Onychophora

Velvet worm Oroperipatus sp., copyright Piotr Naskrecki.

Belongs within: Ecdysozoa.

The Onychophora include the living velvet worms, a group of soft-bodied, multi-legged terrestrial invertebrates, together with a number of their Palaeozoic marine relatives. Synapomorphies of the onychophoran total group include the limbless posterior extension of the lobopodous trunk, undifferentiated posterior appendages and the loss of radially symmetrical circum-oral structures (Smith & Ortega-Hernández 2014).

The little nets
Published 21 January 2008

Among the various components of the Palaeozoic fossil record are several groups of animals described from disarticulated pieces of dermal armour that gave us little idea of what the entire animal originally looked like. In many cases, more complete specimens have been found that show us the animal’s true form. In others, the identity of the original animal remains a near-insoluble mystery.

Microdictyon sclerite, from here.

My subject here is a classic example of just little these isolated elements can sometimes tell us about their source. Microdictyon was first known from tiny roundish to oblong-ish sclerites. The name means “tiny net” and refers to the net-like structure of the sclerites, clearly visible in the example above. Differences in appearance between sclerites led to the description of a number of species (Bengtson et al. 1986), but the identity of the animal bearing them was a complete mystery.

It wasn’t until the discovery of the Chengjiang Biota in China that a fossil showing the soft anatomy of Microdictyon was discovered, and it’s probably fair to say that no-one could have predicted what it looked like:

Microdictyon complete fossil and reconstruction, from Palaeos.

It turns out that Microdictyon was a lobopod, one of a number of Cambrian marine animals not unlike the modern terrestrial onychophorans*. Paired sclerites sat above each pair of legs. It is most likely that these sclerites served a defensive purpose but other functions have also been suggested—Jerzy Dzik (2003), a man who has not been above suggesting heterodox interpretations of Cambrian animals in the past (some of which have even turned out to be accurate), suggested that Microdictyon sclerites were similar in structure to trilobite eyes, and might even be homologous. He therefore reconstructed Microdictyon as an elongate animal with a pair of eyes on each segment!

*“Lobopod” is a collective name for tardigrades and onychophorans, which have a soft body with stumpy tubular legs. Some authors have interpreted tardigrades and onychophorans as forming a monophyletic group, but others hold that the lobopod form is the ancestral grade for panarthropods (the clade joining lobopods and arthropods). In the past, the Cambrian lobopods have been interpreted as stem-onychophorans, but beyond the superficial similarities in appearance, it is not unlikely that the shared features are also plesiomorphic for panarthropods in general and a specific relationship to onychophorans should be regarded sceptically (Liu et al. 2008).

