Hostname: page-component-8448b6f56d-mp689 Total loading time: 0 Render date: 2024-04-24T19:46:43.671Z Has data issue: false hasContentIssue false
  • English
  • Français

The ontogeny of trilobite segmentation: a comparative approach

Published online by Cambridge University Press:  08 April 2016

Nigel C. Hughes ,
Alessandro Minelli  and
Giuseppe Fusco
Nigel C. Hughes
Affiliation:
Department of Earth Sciences, University of California, Riverside, California 92521. E-mail: nigel.hughes@ucr.edu
Alessandro Minelli
Affiliation:
Department of Biology, University of Padua, via U. Bassi 58/B, I-35131, Padua, Italy
Giuseppe Fusco
Affiliation:
Department of Biology, University of Padua, via U. Bassi 58/B, I-35131, Padua, Italy
Get access Rights & Permissions [Opens in a new window]

Abstract

Ontogenetic stages of trilobites have traditionally been recognized on the basis of the development of exoskeletal segmentation. The established protaspid, meraspid, and holaspid phases relate specifically to the development of articulated joints between exoskeletal elements. Transitions between these phases were marked by the first and last appearances of new trunk segment articulations. Here we propose an additional and complementary ontogenetic scheme based on the generation of new trunk segments. It includes an anamorphic phase during which new trunk segments appeared, and an epimorphic phase during which the number of segments in the trunk remained constant. In some trilobites an ontogenetic boundary can also be recognized at the first appearance of morphologically distinct posterior trunk segments. Comparison of the phase boundaries of these different aspects of segment ontogeny highlights rich variation in the segmentation process among Trilobita. Cases in which the onset of the holaspid phase preceded onset of the epimorphic phase are here termed protarthrous, synchronous onset of both phases is termed synarthromeric, and onset of the epimorphic phase before onset of the holaspid phase is termed protomeric. Although these conditions varied among close relatives and perhaps even intraspecifically in some cases, particular conditions may have been prevalent within some clades.

Trilobites displayed hemianamorphic development that was accomplished over an extended series of juvenile and mature free-living instars. Although developmental schedules varied markedly among species, morphological transitions during trilobite development were generally regular, limited in scope, and extended over a large number of instars when compared with those of many living arthropods. Hemianamorphic, direct development with modest change between instars is also seen among basal members of the Crustacea, basal myriapods, pycnogonids, and in some fossil chelicerates. This mode may represent the ancestral condition of euarthropod development.

Type
Articles
Information
Paleobiology , Volume 32 , Issue 4 , Fall 2006 , pp. 602 - 627
Copyright
Copyright © The Paleontological Society 

Access options

Get access to the full version of this content by using one of the access options below. (Log in options will check for institutional or personal access. Content may require purchase if you do not have access.)

References

Literature Cited

Andersson, G. 1976. Post-embryonic development of Lithobius forficatus (Linnaeus), (Chilopoda: Lithobiidae). Entomologica Scandanavica 7:161–168.CrossRefGoogle Scholar
Barrande, J. 1852. Systême Silurien du centre de la Bohême. Iére partie. Recherches paléontologiques. Chez l'auteur et éditeur. Prague and Paris.Google Scholar
