Hostname: page-component-78c5997874-j824f Total loading time: 0 Render date: 2024-11-18T11:23:00.111Z Has data issue: false hasContentIssue false
  • English
  • Français

Wonderful strife: systematics, stem groups, and the phylogenetic signal of the Cambrian radiation

Published online by Cambridge University Press:  08 April 2016

Derek E. G. Briggs  and
Richard A. Fortey
Derek E. G. Briggs
Affiliation:
Department of Geology and Geophysics, Yale University, Post Office Box 208109, New Haven, Connecticut 06520-8109. E-mail: [email protected]
Richard A. Fortey
Affiliation:
Department of Palaeontology, The Natural History Museum, Cromwell Road, London SW7 5BD, United Kingdom. E-mail: [email protected]
Get access Rights & Permissions [Opens in a new window]

Abstract

Gould's Wonderful Life (1989) was a landmark in the investigation of the Cambrian radiation. Gould argued that a number of experimental body plans (“problematica”) had evolved only to become extinct, and that the Cambrian was a time of special fecundity in animal design. He focused attention on the meaning and significance of morphological disparity versus diversity, and provoked attempts to quantify disparity as an evolutionary metric. He used the Burgess Shale as a springboard to emphasize the important role of contingency in evolution, an idea that he reiterated for the next 13 years. These ideas set the agenda for much subsequent research. Since 1989 cladistic analyses have accommodated most of the problematic Cambrian taxa as stem groups of living taxa. Morphological disparity has been shown to be similar in Cambrian times as now. Konservat-Lagerstätten other than the Burgess Shale have yielded important new discoveries, particularly of arthropods and chordates, which have extended the range of recognized major clades still further back in time. The objective definition of a phylum remains controversial and may be impossible: it can be defined in terms of crown or total group, but the former reveals little about the Cambrian radiation. Divergence times of the major groups remain to be resolved, although molecular and fossil dates are coming closer. Although “superphyla” may have diverged deep in the Proterozoic, “explosive” evolution of these clades near the base of the Cambrian remains a possibility. The fossil record remains a critical source of data on the early evolution of multicellular organisms.

Type
Generating Diversity
Information
Paleobiology , Volume 31 , Issue S2 , 2005 , pp. 94 - 112
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

Ahlberg, P. E., ed. 2001. Major events in early vertebrate evolution. Systematics Association Special Volume 61. Taylor and Francis, London.Google Scholar
Aldridge, R. J., Briggs, D. E. G., Clarkson, E. N. K., and Smith, M. P. 1986. The affinities of conodonts—new evidence from the Carboniferous of Edinburgh, Scotland. Lethaia 19:279–291.Google Scholar
Allison, P. A., and Briggs, D. E. G. 1993. Exceptional fossil record: distribution of soft-tissue preservation through the Phanerozoic. Geology 21:527–530.2.3.CO;2>CrossRefGoogle Scholar
Aris-Brosou, S., and Yang, Z-H. 2003. Bayesian models of episodic evolution support a late Precambrian explosive divergence of the Metazoa. Molecular Biology and Evolution 20:1947–1954.CrossRefGoogle ScholarPubMed
Averof, M., and Akam, M. 1995. Insect-crustacean relationships: insights from comparative developmental and molecular studies. Philosophical Transactions of the Royal Society of London B 347:293–303.Google Scholar
Ayala, F. J., Rzhetsky, A., and Ayala, F. J. 1998. Origin of the metazoan phyla: molecular clocks confirm paleontological estimates. Proceedings of the National Academy of Sciences USA 95:606–12.Google Scholar
Bengtson, S., and Yue, Z. 1997. Fossilized metazoan embryos from the earliest Cambrian. Science 277:1645–1648.Google Scholar
Benton, M. J., ed. 1993. The fossil record 2. Chapman and Hall, London.Google Scholar
