Xenusiid

Class Xenusia, the Xenusiids, represents the subset of lobopodian worms that fall in the stem-lineage of Onychophora.[2] Their type genus is Xenusion.[3] They have relatively large, annulated, cylindrical bodies. Their lobopod legs have tubercles at their bases. Some have large frontal appendages,[3] although these may represent taphonomic artefacts.[4] Their mouth is terminal or subterminal, and they are marine.[5] They probably represent a grade (paraphyletic group) rather than a clade (monophyletic group).

Xenusiids Temporal range: Early Cambrian to Carboniferous PreꞒ Ꞓ O S D C P T J K Pg N
Hallucigenia hongmeia
Scientific classification
Kingdom:	Animalia
(unranked):	Panarthropoda
Phylum:	†"Lobopodia"
Class:	†Xenusia Dzik & Krumbiegel, 1989[1]
Orders
See text.

Systematics

Xenusia includes the following orders and families:[5]

Order Archonychophora

• Archonychophora Hou & Bergstrom, 1995[2] – undifferentiated appendages; each segment the same as each other.[6]
  • Luolishaniidae Hou & Bergstrom, 1995 – three small rounded sclerites per segment – these are thorn-shaped spines.[6]
    • Luolishania Hou & Chen 1989
    • Miraluolishania Liu & Shu 2004
  • Paucipodiidae Hou et al., 2004[7] – with no dorsal sclerites, two claws, fine annulation and few segments.
    • Paucipodia Hou et al. 2004

These taxa are only known from the Chengjiang (Cambrian Stage 2 Series 3).

Order Protonychophora

• Protonychophora Hutchinson, 1930 – spiny legs, reduced tail.
  • Aysheaiidae Walcott, 1911 – many claws on each leg. Anterior grasping appendages with long spines.
    • Aysheaia Walcott 1911
  • Xenusiidae Dzik & Krumbiegel, 1989 – >20 leg-bearing segments; paired, rounded sclerites on each segment; spiny legs.
    • Xenusion Pompeckj, 1927
    • Jianshanopodia Liu et al.[3] – from Chengjiang; two rows of tubercles but no obvious sclerites.
    • Hadranax Budd & Peel 1998 – from the Sirius Passet. Lacks obvious dorsal armature, but bears four 'nodes' per row.

Order Scleronychophora

• Scleronychophora Hou & Bergstrom, 1995 – paired sclerites; elongated head sclerites.
  • Eoconchariidae Hou & Shu, 1987 – ~10 leg-bearing segments, small head, sieve-like sclerites, varying in shape along body; curved claws on annulated legs; terminal mouth and anus.
    • Microdictyon Bengtson et al.
    • Quadratapora Hao and Shu, 1987
    • Fusuconcharium Hao and Shu, 1987
  • Hallucigeniidae Conway Morris, 1977 – ~10 leg-bearing segments; large sclerites covering head; sclerites on each segment are long spines.
    • Hallucigenia Conway Morris 1977 – from the Chengjiang, Kaili and Burgess Shale.[8]
  • Cardiodictyidae Hou & Bergstrom, 1995 – many segments (~23); large sclerites cover head; hexagonal sclerite on each body segment.
    • Cardiodictyon Hou et al. 1991 – known from the Chengjiang.[8]

Order Paronychophora

• Paronychophora Hou & Bergstrom, 1995 – short, downwards facing head; papillae on body and legs, some arranged in rows; shield-like sclerites on head; dorsal armature of spines;[9] claw-like jaws; annulated legs [Note: diagnosis modified in [6]].
  • Onychodictyidae Hou & Bergstrom, 1995
    • Onychodictyon Hou et al. 1991

Order unassigned

• Orstenotubulus Maas et al. 2007 – from the Furongian Orsten deposits – with retractable dorsal spines.[10]
• Carbotubulus Haug et al. 2012[11] – from the Mazon creek. Dorwal armature uncertain. Few segments, long limbs.
• Mureropodia[12] from the Stage 2 Murero lagerstatten, Spain.

'Phylum' Onychophora

Onychophora are distinguished by their terrestrial habit, their ventral mouth; their antennae, jaws and oral papillae; they seem to be most closely related to the Paronychophora. Their first fossils are Carboniferous (Helenodora), although they may have had a cryptic earlier history.