Beecher, C. E. 1895. The larval stages of trilobites. American Geologist 16:166–197.Google Scholar
Bird, C., and Clarkson, E. N. K. 2003. Observations on the ontogeny of the upper Cambrian trilobite Peltura scarabaeoides westergaardi Henningsmoen, 1957. Geologiska Föreningens i Stockholm Förhandlingar 125:177–190.Google Scholar
Brandt, D. S. 1996. Epizoans on Flexicalymene (Trilobita) and implications for trilobite paleoecology. Journal of Paleontology 70:442–449.Google Scholar
Brongniart, A. 1822. Corps organisés fossiles nommés trilobites. Pp. 1–65 in Histoire naturelle des Crustacés fossiles sous les rapports zoologiques et géologiques. F.-G. Levrault, Paris.CrossRefGoogle Scholar
Brusca, R. C., and Brusca, G. J. 2003. Invertebrates. Sinauer, Sunderland, Mass.Google Scholar
Budd, G. E. 2001. Why are arthropods segmented? Evolution and Development 3:332–342.Google Scholar
Budd, G. E. 2002. A palaeontological solution to the arthropod head problem. Nature 417:271–275.CrossRefGoogle Scholar
Burmeister, H. 1846. The organization of trilobites. Ray Society, London.Google Scholar
Chatterton, B. D. E. 1971. Taxonomy and ontogeny of Siluro-Devonian trilobites from near Yass, New South Wales. Palaeontographica, Abteilung A 137:1–108.Google Scholar
Chatterton, B. D. E. 1980. Ontogenetic studies of Middle Ordovician trilobites from the Esbataottine Formation, Mackenzie Mountains, Canada. Palaeontographica, Abteilung A 171:1–74.Google Scholar
Chatterton, B. D. E. 1994. Ordovician proetide trilobite Dimeropyge, with a new species from northwestern Canada. Journal of Paleontology 68:541–556.Google Scholar
Chatterton, B. D. E., and Speyer, S. E. 1989. Larval ecology, life history strategies, and patterns of extinction and survivorship among Ordovician trilobites. Paleobiology 15:118–132.Google Scholar
Chatterton, B. D. E., and Speyer, S. E. 1990. Applications of the study of trilobite ontogeny. Pp. 116–136 in Mikulic, D. G., ed. Arthropod paleobiology. Short Courses in Paleontology 3: 116–136. Paleontological Society, Knoxville, Tenn. Google Scholar
Chatterton, B. D. E., and Speyer, S. E. 1997. Ontogeny. Pp. 173–247 in Whittington, H. B. et al. Arthropoda 1, Trilobita. Part O (revised) of Kaesler, R. A., ed. Treatise on invertebrate paleontology. Geological Society of America, Boulder, Colo., and University of Kansas, Lawrence.Google Scholar
Chatterton, B. D. E., Siveter, D. J., Edgecombe, G. E., and Hunt, A. S. 1990. Larvae and relationships of the Calymenina (Trilobita). Journal of Paleontology 64:255–277.CrossRefGoogle Scholar
Chatterton, B. D. E., Edgecombe, G. D., Speyer, S. E., Hunt, A. S., and Fortey, R. A. 1994. Ontogeny and relationships of Trinucleoidea (Trilobita). Journal of Paleontology 68:523–540.Google Scholar
Chatterton, B. D. E., Edgecombe, G. D., Waisfield, B. G., and Vacarri, N. E. 1998. Ontogeny and systematics of Toernquistiidae (Trilobita, Proetida) from the Ordovician of the Argentine Precordillera. Journal of Paleontology 72:273–303.CrossRefGoogle Scholar
Chatterton, B. D. E., Edgecombe, G. D., Vaccari, N. E., and Waisfeld, B. G. 1999. Ontogenies of some Ordovician Telephinidae from Argentina and larval patterns in the Proetida (Trilobita). Journal of Paleontology 73:219–239.CrossRefGoogle Scholar
Cisne, J. L. 1973. Life history of an Ordovician trilobite Triarthrus eatoni . Ecology 54:135–142.Google Scholar
Clarke, J. M., and Ruedemann, R. R. 1912. The Eurypterida of New York. New York State Museum, Memoir 14.Google Scholar
