusually strongly concavely dorsally, as well as to enroll. The musculature suggested omits dorsal longitudinal muscles. It is argued that many illaenids may have been vagrant benthos, particularly adapted for negotiating over irregular surfaces. The view that illaenids lived partly in a burrow is not supported by specimens preserved in such an attitude. The arrangement and varying strength of the terrace ridges on the exoskeleton of Cybantyx is consistent with their having had a sensory function; anastomosing caeca on the fixed cheek and the pleural region of the pygidium are described.

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Adrain, J. M., Chatterton, B. D. E., and Blodgett, R. B. 1995. Silurian trilobites from southwestern Alaska. Journal of Paleontology, 69:723736.CrossRefGoogle Scholar
Archinal, A. 1994. Zur Morphologie, Systematik, Stratigraphie und Ökologie der Gattung Scutellum (Trilobita: Devon). Senckenbergiana lethea, 74:291324.Google Scholar
Bergström, J. 1973. Organization, life and systematics of trilobites. Fossils and Strata, 2:169.Google Scholar
Billings, E. 1859. Descriptions of some new species of trilobites from the Lower and Middle Silurian rocks of Canada. Canadian Naturalist and Geologist, 4:367383.Google Scholar
Billings, E. 1861–65. Geological Survey of Canada. Palaeozoic Fossils, 1:1426.Google Scholar
Bohlin, B. 1949. The Asaphus Limestone in northernmost Öland. Bulletin of the Geological Institution of the University of Uppsala, 33:529570.Google Scholar
Bruton, D. L. 1968. The trilobite genus Panderia from the Ordovician of Scandinavia and the Baltic areas. Norsk Geologisk Tidsskrift, 48:153.Google Scholar
Bruton, D. L., and Owen, A. W. 1988. The Norwegian Upper Ordovician illaenid trilobites. Norsk Geologisk Tidsskrift, 68:241258.Google Scholar
Chatterton, B. D. E. 1980. Ontogenetic studies of Middle Ordovican trilobites from the Esbataottine Formation, Mackenzie Mountains, Canada. Palaeontographica Abteilung A, 171:174.Google Scholar
Chatterton, B. D. E., and Campbell, K. S. W. 1980. Silurian trilobites from near Canberra and some related forms from the Yass basin. Palaeontographica Abteilung A, 167:77119.Google Scholar
Chatterton, B. D. E., and Campbell, M. 1993. Enrolling in trilobites: a review and some new characters. Memoirs of the Association of Australasian Palaeontologists, 15:103123.Google Scholar
Chatterton, B. D. E., and Ludvigsen, R. 1976. Silicified Middle Ordovician trilobites from the South Nahanni River area, District of Mackenzie, Canada. Palaeontographica Abteilung A, 154:1106.Google Scholar
Dalman, J. W. 1827. Om palaeaderna eller de så kallade Trilobiterna. Kongliga Svenska Vetenskaps-Akademiens Handlingar (1826), p. 113162, 226-294.Google Scholar
DeMott, L. L. 1987. Platteville and Decorah trilobites from Illinois and Wisconsin, p. 6398. In Sloan, R. E. (ed.), Middle and Late Ordovician Lithostratigraphy and Biostratigraphy of the Upper Mississippi Valley, Minnesota Geological Survey, Report of Investigations 35.Google Scholar
Fortey, R. A. 1980. The Ordovician trilobites of Spitsbergen. III. Remaining trilobites of the Valhallfonna Formation. Norsk Polarinstitutt, Skrifter number 171:1163.Google Scholar
Fortey, R. A. 1986. The type species of the Ordovician trilobite Symphysurus: systematics, functional morphology and terrace ridges. Paläontologische Zeitschrift 60:255275.CrossRefGoogle Scholar
Fortey, R. A. 1990. Ontogeny, hypostome attachment and trilobite classification. Palaeontology, 33:529576.Google Scholar
Fortey, R. A., and Chatterton, B. D. E. 1988. Classification of the Trilobite Suborder Asaphina. Palaeontology, 31:165222.Google Scholar