Benton, M. J., ed. 2000. Stems, nodes, crown clades, and rank-free lists: is Linnaeus dead? Biological Reviews 75:633–648.Google Scholar
Bourlat, S. J., Nielsen, C., Lockyer, A. E., Littlewood, D. T. J., and Telford, M. J. 2003. Xenoturbella is a deuterostome that eats molluscs. Nature 424:925–928.CrossRefGoogle ScholarPubMed
Brasier, M. J., Cowie, J. W., and Taylor, M. E. 1994. Decision on the Precambrian–Cambrian boundary. Episodes 17:3–8.Google Scholar
Briggs, D. E. G. 1978. The morphology, mode of life and affinities of Canadaspis perfecta (Crustacea: Phyllocarida), Middle Cambrian, Burgess Shale. Philosophical Transactions of the Royal Society of London B 281:439–487.Google Scholar
Briggs, D. E. G. 1992a. Phylogenetic significance of the Burgess Shale crustacean Canadaspis. Acta Zoologica, Stockholm 73:293–300.Google Scholar
Briggs, D. E. G. 1992b. Conodonts—a major extinct group added to the vertebrates. Science 256:1285–1286.Google Scholar
Briggs, D. E. G., and Bartels, C. 2001. New arthropods from the Lower Devonian Hunsrück Slate (Lower Emsian, Rhenish Massif, western Germany). Palaeontology 44:275–303.Google Scholar
Briggs, D. E. G., and Collins, D. 1988. A Middle Cambrian chelicerate from Mount Stephen, British Columbia. Palaeontology 31:779–798.Google Scholar
Briggs, D. E. G., and Morris, S. Conway 1986. Problematica from the Middle Cambrian Burgess Shale of British Columbia. Pp. 167–183in Hoffman, A. and Nitecki, M. H., eds. Problematic fossil taxa. Oxford University Press, Oxford.Google Scholar
Briggs, D. E. G., and Fortey, R. A. 1989. The early radiation and relationships of the major arthropod groups. Science 246:241–243.Google Scholar
Briggs, D. E. G., and Whittington, H. B. 1981. Relationships of arthropods from the Burgess Shale and other Cambrian sequences. Proceedings of the second International Symposium on the Cambrian System. U.S. Geological Survey Open-file Report 81-743:38–41.Google Scholar
Briggs, D. E. G., Fortey, R. A., and Wills, M. A. 1992a. Morphological disparity in the Cambrian. Science 256:1670–1673.Google Scholar
Briggs, D. E. G., Fortey, R. A., and Wills, M. A. 1992b. Cambrian and Recent morphological disparity: response to Foote and Gould, and Lees. Science 258:1817–1818.Google Scholar
Briggs, D. E. G., Siveter, D. J., and Siveter, D. J. 1996. Soft-bodied fossils from a Silurian volcaniclastic deposit. Nature 382:248–250.CrossRefGoogle Scholar
Bromham, L., Rambaut, A., Fortey, R. A., Cooper, A., and Penny, D. 1998. Testing the Cambrian explosion hypothesis using a molecular dating technique. Proceedings of the National Academy of Sciences USA 95:12386–9.CrossRefGoogle ScholarPubMed
Budd, G. E. 1993. A Cambrian gilled lobopod from Greenland. Nature 364:709–711.Google Scholar
Budd, G. E. 1996. The morphology of Opabinia regalis and the reconstruction of the arthropod stem-group. Lethaia 29:1–14.CrossRefGoogle Scholar
Budd, G. E. 1999. Does evolution in body patterning genes drive morphological change—or vice versa? BioEssays 21:326–332.Google Scholar
Budd, G. E. 2002. A palaeontological solution to the arthropod head problem. Nature 417:271–75.Google Scholar
Budd, G. E. 2003. The Cambrian fossil record and the origin of the phyla. Integrative and Comparative Biology 43:157–165.CrossRefGoogle ScholarPubMed
Budd, G. E., and Jensen, S. 2000. A critical appraisal of the fossil record of the bilaterian phyla. Biological Reviews 75:253–295.Google Scholar
Butterfield, N. J. 2003. Exceptional fossil preservation and the Cambrian explosion. Integrative and Comparative Biology 43:166–177.Google Scholar
Carroll, S. B., Weatherbee, S., and Langeland, J. 1995. Homeotic genes and the regulation and evolution of insect wing number. Nature 375:58–61.Google Scholar
Chen, J.-Y., and Huang, D.-Y. 2002. A possible Lower Cambrian chaetognath (arrow worm). Science 298:187.Google Scholar
