Class Xenusia, the Xenusiids, represents the subset of lobopodian worms that fall in the stem-lineage of Onychophora. Their type genus is Xenusion. They have relatively large, annulated, cylindrical bodies. Their lobopod legs have tubercles at their bases. Some have large frontal appendages, although these may represent taphonomic artefacts. Their mouth is terminal or subterminal, and they are marine. They probably represent a grade (paraphyletic group) rather than a clade (monophyletic group).
Xenusia includes the following orders and families:
Archonychophora Hou & Bergstrom, 1995 – undifferentiated appendages; each segment the same as each other.Luolishaniidae Hou & Bergstrom, 1995 – three small rounded sclerites per segment – these are thorn-shaped spines.Luolishania Hou & Chen 1989
Miraluolishania Liu & Shu 2004
Paucipodiidae Hou et al., 2004 – 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).
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. – 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.
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.
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.
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; claw-like jaws; annulated legs [Note: diagnosis modified in ].
Onychodictyidae Hou & Bergstrom, 1995
Onychodictyon Hou et al. 1991
Orstenotubulus Maas et al. 2007 – from the Furongian Orsten deposits – with retractable dorsal spines.
Carbotubulus Haug et al. 2012 – from the Mazon creek. Dorwal armature uncertain. Few segments, long limbs.
Mureropodia from the Stage 2 Murero lagerstatten, Spain.
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.