References

1. Dzik, J.; Krumbiegel, G. N. (1989). "The oldest 'onychophoran' Xenusion: A link connecting phyla?". Lethaia. 22 (2): 169–181. doi:10.1111/j.1502-3931.1989.tb01679.x.
2. Hou, X.; Bergström, J. A. N. (1995). "Cambrian lobopodians-ancestors of extant onychophorans?". Zoological Journal of the Linnean Society. 114: 3–19. doi:10.1111/j.1096-3642.1995.tb00110.x.
3. Jianni Liu Degan Shu, Jian Han, Zhifei Zhang & Xingliang Zhang (2006). "A large xenusiid lobopod with complex appendages from the Lower Cambrian Chengjiang Lagerstätte" (PDF). Acta Palaeontologica Polonica. 51 (2): 215–222.CS1 maint: multiple names: authors list (link)
4. Julián Monge-Nájera & Xianguang Hou; Hou (2002). "Experimental taphonomy of velvet worms (Onychophora) and implications for the Cambrian "explosion, disparity and decimation" model" (PDF). Revista de Biología Tropical. 50 (3–4): 1133–1138. PMID 12947596. Archived from the original (PDF) on June 29, 2011.
5. George Poinar, Jr. (2000). "Fossil onychophorans from Dominican and Baltic amber: Tertiapatus dominicanus n.g., n.sp. (Tertiapatidae n.fam.) and Succinipatopsis balticus n.g., n.sp. (Succinipatopsidae n.fam.) with a proposed classification of the subphylum Onychophora". Invertebrate Biology. 119 (1): 104–109. doi:10.1111/j.1744-7410.2000.tb00178.x. JSTOR 3227105.
6. Ma, X.; Hou, X.; Bergström, J. (2009). "Morphology of Luolishania longicruris (Lower Cambrian, Chengjiang Lagerstätte, SW China) and the phylogenetic relationships within lobopodians". Arthropod Structure & Development. 38 (4): 271–291. doi:10.1016/j.asd.2009.03.001.
7. Xian-Guang Hou, Xiao-Ya Ma, Jie Zhao & Jan Bergström; Ma; Zhao; Bergström (2004). "The lobopodian Paucipodia inermis from the Lower Cambrian Chengjiang fauna, Yunnan, China". Lethaia. 37 (3): 235–244. doi:10.1080/00241160410006555.CS1 maint: multiple names: authors list (link)
8. Whittle, R. J.; Gabbott, S. E.; Aldridge, R. J.; Theron, J. (May 2009). "An Ordovician Lobopodian from the Soom Shale Lagerstätte, South Africa". Palaeontology. 52 (3): 561–567. doi:10.1111/j.1475-4983.2009.00860.x.
9. Liu, J.; Shu, D.; Han, J.; Zhang, Z.; Zhang, X. (2008). "The Lobopod Onychodictyon from the Lower Cambrian Chengjiang Lagerstätte Revisited". Acta Palaeontologica Polonica. 53 (2): 285–292. doi:10.4202/app.2008.0209.
10. Maas, A.; Mayer, G.; Kristensen, R. M.; Waloszek, D. (2007). "A Cambrian micro-lobopodian and the evolution of arthropod locomotion and reproduction". Chinese Science Bulletin. 52 (24): 3385–3392. doi:10.1007/s11434-007-0515-3.
11. Haug, J. T.; Mayer, G.; Haug, C.; Briggs, D. E. G. (2012). "A Carboniferous Non-Onychophoran Lobopodian Reveals Long-Term Survival of a Cambrian Morphotype". Current Biology. 22 (18): 1673–1675. doi:10.1016/j.cub.2012.06.066. PMID 22885062.
12. Gámez Vintaned, J. A.; Liñán, E.; Zhuravlev, A. (29 June 2011). "A New Early Cambrian Lobopod-Bearing Animal (Murero, Spain) and the Problem of the Ecdysozoan Early Diversification". Evolutionary Biology – Concepts, Biodiversity, Macroevolution and Genome Evolution. pp. 193–219. doi:10.1007/978-3-642-20763-1_12. ISBN 978-3-642-20762-4.