Chen, J.-Y., and Li, C.-W. 1997. Early Cambrian chordate from Chengjiang, China. Bulletin of the National Museum of Natural Science 10:257–273.Google Scholar
Chen, J.-Y., and Zhou, G.-Q. 1997. Biology of the Chengjiang fauna. Bulletin of the National Museum of Natural Science 10:11–105.Google Scholar
Chen, J.-Y., Hou, X.-G., and Li, G.-X. 1989. Early Cambrian netted scale-bearing worm-like sea animal. Acta Palaeontologica Sinica 29:402–414.Google Scholar
Chen, J.-Y., Dzik, J., Edgecombe, G. E., Ransköld, L., and Zhou, G.-Q. 1995. A possible Early Cambrian chordate. Nature 377:720–722.Google Scholar
Chen, J.-Y., Huang, D.-Y., and Li, C.-W. 1999. An early Cambrian craniate-like chordate. Nature 402:518–522.Google Scholar
Chen, J.-Y., Huang, D.-Y., Peng, Q.-Q., Chi, H.-M., Wang, X.-Q., and Feng, M. 2003. The first tunicate from the Early Cambrian of South China. Proceedings of the National Academy of Sciences USA 100:8314–8318.Google Scholar
Collins, D. 1996. The “evolution” of Anomalocaris and its classification in the arthropod class Dinocarida (nov.) and order Radiodonta (nov.). Journal of Paleontology 70:280–93.CrossRefGoogle Scholar
Morris, S. Conway 1977a. A new metazoan from the Cambrian Burgess Shale, British Columbia. Palaeontology 20:623–640.Google Scholar
Morris, S. Conway 1977b. Fossil priapulid worms. Special Papers in Palaeontology 20:1–98.Google Scholar
Morris, S. Conway 1998. The crucible of creation: the Burgess Shale and the rise of animals. Oxford University Press, Oxford.Google Scholar
Morris, S. Conway 2000. The Cambrian “explosion”: slow fuse or megatonnage? Proceedings of the National Academy of Sciences USA 97:4426–9.Google Scholar
Morris, S. Conway 2003a. Life's solution: inevitable humans in a lonely universe. Cambridge University Press, Cambridge.Google Scholar
Morris, S. Conway 2003b. The Cambrian “explosion” of metazoans and molecular biology: would Darwin be satisfied? International Journal of Developmental Biology 47:505–515.Google Scholar
Morris, S. Conway, and Gould, S. J. 1998. Showdown on the Burgess Shale. [The Challenge by Simon Conway Morris and the Reply by Stephen Jay Gould.]. Natural History 107:48–55.Google Scholar
Morris, S. Conway, and Peel, J. S. 1995. Articulated halkieriids from the Lower Cambrian of North Greenland and their role in early protostome evolution. Philosophical Transactions of the Royal Society of London B 347:304–358.Google Scholar
Cooper, A., and Fortey, R. A. 1998. Evolutionary explosions and the phylogenetic fuse. Trends in Ecology and Evolution 13:151–6.Google Scholar
Cutler, D. J. 2000. Estimating divergence times in the presence of an overdispersed molecular clock. Molecular Biology and Evolution 17:1647–60.Google Scholar
Dewel, R. A., and Dewel, W. C. 1998. The place of tardigrades in arthropod evolution. Pp. 109–123in Fortey, R. A. and Thomas, R. H., eds. Arthropod relationships. Chapman and Hall, London.Google Scholar
Droser, M. L., Jensen, S., and Gehling, J. G. 2002. Trace fossils and substrates of the terminal Proterozoic-Cambrian transition: implications for the record of early bilaterians and sediment mixing. Proceedings of the National Academy of Sciences USA 99:12572–12576.Google Scholar
Dzik, J. 1995. Yunnanozoon and the ancestry of the chordates. Acta Palaeontologica Polonica 40:341–360.Google Scholar
Edgecombe, G. D., ed. 1998. Arthropod fossils and phylogeny. Columbia University Press, New York.Google Scholar
Fedonkin, M. A. 2003. The origin of the Metazoa in the light of the Proterozoic fossil record. Paleontological Research 7:9–41.Google Scholar
Foote, M., and Gould, S. J. 1992. Cambrian and Recent morphological disparity. Science 258:1816.Google Scholar
Fortey, R. A. 1989. The collection connection. [Review of Wonderful Life.]Nature 342:303.Google Scholar
Fortey, R. A., Briggs, D. E. G., and Wills, M. A. 1996. The Cambrian evolutionary ‘explosion’: decoupling cladogenesis from morphological disparity. Biological Journal of the Linnean Society 57:13–33.Google Scholar