Systematics of Onychophora
<==Onychophora [Archonychophora, Polylobopoda, Xenusia]
    |  i. s.: Ooperipatus viridimaculataO81
    |         Cephalofovea tomahmontisMS98
    |         Cantabria Clausen & Álvaro 2006CA06
    |           `--*C. labyrinthica Clausen & Álvaro 2006CA06
    |         Fusuconcharium Hao & Shu 1987CA06
    |         Quadratopora Hao & Shu 1987CA06
    |--Onychodictyon gracilisSO-H14
    `--+--Paucipodia Chen, Zhou & Ramsköld 1995SO-H14, HM04 [Paucipodiidae]
       |    `--*P. inermis Chen, Zhou & Ramsköld 1995HM04
       |--Microdictyon Bengtson, Matthews & Missarzhevsky 1981SO-H14, C12 [Microdictyonidae]
       |    |--M. robisoni Bengtson et al. 1986WS93
       |    |--M. sinicum Chen, Hou & Lu 1989HM04
       |    |--M. sphaeroides Hinz 1987L95
       |    `--M. tenuiporatum Bengtson et al. 1986WS93
       |--+--Diania Liu, Steiner et al. 2011SO-H14, LS11
       |  |    `--*D. cactiformis Liu, Steiner et al. 2011LS11
       |  `--Xenusion Pompeckj 1927SO-H14, H62
       |       `--*X. auerwaldae Pompeckj 1927H62, HM04
       `--+--Cardiodictyon catenulum Hou, Ramsköld & Bergström 1991SO-H14, HM04
          `--+--Hallucigenia [Hallucigeniidae]SO-H14
             |    |--*H. sparsa (Walcott 1914)C12, SO-H14, HM04 [=Canadia sparsaCM98]
             |    |--H. fortis Hou & Bergström 1995SO-H14, HM04
             |    `--+--H. hongmeiaSO-H14
             |       `--+--Miraluolishania haikouensis Liu et al. 2004LS11, C12
             |          `--Luolishania [Luolishaniidae]SO-H14
             |               `--L. longicruris Hou & Chen 1989HM04
             `--+--OrstenotubulusSO-H14
                `--+--AntennacanthopodiaSO-H14
                   `--+--IlyodesSO-H14
                      `--EuonychophoraGE05
                           |--Helenodora inopinata Thompson & Jones 1980GE05, W93
                           `--+--SuccinipatopsisLSE13
                              |--TertiapatusLSE13
                              `--+--PeripatopsidaeGE05
                                 |    |--Peripatoides novaezealandiaeRS10
                                 |    |--Ooperipatellus nanus Ruhberg 1985RS04
                                 |    |--PeripatopsisGEW01
                                 |    |    |--P. albaL31
                                 |    |    |--P. capensisGD00
                                 |    |    |--P. overbergiensisBSL19
                                 |    |    `--P. sedgwickiSK14
                                 |    `--EuperipatoidesRS10
                                 |         |--E. kanangrensisDH08
                                 |         |--E. leukartiGD00
                                 |         `--E. rowelliRS10
                                 `--PeripatidaeGE05
                                      |  i. s.: Cretoperipatus burmiticusGE05
                                      `--PeripatinaeZ54b
                                           |--MesoperipatusL88
                                           |--Speleoperipatus speleusL88, MS98
                                           |--Heteroperipatus Zilch 1954Z54a
                                           |    `--*H. engelhardi Zilch 1954Z54a
                                           |--Oroperipatus Cockerell 1908Z54b
                                           |    |--O. bluntschlii Fuhrmann 1915Z54b
                                           |    |--O. koepckei Zilch 1954Z54b
                                           |    |--O. omeyrus Marcus 1952Z54b
                                           |    |--‘Peripatus’ peruanus Grube 1876Z54b
                                           |    |--O. peruvianus (Brues 1917)Z54b
                                           |    |--O. sorotanusZ54b
                                           |    `--O. weyrauchi Marcus 1952Z54b
                                           `--Peripatus Guilding 1826Z54a
                                                |  i. s.: P. amboinensisA99
                                                |         P. clarki Dunn 1943Z54a
                                                |         P. horstiA99
                                                |         P. intermedius Bouvier 1901Z54a
                                                |         P. jamaicensisA99
                                                |         P. leuckarti [incl. P. leuckarti var. orientalis]S96
                                                |         P. oviparusS96
                                                `--P. (Epiperipatus Clark 1913)Z54a
                                                     |--‘Epiperipatus’ biolleyiGR98
                                                     `--‘Epiperipatus’ imthurmiMS98

*Type species of generic name indicated

References

[A99] Ax, P. 1999. Das System der Metazoa II. Ein Lehrbuch der phylogenetischen Systematik. Gustav Fisher Verlag: Stuttgart (translated: 2000. Multicellular Animals: The phylogenetic system of the Metazoa vol. 2. Springer).

[BSL19] Ballesteros, J. A., C. E. Santibáñez López, Ľ. Kováč, E. Gavish-Regev & P. P. Sharma. 2019. Ordered phylogenomic subsampling enables diagnosis of systematic errors in the placement of the enigmatic arachnid order Palpigradi. Proceedings of the Royal Society of London Series B—Biological Sciences 286: 20192426.

Bengtson, S., S. C. Matthews & V. V. Missarzhevsky. 1986. The Cambrian netlike fossil Microdictyon. In: Hoffman, A., & M.H. Nitecki (eds) Problematic Fossil Taxa pp. 97–115. Oxford University Press, New York.

[C12] Chen, J.-Y. 2012. Evolutionary scenario of the early history of the animal kingdom: evidence from Precambrian (Ediacaran) Weng’an and Early Cambrian Maotianshan biotas, China. In: Talent, J. A. (ed.) Earth and Life: Global biodiversity, extinction intervals and biogeographic perturbations through time pp. 239–379. Springer.

[CA06] Clausen, S., & J. J. Álvaro. 2006. Skeletonized microfossils from the Lower–Middle Cambrian transition of the Cantabrian Mountains, northern Spain. Acta Palaeontologica Polonica 51 (2): 223–238.

[CM98] Conway Morris, S. 1998. The Crucible of Creation. Oxford University Press: Oxford.

[DH08] Dunn, C. W., A. Hejnol, D. Q. Matus, K. Pang, W. E. Browne, S. A. Smith, E. Seaver, G. W. Rouse, M. Obst, G. D. Edgecombe, M. V. Sørensen, S. H. D. Haddock, A. Schmidt-Rhaesa, A. Okusu, R. M. Kristensen, W. C. Wheeler, M. Q. Martindale & G. Giribet. 2008. Broad phylogenomic sampling improves resolution of the animal tree of life. Nature 452: 745–749.

Dzik, J. 2003. Early Cambrian lobopodian sclerites and associated fossils from Kazakhstan. Palaeontology 46 (1): 93–112.