Fortey, R. A., and Owens, R. M. 1987. The Arenig Series in South Wales. Bulletin of the British Museum (Natural History), Geology, 41:69307.Google Scholar
Fortey, R. A., and Owens, R. M. 1990. Trilobites, p. 121142. In McNamara, K. J. (ed.), Evolutionary Trends. Belhaven, London.Google Scholar
Hammann, W. 1992. The Ordovician trilobites from the Iberian Chains in the Province of Aragón, N. E. Spain. I. The trilobites of the Cystoid Limestone (Ashgill Series). Beringeria, 6:1219.Google Scholar
Harper, D. A. T. 1982. The stratigraphy of the Drummuck Group (Ashgill), Girvan. Geological Journal, 17:251277.CrossRefGoogle Scholar
Hawle, I., and Corda, A. J. C. 1847. Prodrom einer Monographie der böhmischen Trilobiten. Abhandlungen Koeniglichen Boehmischen Gesellschaft der Wissenschaften, Prague, 176 p.Google Scholar
Holm, G. 1882. De Svenska arterna af trilobitsläget Illaneus (Dalman). Kongliga Svenska Vetenskaps-Akademiens Handlingar, 7:1148.Google Scholar
Holm, G. 1886. Die ostbaltischen Illaeniden. In F. Schmidt, Revision der ostbaltischen Silurischen trilobiten, 3. Mémoires de L'Académie Impériale des Sciences de St.-Pétersbourg, Série 7, 33:173 p.Google Scholar
Hupé, P. 1955. A propos des organes de Pander des trilobites. Bulletin de la Société géologique de France, 6th Série, 4:701711.Google Scholar
Jaanusson, V. 1954. Zur morphologie und taxonomie der Illaeniden. Arkiv für Mineralogi och Geologi, 1:545583.Google Scholar
Jaanusson, V. 1957. Unterordovizische Illaeniden aus Skandinavien. Bulletin of the Geological Institution of the University of Uppsala, 37:79165.Google Scholar
James, N. P., and Stevens, R. K. 1986. Stratigraphy and correlation of the Cambro-Ordovician Cow Head Group, western Newfoundland. Geological Survey of Canada, Bulletin 366:1143.Google Scholar
Kjerulf, R. 1865. Veiviser ved geologisker Excursioner i Christiania Omegn. Universitets-program seminar 2, iv + 43 p.Google Scholar
Kobayashi, T. 1935. The Cambro-Ordovician Formations and Faunas of South Chosen. Paleontology. Part III, Cambrian faunas of South Chosen. Journal of the Faculty of Science, Imperial University of Tokyo, Section II, Geology, Mineralogy, Geography, Seismology, 4:49344.Google Scholar
Lane, P. D. 1972. New trilobites from the Silurian of north-east Greenland, with a note on trilobite faunas in pure limestones. Palaeontology, 15:336364.Google Scholar
Lane, P. D., and Owens, R. M. 1982. Silurian trilobites from Kap Schuchert, Washington Land, western North Greenland. Rapport Grønlands Geologiske Undersøgelse, 108:4169.Google Scholar
Lane, P. D., and Thomas, A. T. 1983. A review of the trilobite Suborder Scutelluina, p. 141160. In Briggs, D. E. G. and Lane, P. D. (eds.), Trilobites and other early arthropods: papers in honour of Professor H. B. Whittington, F. R. S., Special Papers in Palaeontology, 30.Google Scholar
Yanhao, Lu and Zhou, T. R. 1990. Trilobites across the Cambrian-Ordovician Boundary of the transitional region of Sandu, southeastern Guizhou. Palaeontologia Cathayana, 5:184.Google Scholar
Ludvigsen, R., and Chatterton, B. D. E. 1980. The ontogeny of Failleana and the origin of the Bumastinae (Trilobita). Geological Magazine 117:471478.CrossRefGoogle Scholar
Ludvigsen, R., and Tripp, R. P. 1990. Silurian trilobites from the Northern Yukon Territory. Life Sciences Contributions, Royal Ontario Museum, 153, 58 p.Google Scholar
Mikulic, D. G. 1981. Trilobites in Paleozoic carbonate buildups. Lethaia 14:4556.CrossRefGoogle Scholar
Miller, J. M. 1975. Structure and function of trilobite terrace lines. Fossils and Strata, 4:155178.Google Scholar