Fortey, R. A., Jackson, J., and Strugnell, J. 2003. Phylogenetic “fuses” and evolutionary explosions: conflicting evidence and critical tests. Pp. 41–65in Donoghue, P., ed. Molecular evolution. Chapman and Hall.Google Scholar
Gee, H. 2001. On being vetulicolian. Nature 414:407–409.CrossRefGoogle ScholarPubMed
Gehling, J. G. 1991. The case for Ediacaran fossil roots to the Metazoan tree. Pp. 181–224in Radhakrishna, B. P., ed. The world of Martin F. Glaessner. Geological Society of India Memoir No. 20. Bangalore.Google Scholar
Gehling, J. G., and Rigby, J. K. 1996. Long expected sponges from the Neoproterozoic Ediacara fauna of South Australia. Journal of Paleontology 70:185–195.Google Scholar
Giribet, G., Edgecombe, G. D., and Wheeler, W. C. 2001. Arthropod phylogeny based on eight molecular loci and morphology. Nature 413:157–161.Google Scholar
Glaessner, M. F. 1984. The dawn of animal life: a biohistorical study. Cambridge University Press, Cambridge.Google Scholar
Gould, S. J. 1989. Wonderful life: the Burgess Shale and the nature of history. Norton, New York.Google Scholar
Gould, S. J. 1991. The disparity of the Burgess Shale arthropod fauna and the limits of cladistic analysis: why we must strive to quantify morphospace. Paleobiology 17:411–423.Google Scholar
Gould, S. J. 2002. The structure of evolutionary theory. Belknap Press of Harvard University Press, Cambridge.Google Scholar
Hedges, S. B., and Kumar, S. 2003. Genomic clocks and evolutionary timescales. Trends in Genetics 19:200–206.Google Scholar
Hoffman, P. F., Kaufman, A. J., Halverson, G. P., and Schrag, D. P. 1998. A Neoproterozoic snowball earth. Science 281:1342–1346.Google Scholar
Holland, N. D., and Chen, J.-Y. 2001. Origin and early evolution of the vertebrates: new insights from advances in molecular biology, anatomy, and palaeontology. BioEssays 23:142–151.3.0.CO;2-5>CrossRefGoogle ScholarPubMed
Hou, X.-G. 1987. Early Cambrian large bivalved arthropods from Chengjiang, Eastern Yunnan. Acta Paleontological Sinica 26:286–297.Google Scholar
Hou, X.-G., and Bergström, J. 1997. Arthropods of the Chengjiang fauna, southwest China. Fossils and Strata 45:1–116.Google Scholar
Hou, X.-G., Ramsköld, L., and Bergström, J. 1991. Composition and preservation of the Chengjiang fauna—a Lower Cambrian soft-bodied biota. Zoologica Scripta 20:395–411.Google Scholar
Hou, X.-G., Bergström, J., and Ahlberg, P. 1995. Anomalocaris and other large animals in the Lower Cambrian Chengjiang Fauna of southwestern China. Geologiska Föreningens Förhandlingar 117:163–183.Google Scholar
Hou, X.-G., Aldridge, R. J., Siveter, D. J., Siveter, D. J., and Xiang-hong, F. 2002. New evidence on the anatomy and phylogeny of the earliest vertebrates. Proceedings of the Royal Society of London B 269:1865–1869.Google Scholar
Huelsenbeck, J. P., Ronquist, F., Neilsen, R., and Bollback, J. P. 2001. Bayesian inference of phylogeny and its impact on evolutionary biology. Science 294:2310–4.Google Scholar
Hwang, U. W., Friedrich, M., and Tautz, D. 2001. Mitochondrial protein joins myriapods with chelicerates. Nature 413:154–7.Google Scholar
Jefferies, R. P. S. 1979. The origin of chordates—a methodological essay. In House, M. R., ed. The origin of major invertebrate groups. Systematics Association Special Volume 12:443–477. Academic Press, London.Google Scholar
Lacalli, T. C. 2002. Vetulicolians—are they deuterostomes? Chordates? BioEssays 24:208–211.Google Scholar
Lee, M. S. Y. 1992. Cambrian and Recent morphological disparity. Science 258:1816–1817.Google Scholar
Levinton, J. S. 2001. Genetics, paleontology, and macroevolution, 2d ed.Cambridge University Press, Cambridge.Google Scholar
Lofgren, A. S., Plotnick, R. E., and Wagner, P. J. 2003. Morphological diversity of Carboniferous arthropods and insights on disparity patterns through the Phanerozoic. Paleobiology 29:349–368.Google Scholar