Clarkson, E. N. K., and Ahlberg, P. 2002. Ontogeny and structure of a new, miniaturised and spiny olenid trilobite from southern Sweden. Palaeontology 45:1–22.Google Scholar
Clarkson, E. N. K., and Zhang, X. G. 1991. Ontogeny of the Carboniferous trilobite Paladin eichwaldi shunnerensis (King 1914). Transactions of the Royal Society of Edinburgh (Earth Sciences) 82:277–295.CrossRefGoogle Scholar
Clarkson, E. N. K., Ahlgren, J., and Taylor, C. M. 2003. Structure, ontogeny, and moulting of the olenid trilobite Ctenopyge (Eoctenopyge) angusta Westergård, 1922 from the Upper Cambrian of Västergötland, Sweden. Palaeontology 46:1–27.Google Scholar
Clarkson, E. N. K., Ahlgren, J., and Taylor, C. M. 2004. Ontogeny, structure and functional morphology of some spiny Ctenopyge species (Trilobita) from the upper Cambrian of Västergötland, Sweden. Transactions of the Royal Society of Edinburgh (Earth Sciences) 94(for 2003):115–143.Google Scholar
Cotton, T. J., and Braddy, S. J. 2004. The phylogeny of arachnomorph arthropods and the origin of the Chelicerata. Transactions of the Royal Society of Edinburgh (Earth Sciences) 94(for 2003):169–193.Google Scholar
Cotton, T. J., and Fortey, R. A. 2005. Comparative morphology and relationships of the Agnostida. Pp. 95–136 in Koenemann, and Jenner, 2005.Google Scholar
Crônier, C., Renaud, S., Feist, R., and Auffray, J.-C. 1998. Ontogeny of Trimerocephalus levievrei (Trilobita, Phacopida), a representative of the Late Devonian phacopine paedomorphocline: a morphometric approach. Paleobiology 24:359–370.Google Scholar
Crônier, C., Bartzsch, K., Weyer, D., and Feist, R. 1999. Larval morphology and ontogeny of a late Devonian phacopid with reduced sight from Thuringia, Germany. Journal of Paleontology 73:240–255.Google Scholar
Dathe, H. H. 2003. Lehrbuch der Speziellen Zoologie/begründed von Alfred Kaestner. Band 1. In Gruner, H.-E., ed. Wirbellose Tiere. Bandteil 5: Insecta, 2d ed. Spektrum, Heidelberg.Google Scholar
Demange, J.-M. 1967. Recherches sur la segmentation du tronc des Chilopodes et des Diplopodes Chilognathes (Myriapodes). Mémoires du Muséum National d'Histoire Naturelle, Paris A 4:1–188.Google Scholar
Dunlop, J. A., and Arango, C. P. 2005. Pycnogonid affinities: a review. Journal of Zoological Systematics and Evolutionary Research 43:8–21.Google Scholar
Edgecombe, G. D., and Ramsköld, L. 1999. Relationships of Cambrian Arachnata and the systematic position of Trilobita. Journal of Paleontology 73:263–287.Google Scholar
Edgecombe, G. D., Chatterton, B. D. E., Vaccari, N. E., and Waisfeld, B. G. 1997. Ontogeny of the proetoid trilobite Stenoblepharum, and relationships of a new species from the Upper Ordovician of Argentina. Journal of Paleontology 71:419–433.Google Scholar
Edgecombe, G. D., Giribet, G., and Wheeler, W. C. 1999. Filogenia de Chilopoda: combinando sequencias de los genes ribosómicos 18S y 28S y morfología. Boletín de la Sociedad Entomológica Aragonesa 26:293–331.Google Scholar
Enghoff, H., Dohle, W., and Blower, J. G. 1993. Anamorphosis in millipedes (Diplopoda): the present state of knowledge and phylogenetic considerations. Zoological Journal of the Linnean Society 109:103–234.CrossRefGoogle Scholar
Evitt, W. R. 1961. Early ontogeny in the trilobite family Asaphidae. Journal of Paleontology 35:986–995.Google Scholar
Feist, R. 1970. Breviscutellum (Meridioscutellum) n. sg. (Trilobite) et son développement larvaire. Geobios 3:41–73.Google Scholar
Fortey, R. A., and Morris, S. F. 1978. Discovery of nauplius-like trilobite larvae. Palaeontology 21:823–833.Google Scholar
