Extant arthropods are diverse and ubiquitous, forming a major constituent of most modern ecosystems. Evidence from early Palaeozoic Konservat Lagerstätten indicates that this has been the case since the Cambrian. Despite this, the details of arthropod origins remain obscure, although most hypotheses regard the first arthropods as benthic predators or scavengers such as the fuxianhuiids or megacheirans (‘great-appendage’ arthropods). Here, we describe a new arthropod from the Tulip Beds locality of the Burgess Shale Formation (Cambrian, series 3, stage 5) that possesses a weakly sclerotized thorax with filamentous appendages, encased in a bivalved carapace, and a strongly sclerotized, elongate abdomen and telson. A cladistic analysis resolved this taxon as the basal-most member of a paraphyletic grade of nekto-benthic forms with bivalved carapaces. This grade occurs at the base of Arthropoda (panarthropods with arthropodized trunk limbs) and suggests that arthrodization (sclerotization and jointing of the exoskeleton) evolved to facilitate swimming. Predatory and fully benthic habits evolved later in the euarthropod stem-lineage and are plesiomorphically retained in pycnogonids (sea spiders) and euchelicerates (horseshoe crabs and arachnids).

References

1
Edgecombe G. D. . 2010 Arthropod phylogeny: an overview from the perspectives of morphology, molecular data and the fossil record. Arthropod Struct. Dev. 39, 74–87.doi: 10.1016/j.asd.2009.10.002 (doi:10.1016/j.asd.2009.10.002). Crossref, PubMed, Web of Science, Google Scholar
2
Budd G. E.& Telford M. J. . 2009 The origin and evolution of arthropods. Nature 457, 812–817.doi: 10.1038/nature07890 (doi:10.1038/nature07890). Crossref, PubMed, Web of Science, Google Scholar
3
Legg D. A., Ma X., Wolfe J. M., Ortega-Hernández J., Edgecombe G. D.& Sutton M. D. . 2011 Lobopodian phylogeny reanalysed. Nature 476, E2–E3.doi: 10.1038/nature10267 (doi:10.1038/nature10267). Crossref, PubMed, Web of Science, Google Scholar
4
Briggs D. E. G., Erwin D. H.& Collier F. J. . 1994 The fossils of the Burgess Shale. Washington, DC: Smithsonian Institution. Google Scholar
5
Hou X. G., Aldridge R. J., Bergström J., Siveter D. J., Siveter D. J.& Feng X. H. . 2004 The Cambrian fossils of Chengjiang, China: the flowering of early animal life. Oxford, UK: Blackwell. Google Scholar
6
Collins D. . 1996 The ‘evolution’ of Anomalocaris and its classification in the arthropod class Dinocarida (nov.) and order Radiodonta (nov.). J. Paleontol. 70, 280–293. Crossref, Web of Science, Google Scholar
7
Daley A. C., Budd G. E., Caron J.-B., Edgecombe G. D.& Collins D. . 2009 The Burgess Shale anomalocaridid Hurdia and its significance for early euarthropod evolution. Science 323, 1597–1600.doi: 10.1126/science.1169514 (doi:10.1126/science.1169514). Crossref, PubMed, Web of Science, Google Scholar
8
Budd G. E. . 2002 A palaeontological solution to the arthropod head problem. Nature 417, 271–275.doi: 10.1038/417271a (doi:10.1038/417271a). Crossref, PubMed, Web of Science, Google Scholar
9
Kühl G., Briggs D. E. G.& Rust J. . 2009 A great-appendage arthropod with a radial mouth from the Lower Devonian Hunsrück Slate, Germany. Science 323, 771–773.doi: 10.1126/science.1166586 (doi:10.1126/science.1166586). Crossref, PubMed, Web of Science, Google Scholar
10
Bergström J., Hou X.-G., Zhang X.& Clausen S. . 2008 A new view of the Cambrian arthropod Fuxianhuia. GFF 130, 189–201. Crossref, Web of Science, Google Scholar
11
Hou X.-G.& Bergström J. . 1997 Arthropods of the Lower Cambrian Chengjiang fauna, southwest China. Fossils and Strata 45, 1–116. Google Scholar
12
Waloszek D., Chen J.-Y., Maas A.& Wang X.-Q. . 2005 Early Cambrian arthropods—new insights into arthropod head and structural evolution. Arthropod Struct. Dev. 34, 189–205.doi: 10.1016/j.asd.2005.01.005 (doi:10.1016/j.asd.2005.01.005). Crossref, Web of Science, Google Scholar
13
Bousfield E. L. . 1995 A contribution to the natural classification of Lower and Middle Cambrian arthropods: food-gathering and feeding mechanisms. Amphipacifica 2, 3–34. Google Scholar
14