Lynch, M. 1999. The age and relationships of the major animal phyla. Evolution 53:319–25.CrossRefGoogle ScholarPubMed
Mallatt, J., and Chen, J.-Y. 2003. Fossil sister group of craniates: predicted and found. Journal of Morphology 258:1–31.Google Scholar
Mallatt, J., and Sullivan, J. 1998. 28S and 18S rDNA sequences support the monophyly of lampreys and hagfishes. Molecular Biology and Evolution 15:1706–1718.Google Scholar
Mallatt, J., Chen, J.-Y., and Holland, N. D. 2003. Comment on “A new species of Yunnanozoan with implications for deuterostome evolution.” Science 300:1372c.Google Scholar
Müller, K. J., Walossek, D., and Zakharov, A. 1995. “Orsten” type phosphatized soft-integument preservation and a new record from the Middle Cambrian Kuonamka Formation in Siberia. Neues Jahrbuch für Geologie und Paläontologie, Abhandlungen 197:101–118.Google Scholar
Patterson, C. 1981. Significance of fossils in determining evolutionary relationships. Annual Review of Ecology and Systematics 12:195–223.Google Scholar
Pennisi, E. 2003. Modernizing the tree of life. Science 300:1692–1697.Google Scholar
Peterson, K. J., Lyons, J. B., Nowak, K. S., Takacs, C. M., Wargo, M. J., and McPeek, M. A. 2004. Estimating metazoan divergence times with a molecular clock. Proceedings of the National Academy of Sciences USA 101:6536–6541.Google Scholar
Peterson, K. J., McPeek, M. A., and Evans, D. A. D. 2005. Tempo and mode of early animal evolution: inferences from rocks, Hox, and molecular clocks. [This volume.]Google Scholar
Ramsköld, L. 1992. The second leg row of Hallucigenia discovered. Lethaia 25:221–224.Google Scholar
Ramsköld, L., and Chen, J.-Y. 1998. Cambrian lobopodians: morphology and phylogeny. Pp. 107–150in Edgecombe, 1998.Google Scholar
Ramsköld, L., and Hou, X.-G. 1991. New early Cambrian animal and onychophoran affinities of enigmatic metazoans. Nature 351:225–227.Google Scholar
Ronshaugen, M., McGinnis, N., and McGinnis, W. 2002. Hox protein mutation and macroevolution of the insect body plan. Nature 415:914–917.Google Scholar
Ruiz-Trillo, I., Paps, J., Loukota, M., Ribera, C., Jondelius, U., Baguna, J., and Riutort, M. 2002. A phylogenetic analysis of myosin heavy chain type II sequences corroborates that Acoela and Nemertodermatida are basal bilaterians. Proceedings of the National Academy of Sciences USA 99:11246–11251.CrossRefGoogle ScholarPubMed
Sanderson, M. J. 2002. Estimating absolute rates of molecular evolution and divergence times: a penalized likelihood approach. Molecular Biology and Evolution 19:101–109.Google Scholar
Seilacher, A. 1989. Vendozoa—organismic construction in the Proterozoic biosphere. Lethaia 22:229–239.CrossRefGoogle Scholar
Seilacher, A., Grazhdankin, D., and Legouta, A. 2003. Ediacaran biota: the dawn of animal life in the shadow of giant protists. Paleontological Research 7:43–54.Google Scholar
Selden, P. A., and Dunlop, J. A. 1998. Fossil taxa and relationships of chelicerates. Pp. 303–331in Edgecombe, 1998.Google Scholar
Shoemaker, J. S., Painter, I. S., and Weir, B. S. 1999. Bayesian statistics in genetics. a guide for the uninitiated. Trends in Genetics 15:354–8.Google Scholar
Shu, D.-G. 2003. A paleontological perspective of vertebrate origin. Chinese Science Bulletin 48:725–735.Google Scholar
Shu, D.-G., and Morris, S. Conway 2003. Response to comment on “A new species of Yunnanozoan with implications for deuterostome evolution.” Science 300:1372d.Google Scholar
Shu, D.-G., Morris, S. Conway, and Zhang, X.-L. 1996a. A Pikaia-like chordate from the Lower Cambrian of China. Nature 384:157–158.Google Scholar
Shu, D.-G., Zhang, X., and Chen, L. 1996b. Reinterpretation of Yunnanozoon as the earliest known hemichordate. Nature 380:428–430.Google Scholar
Shu, D.-G., Luo, H.-L., Morris, S. Conway, Zhang, X.-L., Hu, S.-X., Chen, L., Han, J., Zhu, M., Li, Y., and Chen, L.-Z. 1999. Lower Cambrian vertebrates from south China. Nature 402:42–46.Google Scholar