Fortey, R. A., and Owens, R. M. 1991. A trilobite fauna from the highest Shineton Shales in Shropshire, and the correlation of the latest Tremadoc. Geological Magazine 128:437–464.Google Scholar
Fortey, R. A., and Whittington, H. B. 1989. The Trilobita as a natural group. Historical Biology 2:125–138.CrossRefGoogle Scholar
Fusco, G. 2005. Trunk segment numbers and sequential segmentation in myriapods. Evolution and Development 7:608–617.Google Scholar
Fusco, G., Hughes, N. C., Webster, M., and Minelli, A. 2004. Exploring developmental modes in a fossil arthropod: growth and trunk segmentation of the trilobite Aulacopleura konincki . American Naturalist 163:167–183.Google Scholar
Gould, S. J. 1977. Ontogeny and phylogeny. Belknap Press of Harvard University Press, Cambridge.Google Scholar
Haase, E. 1880. Schlesiens Chilopoden. I. Chilopoda anamorpha. . A. Neumann, Breslau.Google Scholar
Harrington, H. J., Moore, R. C., and Stubblefield, C. J. 1959. Morphological terms applied to Trilobita. Pp. O117–O126 in Harrington, H. J. et al. Arthropoda 1. Part O of Moore, R. C., ed. Treatise on invertebrate paleontology. Geological Society of America, New York, and University of Kansas, Lawrence.Google Scholar
Hartnoll, R. G. 1982. Growth. Pp. 111–196 in Abele, L. G., ed. Biology of the Crustacea, Vol. 2. Embryology, morphology and genetics. Academic Press, New York.Google Scholar
Hawle, I., and Corda, A. J. C. 1847. Prodrom einer Monographie der böhmischen Trilobiten. J. G. Calve, Prague.Google Scholar
Hou, X., and Bergström, J. 1997. Arthropods from the lower Cambrian Chenjiang fauna, southwest China. Fossils and Strata 45:1–116.Google Scholar
Hughes, N. C. 1994. Ontogeny, intraspecific variation, and systematics of the Late Cambrian trilobite Dikelocephalus . Smithsonian Contributions to Paleobiology 79:1–89.Google Scholar
Hughes, N. C. 2003a. Trilobite body patterning and the evolution of arthropod tagmosis. BioEssays 25:386–395.CrossRefGoogle ScholarPubMed
Hughes, N. C. 2003b. Trilobite tagmosis and body patterning from morphological and developmental perspectives. Integrative and Comparative Biology 41:185–206.Google Scholar
Hughes, N. C. 2005. Trilobite construction: building a bridge across the micro and macroevolutionary divide. Pp. 138–158 in Briggs, D. E. G., ed. Evolving form and function: fossils and development. Peabody Museum of Natural History, Yale University, New Haven, Conn. Google Scholar
Hughes, N. C., and Chapman, R. E. 1995. Growth and variation in the Silurian proetide trilobite Aulacopleura konincki and its implications for trilobite paleobiology. Lethaia 28:333–353.Google Scholar
Hunt, A. S. 1967. Growth, variation, and instar development of an agnostid trilobite. Journal of Paleontology 41:203–208.Google Scholar
Janssen, R., Prpic, N., and Damen, W. G. M. 2004. Gene expression suggests decoupled dorsal and ventral segmentation in the millipede Glomeris marginata . Developmental Biology 268:89–104.Google Scholar
Janssen, R., Prpic, N., and Damen, W. G. M. 2006. A review of the correlation of tergites, sternites, and leg pairs in diplopods. Frontiers in Zoology. doi: 10.1186/1742-9994-3-2.CrossRefGoogle Scholar
Jell, P. A. 1975. Australian Middle Cambrian eodiscoids with a review of the superfamily. Palaeontographica, Abteilung A 150:1–97.Google Scholar
Kácha, P., and Šaric, R. 1991. Ontogeny of the trilobite Kosovopeltis svobodai Šnadjr from the Bohemian Silurian. Vestník Ústredního Ústavu Geologického 66:257–273.Google Scholar
Kim, K., Sheets, H. D., Haney, R. A., and Mitchell, C. E. 2002. Morphometric analysis of the ontogeny and allometry of the Middle Ordovician trilobite Triarthrus becki . Paleobiology 28:364–377.Google Scholar