[GD00] Giribet, G., D. L. Distel, M. Polz, W. Sterrer & W. C. Wheeler. 2000. Triploblastic relationships with emphasis on the acoelomates and the position of Gnathostomulida, Cycliophora, Plathelminthes, and Chaetognatha: a combined approach of 18S rDNA sequences and morphology. Systematic Biology 49: 539–562.

[GEW01] Giribet, G., G. D. Edgecombe & W. C. Wheeler. 2001. Arthropod phylogeny based on eight molecular loci and morphology. Nature 413: 157–161.

[GR98] Giribet, G., & C. Ribera. 1998. The position of arthropods in the animal kingdom: a search for a reliable outgroup for internal arthropod phylogeny. Molecular Phylogenetics and Evolution 9: 481–488.

[GE05] Grimaldi, D., & M. S. Engel. 2005. Evolution of the Insects. Cambridge University Press: New York.

[H62] Häntzschel, W. 1962. Trace fossils and problematica. In: Moore, R. C. (ed.) Treatise on Invertebrate Paleontology pt W. Miscellanea: Conodonts, Conoidal Shells of Uncertain Affinities, Worms, Trace Fossils and Problematica pp. W177–W245. Geological Society of America, and University of Kansas Press.

[HM04] Hou, X.-G., X.-Y. Ma, J. Zhao & J. Bergström. 2004. The lobopodian Paucipodia inermis from the Lower Cambrian Chengjiang fauna, Yunnan, China. Lethaia 37: 235–244.

[L95] Landing, E. 1995. Upper Placentian–Branchian series of mainland Nova Scotia (middle–upper Lower Cambrian): faunas, paleoenvironments, and stratigraphic revision. Journal of Paleontology 69 (3): 475–495.

[L31] Lawrence, R. F. 1931. The harvest-spiders (Opiliones) of South Africa. Annals of the South African Museum 29 (2): 341–508.

[LSE13] Legg, D. A., M. D. Sutton & G. D. Edgecombe. 2013. Arthropod fossil data increase congruence of morphological and molecular phylogenies. Nature Communications 4: 2485.

[L88] Liebherr, J. K. 1988. General patterns in West Indian insects, and graphical biogeographic analysis of some circum-Caribbean Platynus beetles (Carabidae). Systematic Zoology 37 (4): 385–409.

Liu, J., D. Shu, J. Han, Z. Zhang & X. Zhang. 2008. Origin, diversification, and relationships of Cambrian lobopods. Gondwana Research 14: 277–283.

[LS11] Liu, J., M. Steiner, J. A. Dunlop, H. Keupp, D. Shu, Q. Ou, J. Han, Z. Zhang & X. Zhang. 2011. An armoured Cambrian lobopodian from China with arthropod-like appendages. Nature 470: 526–530.

[MS98] Margulis, L., & K. V. Schwartz. 1998. Five Kingdoms: An Illustrated Guide to the Phyla of Life on Earth 3rd ed. W. H. Freeman and Company: New York.

[O81] O’Brien, C. O. 1981. A. A. Book of New Zealand Wildlife: A guide to the native and introduced animals of New Zealand. Lansdowne Press: Auckland.

[RS04] Regier, J. C., J. W. Shultz, R. E. Kambic & D. R. Nelson. 2004. Robust support for tardigrade clades and their ages from three protein-coding nuclear genes. Invertebrate Biology 123 (2): 93–100.

[RS10] Regier, J. C., J. W. Shultz, A. Zwick, A. Hussey, B. Ball, R. Wetzer, J. W. Martin & C. W. Cunningham. 2010. Arthropod relationships revealed by phylogenomic analysis of nuclear protein-coding sequences. Nature 463: 1079–1083.

[SK14] Sharma, P. P., S. T. Kaluziak, A. R. Pérez-Porro, V. L. González, G. Hormiga, W. C. Wheeler & G. Giribet. 2014. Phylogenomic interrogation of Arachnida reveals systemic conflicts in phylogenetic signal. Molecular Biology and Evolution 31 (11): 2963–2984.

[SO-H14] Smith, M. R., & J. Ortega-Hernández. 2014. Hallucigenia‘s onychophoran-like claws and the case for Tactopoda. Nature 514: 363–366.

[S96] Steel, T. 1896. Observations on peripatus. Proceedings of the Linnean Society of New South Wales 21 (1): 94–103.

[W93] Wills, M. A. 1993. Miscellania. In: Benton, M. J. (ed.) The Fossil Record 2 pp. 555–560. Chapman & Hall: London.

[WS93] Wills, M. A., & J. J. Sepkoski Jr. 1993. Problematica. In: Benton, M. J. (ed.) The Fossil Record 2 pp. 543–554. Chapman & Hall: London.

[Z54a] Zilch, A. 1954a. Onychophoren aus El Salvador. Senckenbergiana Biologica 35: 147–150.

[Z54b] Zilch, A. 1954b. Ein neuer Oroperipatus aus Peru (Onychophora). Senckenbergiana Biologica 35: 151–154.

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