Shu, D.-G., Chen, L., Han, J., and Zhang, X.-L. 2001a. An Early Cambrian tunicate from China. Nature 411:472–473.Google Scholar
Shu, D.-G., Morris, S. Conway, Han, J., Chen, L., Zhang, X.-L., Zhang, Z.-F., Liu, H.-Q., Li, Y., and Liu, J.-N. 2001b. Primitive deuterostomes from the Chengjiang Lagerstätte (Lower Cambrian, China). Nature 414:419–424.Google Scholar
Shu, D.-G., Morris, S. Conway, Han, J., Zhang, Z.-F., Yasui, K., Janvier, P., Chen, L., Zhang, X.-L., Liu, J.-N., Li, Y., and Liu, H.-Q. 2003a. Head and backbone of the Early Cambrian vertebrate Haikouichthys. Nature 421:526–529.Google Scholar
Shu, D.-G., Morris, S. Conway, Zhang, Z.-F., Liu, J.-N., Chen, L., Zhang, X.-L., Yasui, K., and Li, Y. 2003b. A new species of yunnanozoan with implications for deuterostome evolution. Science 299:1380–1384.Google Scholar
Simonetta, A. M., and Insom, E. 1993. New animals from the Burgess Shale (Middle Cambrian) and their possible significance for the understanding of the Bilateria. Bollettino Zoologica 60:97–107.Google Scholar
Siveter, D. J., Williams, M., and Waloszek, D. 2001. A phosphatocopid crustacean with appendages from the Lower Cambrian. Science 293:479–80.CrossRefGoogle ScholarPubMed
Smith, A. B., and Peterson, K. J. 2002. Dating the time of origin of major clades: molecular clocks and the fossil record. Annual Review of Earth and Planetary Sciences 30:65–88.Google Scholar
Smith, M. P., Sansom, I. J., and Cochrane, K. D. 2001. The Cambrian origin of vertebrates. Pp. 67–84in Ahlberg, 2001.Google Scholar
Sutton, M. D., Briggs, D. E. G., Siveter, D. J., and Siveter, D. J. 2001. An exceptionally preserved vermiform mollusc from the Silurian of England. Nature 410:461–3.Google Scholar
Takezaki, N., Figueroa, F., Zaleska-Rutczynska, Z., and Klein, J. 2003. Molecular phylogeny of early vertebrates: monophyly of the agnathans as revealed by sequences of 35 genes. Molecular Biology and Evolution 20:287–292.Google Scholar
Walossek, D. 1999. On the Cambrian diversity of Crustacea. Pp. 3–27in Schram, F. R. and von Vaupel Klein, J. C., eds. Proceedings of the Fourth International Crustacean Congress, Amsterdam, The Netherlands, Vol. 1. Brill, Leiden.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
Wang, D. Y-C., Kumar, S., and Hedges, S. B. 1999. Divergence time estimates for the early history of animal phyla and the origin of plants, animals and fungi. Proceedings of the Royal Society of London B 266:163–71.Google Scholar
Wheeler, W. C. 1998. Molecular systematics and arthropods. Pp. 9–32in Edgecombe, 1998.Google Scholar
Whittington, H. B., and Briggs, D. E. G. 1985. The largest Cambrian animal, Anomalocaris, Burgess Shale, British Columbia. Philosophical Transactions of the Royal Society of London B 309:569–609.Google Scholar
Wills, M. A. 1998. Cambrian and recent disparity: the picture from priapulids. Paleobiology 24:177–199.Google Scholar
Wills, M. A., and Fortey, R. A. 2000. The shape of life: how much is written in stone? BioEssays 22:1142–1152.Google Scholar
Wills, M. A., Briggs, D. E. G., and Fortey, R. A. 1994. Disparity as an evolutionary index: a comparison of Cambrian and Recent arthropods. Paleobiology 20:93–130.Google Scholar
Wills, M. A., Briggs, D. E. G., and Fortey, R. A. 1998. An arthropod phylogeny based on fossil and Recent taxa. Pp. 33–106in Edgecombe, 1998.Google Scholar
Winnepenninckx, B. M. H., Backeljau, T., and Kristensen, R. M. 1998. Relations of the new phylum Cycliophora. Nature 393:636–637.Google Scholar
Wray, G. A., Levinton, J. S., and Shapiro, L. H. 1996. Molecular evidence for deep Precambrian divergences among metazoan phyla. Science 274:568–72.Google Scholar
Xiao, S., and Knoll, A. H. 2000. Phosphatized animal embryos from the Neoproterozoic Doushantuo Formation at Weng'an, Guizhou, South China. Journal of Paleontology 74:767–788.Google Scholar