Koenemann, S., and Jenner, R. A., eds. 2005. Crustacea and arthropod phylogeny. Crustacean issues, Vol. 16. CRC Press, Boca Raton, Fla.Google Scholar
Kopaska-Merkel, D. C. 1987. Ontogeny and evolution of an Ordovician trilobite. SEPM Midyear Meeting Abstracts 4:43–44. Society for Sedimentary Geology, Tulsa, Okla.Google Scholar
Lee, D.-C., and Chatterton, B. D. E. 1997. Ontogenies of trilobites from the lower Ordovician Garden City Formation of Idaho and their implications for the phylogeny of the Cheirurina. Journal of Paleontology 71:683–702.Google Scholar
Lee, D.-C., and Chatterton, B. D. E. 2005. Protaspides of Upper Cambrian Aphelaspis (Ptychopariida, Trilobita) and related species with their taxonomic implications. Palaeontology 48:1351–1375.CrossRefGoogle Scholar
Lerosey-Aubril, R., and Feist, R. 2005a. First Carboniferous protaspid larvae (Trilobita). Journal of Paleontology 79:702–718.Google Scholar
Lerosey-Aubril, R., and Feist, R. 2005b. Ontogeny of a new crytosymboline trilobite from the Famennian of Morocco. Acta Palaeontologica Polonica 50:449–464.Google Scholar
Lewis, J. G. E. 1981. The biology of centipedes. Cambridge University Press, Cambridge.Google Scholar
Matthew, G. F. 1896. Faunas of the Paradoxides beds in Eastern North America, No. 1. Transactions of the New York Academy of Sciences 15:192–247.Google Scholar
Maxmen, A., Browne, W. E., Martindale, M. Q., and Giribet, G. 2005. Neuroanatomy of sea spiders implies an appendicular origin of the protocerebral segment. Nature 437:1144–1148.Google Scholar
McNamara, K. J., and Rudkin, D. M. 1984. Techniques of trilobite exuviation. Lethaia 17:153–173.Google Scholar
McNamara, K. J., Yu, F., and Zhou, Z. 2003. Ontogeny and heterochrony in the oryctocephalid trilobite Arthricocephalus from the Early Cambrian of China. Special Papers in Palaeontology 70:103–126. Palaeontological Society, London Google Scholar
McNamara, K. J., Yu, F., and Zhou, Z. 2006. Ontogeny and heterochrony in the early Cambrian oryctocephalid trilobites Changaspis, Duyunaspis and Balangia from China. Palaeontology 49:1–19.Google Scholar
Minelli, A. 1992. Towards a new comparative morphology of myriapods. Berichte des Naturwissenschaftlich-Medizinischen Vereins in Innsbruck 10(Suppl.):37–46.Google Scholar
Minelli, A. 2003. The development of animal form: ontogeny, morphology, and evolution. Cambridge University Press, Cambridge.Google Scholar
Minelli, A., and Fusco, G. 2004. Evo-devo perspectives on segmentation: model organisms and beyond. Trends in Ecology and Evolution 19:423–429.Google Scholar
Minelli, A., and Peruffo, B. 1991. Developmental pathways, homology and homonomy in metameric animals. Journal of Evolutionary Biology 4:429–445.Google Scholar
Minelli, A., Foddai, D., Pereira, L. A., and Lewis, J. G. E. 2000. The evolution of segmentation of centipede trunk and appendages. Journal of Zoological Systematics and Evolutionary Research 38:103–117.Google Scholar
Minelli, A., Fusco, G., and Hughes, N. C. 2003. Tagmata and segment specification in trilobites. Special Papers in Palaeontology 70:31–43.Google Scholar
Minelli, A., Brena, C., Deflorian, G., Maruzzo, D., and Fusco, G. 2006. From embryo to adult—beyond the conventional periodization of arthropod development. Development, Genes, and Evolution 216:373–383.Google Scholar
Müller, K. J., and Walossek, D. 1986. Arthropod larvae from the Upper Cambrian of Sweden. Transactions of the Royal Society of Edinburgh (Earth Sciences) 77:157–179.Google Scholar