Morris, S. F., and Tripp, R. P. 1986. Lectotype selections for Ordovician trilobites from the Girvan District, Strathclyde. Bulletin of the British Museum of Natural History (Geology), 40:161176.Google Scholar
Moore, R. C. 1959. Treatise on Invertebrate Paleontology, Part O, Arthropoda 1. Geological Society of America and University of Kansas Press. Lawrence, 560 p.Google Scholar
Owen, A. W. 1981. The Ashgill trilobites of the Oslo region, Norway. Palaeontographica Abteilung A, 175:188.Google Scholar
Owen, A. W., and Bruton, D. L. 1980. Late Caradoc-early Ashgill trilobites of the central Oslo region. Paleontological Contributions from the University of Oslo, no. 245, 62 p.Google Scholar
Rabano, A. W. 1989. Trilobites del Ordovicico Medio del sector meridional de la zona Centroibérica española; parte IV: Phacopina, Scutelluina, Odontopleurida y Lichida. Boletin Geologico y Minero, 100:9711032.Google Scholar
Rabano, A. W., and Gutiérrez-Marco, J. C. 1983. Revisión del género Ectillaenus Salter, 1867 (Trilobita, Illaenina) en el Ordovicico de la peninsula Ibérica. Boletin Real Sociedad Española de Historia Natural (Geológica), 81:225246.Google Scholar
Raymond, P. E. 1905. The trilobites of the Chazy Limestone. Annals of the Carnegie Museum, 3:328386.Google Scholar
Raymond, P. E. 1925. Some trilobites of the lower Middle Ordovician of eastern North America. Bulletin of the Museum of Comparative Zoology at Harvard College, 67:1180.Google Scholar
Richter, R. 1920. Beitrâge zur Kenntnis devonischer Trilobiten. Über die Organisation von Harpes. Abhandlungen der Senckenbergischen Naturforschenden Gesellschaft, 37:177218.Google Scholar
Rosova, A. V. 1964. Biostratigraphy and descriptions of trilobites of Middle and Upper Cambrian of the northwestern Siberian platform. Akademija Nauk SSSR, Sibiskoe Otdednie, Institut Geologii i Geofiziki, Moscow, 148 pp. (In Russian)Google Scholar
Rosova, A. V. 1977. Some Upper Cambrian and Lower Ordovician trilobites from the Rybnaya, Khantayka, Kurryka and Letnyaya River basins, pp. 5484, In Zhuravleva, I. T. and Rozova, A. V. (eds.), Biostratigraphy and fauna of the Upper Cambrian and its boundary beds, Novosibirsk, Trudy Instituta Geologii I Geofiziki, 313, 355 pp. (In Russian)Google Scholar
Ross, R. J., and James, N. P. 1987. Brachiopod stratigraphy of the Middle Ordovician Cow Head and Table Head groups, western Newfoundland. Canadian Journal of Earth Sciences, 24:7095.CrossRefGoogle Scholar
Rudolph, F. 1992. Kopfmuskulatur bei Trilobiten-Rekonstruktion, Funktionsmorphologie und phylogenetisch-systematische Schlussfologerungen. Diplom-Arbeit aus dem Zoologischen Institut der Christian Albrechts Universität Kiel, 115 p.Google Scholar
Salter, J. W. 1867. A monograph of British trilobites, part IV, Silurian. Palaeontographical Society, London, p. 177216.Google Scholar
Shaw, F. C. 1968. Early Middle Ordovician Chazy trilobites of New York. New York State Museum and Science Service, Memoir 17:vii + 162 p.Google Scholar
Shaw, F. C. 1974. Simpson Group (Middle Ordovician) trilobites of Oklahoma. The Paleontological Society, Memoir 6 (Journal of Paleontology, 48[5] supplement), 54 p.Google Scholar
Shergold, J. H., and Sdzuy, K. 1984. Cambrian and early Tremadocian trilobites from Sultan Dag, Central Turkey. Senckenbergiana lethaea, 65:51135.Google Scholar
Siegried, P. 1939. Von der lebenweise der Trilobiten der Gattung Illaenus Dalm. Beitrage der Kunde Estlands, Naturwissenschaftliche Reihe, 1:4348.Google Scholar
Šnajdr, M. 1957. Classification of the family Illaenidae (Hawle et Corda) in the Lower Paleozoic of Bohemia. Sborník Ústředního Ústavu Geologického, oddíl paleontologický, 23:125284. (In Czech, English summary)Google Scholar