Rota-Stabelli O., et al. 2011 A congruent solution to arthropod phylogeny: phylogenomics, microRNAs and morphology support Mandibulata. Proc. R. Soc. B 278, 298–306.doi: 10.1098/rspb.2010.0590 (doi:10.1098/rspb.2010.0590). Link, Web of Science, Google Scholar
15
Goloboff P. A., Farris J. S.& Nixon K. C. . 2008 TNT, a free program for phylogenetic analysis. Cladistics 24, 774–786.doi: 10.1111/j.1096-0031.2008.00217.x (doi:10.1111/j.1096-0031.2008.00217.x). Crossref, Web of Science, Google Scholar
16
Goloboff P. A. . 1993 Estimating character weights during tree search. Cladistics 9, 83–91.doi: 10.1111/j.1096-0031.tb00209.x (doi:10.1111/j.1096-0031.tb00209.x). Crossref, Web of Science, Google Scholar
17
Nixon K. C. . 1999 The Parsimony Ratchet, a new method for rapid parsimony analysis. Cladistics 15, 407–414.doi: 10.1111/j.1096-0031.1999.tb00277.x (doi:10.1111/j.1096-0031.1999.tb00277.x). Crossref, Web of Science, Google Scholar
18
Goloboff P. A. . 1999 Analyzing large data sets in reasonable times: solutions for composite optima. Cladistics 15, 415–428.doi: 10.1111/j.1096-0031.1999.tb00278.x (doi:10.1111/j.1096-0031.1999.tb00278.x). Crossref, Web of Science, Google Scholar
19
Fletcher T. P.& Collins D. . 1998 The Middle Cambrian Burgess Shale and its relationship to the Stephen Formation in the southern Canadian Rocky Mountains. Can. J. Earth Sci. 35, 413–436.doi: 10.1139/e97-120 (doi:10.1139/e97-120). Crossref, Web of Science, Google Scholar
20
O'Brien L. J.& Caron J.-B. . 2012 A new stalked filter-feeder from the Middle Cambrian Burgess Shale, British Columbia, Canada. PLoS ONE 7, 1–21.doi: 10.1371/journal.pone.0029233 (doi:10.1371/journal.pone.0029233). Crossref, Web of Science, Google Scholar
21
Briggs D. E. G. . 1981 The arthropod Odaraia alata Walcott, Middle Cambrian, Burgess Shale, British Columbia. Phil. Trans. R. Soc. Lond. B 291, 541–582.doi: 10.1098/rstb.1981.0007 (doi:10.1098/rstb.1981.0007). Link, Web of Science, Google Scholar
22
Fu D.& Zhang X. . 2011 A new arthropod Jugatacaris agilis n. gen. n. sp. from the Early Cambrian Chengjiang Biota, South China. J. Paleontol. 85, 567–586.doi: 10.1666/09-173.1 (doi:10.1666/09-173.1). Crossref, Web of Science, Google Scholar
23
Hou X.-G., Bergström J.& Xu G.-H. . 2004 The Lower Cambrian crustacean Pectocaris from the Chengjiang Biota, Yunnan, China. J. Paleontol. 78, 700–708.doi: 10.1666/0022-3360(2004)078<0700:TLCCPF>2.0.CO;2 (doi:10.1666/0022-3360(2004)078<0700:TLCCPF>2.0.CO;2). Crossref, Web of Science, Google Scholar
24
Briggs D. E. G. . 1977 Bivalved arthropods from the Cambrian Burgess Shale of British Columbia. Palaeontology 20, 595–621. Google Scholar
25
Briggs D. E. G. . 1978 The morphology, mode of life, and affinities of Canadaspis perfecta (Crustacea: Phyllocarida), Middle Cambrian, Burgess Shale, British Columbia. Phil. Trans. R. Soc. Lond. B 281, 439–487.doi: 10.1098/rstb.1978.0005 (doi:10.1098/rstb.1978.0005). Link, Web of Science, Google Scholar
26
Wills M. A., Briggs D. E. G., Fortey R. A., Wilkinson M.& Sneath P. H. A. . 1998 An arthropod phylogeny based on fossil and Recent taxa. In Arthropod fossils and phylogeny (ed. & Edgecombe G. D. ), pp. 33–105. New York, NY: Columbia University Press. Google Scholar
27
Budd G. E. . 2001 Ecology of nontrilobite arthropods and lobopods in the Cambrian. The ecology of the Cambrian radiation (eds , Zhuravlev A. Yu.& Riding R. ), pp. 404–427. New York, NY: Columbia University Press. Google Scholar
28
Butterfield N. J. . 2002 Leanchoilia guts and the interpretation of three-dimensional structures in Burgess Shale-type fossils. Paleobiology 28, 155–171.doi: 10.1666/0094-8373(2002)028<0155:LGATIO>2.0.CO;2 (doi:10.1666/0094-8373(2002)028<0155:LGATIO>2.0.CO;2). Crossref, Web of Science, Google Scholar
29
Zhu M.-Y., Vannier J., Van Iten H.& Zhao Y.-L. . 2004 Direct evidence for predation on trilobites in the Cambrian. Proc. R. Soc. Lond. B 271, S277–S280.doi: 10.1098/rsbl.2004.0194 (doi:10.1098/rsbl.2004.0194). Link, Web of Science, Google Scholar
30
Budd G. E. . 2001 Why are arthropods segmented? Evol. Dev. 3, 332–342.doi: 10.1046/j.1525-142X.2001.01041.x (doi:10.1046/j.1525-142X.2001.01041.x). Crossref, PubMed, Web of Science, Google Scholar