Müller, K. J., and Walossek, D. 1987. Morphology, ontogeny, and life habit of Agnostus pisiformis from the Upper Cambrian of Sweden. Fossils and Strata 19:1–124.Google Scholar
Palmer, A. R. 1962. Comparative ontogeny of some opisthoparian, gonatoparian, and proparian Upper Cambrian trilobites. Journal of Paleontology 36:87–96.Google Scholar
Paterson, J. R., and Edgecombe, G. D. 2006. The Early Cambrian trilobite family Emuellidae Pocock, 1970: systematic position and revision of Australian species. Journal of Paleontology 80:496–513.Google Scholar
Peel, A., Chipman, A. D., and Akam, M. 2005. Arthropod segmentation: macroevolution at the molecular level. Nature Reviews Genetics 6:905–916.Google Scholar
Peng, S., Babcock, L. E., and Lin, H. 2005. Polymerid trilobites from the Cambrian of northwestern Hunan, China, Vol. 2. Ptychopariida, Eodiscida, and undetermined forms. Science Press, Beijing.Google Scholar
Pocock, K. J. 1970. The Emuellidae, a new family of trilobites from the Lower Cambrian of South Australia. Palaeontology 13:522–562.Google Scholar
Ramsköld, L., and Edgecombe, G. D. 1991. Trilobite monophyly revisited. Historical Biology 4:267–283.Google Scholar
Ramsköld, L., and Edgecombe, G. D. 1994. Revision of the Silurian encrinurine trilobite Wallacia Lamont 1978, with species from Gotland and Canada. Paläontologische Zeitschrift 68:89–115.Google Scholar
Raw, F. 1925. The development of Leptoplastus salteri (Calloway) and of other trilobites (Olenidae, Ptychoparidae, Conocoryphidae, Paradoxidae, Phacopidae, and Mesonacidae). Journal of the Geological Society, London 81:223–324.Google Scholar
Raw, F. 1927. The ontogenies of trilobites, and their significance, Part 2. American Journal of Science 14:131–151.CrossRefGoogle Scholar
Richter, R. 1925. Von Bau und Leben der Trilobiten. V. Die Segmentbildung der Trilobiten, verglichen mit anderen Tiergruppen. Zentralblatt für Mineralogie, Geologie und Paläontologie 4:104–122.Google Scholar
Robison, R. A. 1967. Ontogeny of Bathyuriscus fimbriatus and its bearing on the affinities of corynexochid trilobites. Journal of Paleontology 41:213–221.Google Scholar
Salter, J. W. 1864. A monograph of British trilobites, Part 1. Monograph volume for 1862:1–80. Palaeontographical Society, London.Google Scholar
Scholtz, G., and Edgecombe, G. D. 2005. Heads, Hox, and the phylogenetic position of trilobites. Pp. 139–165 in Koenemann, and Jenner, 2005.Google Scholar
Schram, F. R. 1982. The fossil record and evolution of Crustacea. Pp. 93–147 in Abele, L. G., ed. Biology of the Crustacea, Vol. 1. Systematics, the fossil record, and biogeography. Academic Press, New York.Google Scholar
Schram, F. R. 1986. Crustacea. Oxford University Press, New York.Google Scholar
Simpson, A. G., Hughes, N. C., Kopaska-Merkel, D. C., and Ludvigsen, R. 2005. Development of the caudal exoskeleton of the pliomerid trilobite Hintzeia plicamarginis new species. Evolution and Development 7:528–541.Google Scholar
Snodgrass, R. E. 1956. Crustacean metamorphosis. Smithsonian Miscellaneous Collections 131(10):1–78.Google Scholar
Speyer, S. E., and Chatterton, B. D. E. 1989. Trilobite larvae and larval ecology. Historical Biology 3:27–60.Google Scholar
Speyer, S. E., and Chatterton, B. D. E. 1990. Trilobite larvae, larval ecology, and developmental paleobiology. In Mikulic, D. G., ed. Short Courses in Paleontology 3:137–156. Paleontological Society, Knoxville, Tenn. Google Scholar
Stein, M., Waloszek, D., and Maas, A. 2005. Oelandocaris oelandica and the stem lineage of Crustacea. Pp. 55–71 in Koenemann, and Jenner, 2005.Google Scholar