Šnajdr, M. 1960. Studie o čeledi Scutelluidae (Trilobitae). Rozpravy, Ústředního Ústavu Geologického, 26:1263. (In Czech, English summary)Google Scholar
Šnajdr, M. 1990. Bohemian Trilobites. Geological Survey, Prague, 265 pp.Google Scholar
Thomas, A. T. 1978. British Wenlock trilobites, Part 1. Palaeontographical Society Monographs, 132:156.Google Scholar
Thomas, A. T. 1979. Trilobite associations in the British Wenlock, p. 447451. In Harris, A. L., Holland, C. H., and Leake, B. E. (eds.), The Caledonides of the British Isles-reviewed. Geological Society of London, Scottish Academic Press.Google Scholar
Westrop, S. R. 1983. The life habits of the Ordovician illaenine trilobite Bumastoides. Lethaia, 16:1524.CrossRefGoogle Scholar
Westrop, S. R., and Ludvigsen, R. 1983. Systematics and paleoecology of Upper Ordovician trilobites from the Selkirk Member of the Red River Formation, southern Manitoba. Manitoba Department of Energy and Mines, Mineral Resources Division, Geological Report GR 82-2:151.Google Scholar
Whittard, W. F. 1961. The Ordovician trilobites of the Shelve Inlier, Part 6. Palaeontographical Society Monograph, 115:197228.Google Scholar
Whittington, H. B. 1950. Sixteen Ordovician genotype trilobites. Journal of Paleontology, 24:531565.Google Scholar
Whittington, H. B. 1954. Ordovician trilobites from Silliman's Fossil Mount, p. 119149. In Miller, A. K., Youngquist, W., and Collinson, C., Ordovician Cephalopod Fauna of Baffin Island, Geological Society of America, Memoir 62.Google Scholar
Whittington, H. B. 1963. Middle Ordovician trilobites from Lower Head, western Newfoundland. Bulletin of the Museum of Comparative Zoology at Harvard College, 129:1118.Google Scholar
Whittington, H. B. 1965. Trilobites of the Ordovician Table Head Formation, western Newfoundland. Bulletin of the Museum of Comparative Zoology, Harvard University, 132:277441.Google Scholar
Whittington, H. B. 1966. Phylogeny and distribution of Ordovician trilobites. Journal of Paleontology, 40:696737.Google Scholar
Whittington, H. B. 1980. Exoskeleton, moult stage, appendage morphology, and habits of the Middle Cambrian trilobite Olenoides serratus. Palaeontology, 23:171204.Google Scholar
Whittington, H. B. 1988. Hypostomes of post-Cambrian trilobites. New Mexico Bureau of Mines and Mineral Resources, Memoir 44:321339.Google Scholar
Whittington, H. B. 1990. Articulation and exuviation in Cambrian trilobites. Philosophical Transactions of the Royal Society, London, B329:2746.CrossRefGoogle Scholar
Whittington, H. B. 1993a. Anatomy of the Ordovician trilobite Placoparia. Philosophical Transactions of the Royal Society, London, B339:109118.CrossRefGoogle Scholar
Whittington, H. B. 1993b. Morphology, anatomy and habits of the Silurian homalonotid trilobite Trimerus. Memoirs of the Association of Australasian Palaeontologists, 15:6983.Google Scholar
Whittington, H. B. 1995. Oryctocephalid trilobites from the Cambrian of North America. Palaeontology, 38:543562.Google Scholar
Whittington, H. B., and Almond, J. E. 1987. Appendages and habits of the Upper Ordovician trilobite Triarthrus eatoni. Philosophical Transactions of the Royal Society, London, B317:146.CrossRefGoogle Scholar
Whittington, H. B., and Evitt, W. R. 1954. Silicified Middle Ordovician trilobites. Geological Society of America, Memoir 59, 137 p.Google Scholar
Zhou, Zhi-yi, and Fortey, R. A. 1986. Ordovician trilobites from north and northeast China. Palaeontographica Abteilung A, 192:157210.Google Scholar
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