Stubblefield, C. J. 1926. Notes on the development of a trilobite, Shumardia pusilla (Sars). Zoological Journal of the Linnean Society of London 35:345–372.CrossRefGoogle Scholar
Tripp, R. P., and Evitt, W. R. 1981. Silicified Lichidae (Trilobita) from the Middle Ordovician of Virginia. Geological Magazine 118:665–677.CrossRefGoogle Scholar
Verhoeff, K. W. 1905. Über die Entwickelungsstufen der Steinläufer Lithobiiden und Beiträge zur Kenntnis der Chilopoden. Zoologische Jahrbücher, Abteilung für Systematik 8:195–298.Google Scholar
Walcott, C. D. 1916. Cambrian geology and paleontology: Cambrian trilobites. Smithsonian Miscellaneous Collections 64:303–456.Google Scholar
Walossek, D. 1993. The Upper Cambrian Rehbachiella and the phylogeny of Branchiopoda and Crustacea. Fossils and Strata 32:1–202.Google Scholar
Waloszek, D., and Dunlop, J. A. 2002. A larval sea spider (Arthropoda: Pycnogonida) from the Upper Cambrian ‘Orsten’ of Sweden and the phylogenetic position of pycnogonids. Palaeontology 45:421–446.Google Scholar
Waloszek, D., and Maas, A. 2005. The evolutionary history of crustacean segmentation: a fossil-based perspective. Evolution and Development 7:515–527.Google Scholar
Walossek, D., and Müller, K. J. 1990. Upper Cambrian stem-lineage crustaceans and their bearing on the monophyletic origin of Crustacea and the position of Agnostus . Lethaia 23:409–427.Google Scholar
Walossek, D., and Müller, K. J. 1998. Early arthropod phylogeny in the light of the Cambrian “Orsten” fossils. Pp. 185–231 in Edgecombe, G. D., ed. Arthropod fossils and phylogeny. Columbia University Press, New York.Google Scholar
Whittington, H. B. 1956. Silicified Middle Ordovician trilobites: the Odontopleuridae. Bulletin of the Museum of Comparative Zoology, Harvard University 114:155–288.Google Scholar
Whittington, H. B. 1957. Ontogeny of Elliptocephala, Paradoxides, Sao, Blainia, and Triarthrus (Trilobita). Journal of Paleontology 31:934–945.Google Scholar
Whittington, H. B. 1959. Ontogeny of Trilobita. Pp. O127–O144 in Harrington, H. J. et al. Arthropoda 1. Part O of Moore, R. C., ed. Treatise on invertebrate paleontology Geological Society, New York, and University of Kansas, Lawrence.Google Scholar
Whittington, H. B., and Evitt, W. R. 1954. Silicified Middle Ordovician trilobites. Geological Society of America Memoir 59:1–137.Google Scholar
Whittington, H. B., and Kelly, S. R. A. 1997. Morphological terms applied to Trilobita. Pp. 313–329 in Whittington, H. B. et al. Arthropoda 1. Trilobita. Part O (revised) of Kaesler, R. L., ed. Treatise on invertebrate paleontology. Geological Society of America, Boulder, and University of Kansas, Lawrence.Google Scholar
Williamson, D. I. 1982. Larval morphology and diversity. Pp. 43–110 in Abele, L. G., ed. The biology of Crustacea, Vol. 2. Embryology, morphology and genetics. Academic Press, New York.Google Scholar
Zhang, X.-G. 1989. Ontogeny of an Early Cambrian eodiscoid trilobite from Henan, China. Lethaia 22:13–29.Google Scholar
Zhang, X.-G., and Clarkson, E. N. K. 1993. Ontogeny of the eodiscid trilobite Shizhudiscus longquanensis from the Lower Cambrian of China. Palaeontology 36:785–806.Google Scholar
Zhang, X.-G., and Pratt, B. R. 1999. Early Cambrian trilobite larvae and ontogeny of Ichangia ichangensis Chang 1957 (Protoleridae) from Henan, China. Journal of Paleontology 73:117–128.Google Scholar
Zhang, X.-L., Han, J., Zhang, Z.-F., Liu, H.-Q., and Shu, D.-G. 2003. Reconsideration of the supposed naraoiid larva from the early Cambrian Chengjiang lagerstätte, south China. Palaeontology 46:447–465.Google Scholar