Xenusia (Order: Scleronychophora, stem group onychophorans)
Hallucigenia is regarded as a member of the “lobopodans,” a group of vermiform Cambrian organisms possessing pairs of leg-like extensions of the body. The affinities of these animals are controversial; they have been placed at the base of a clade comprised of anomalocaridids and arthropods (Budd, 1996), or in a stem-group to modern onychophorans (Ramsköld and Chen, 1998).
Hallucigenia – from the Latin hallucinatio, “wandering of the mind,” after the bizarreness of the animal.
sparsa – from the Latin sparsus, “rare, or scattered,” reflecting the rarity of the specimens available in the original study.
Holotype – USNM 83935 in the National Museum of Natural History, Smithsonian Institution, Washington, DC, USA.
Burgess Shale and vicinity: none.
Other deposits: H. fortis from the Middle Cambrian Chengjiang biota (Hou and Bergström 1995).
Middle Cambrian, Bathyuriscus-Elrathina Zone (approximately 505 million years ago).
The Walcott and Raymond Quarries on Fossil Ridge. The Tulip Beds (S7) on Mount Stephen.
Brief history of research:
Hallucigenia was originally described as “Canadia sparsa” by Walcott (1911) in a review of various Burgess Shale “annelids.” One specimen was illustrated twenty years later (Walcott, 1931), but the first thorough study of this animal wasn’t published until Conway Morris (1977) demonstrated that it did not belong to the genus Canadia or to the annelids at all. His reconstruction showed a bizarre animal walking on spines, with dorsal tentacles interpreted as a feeding apparatus (Conway Morris, 1977). The new genus name Hallucigenia was coined in reference to this “dreamlike” appearance and also reflected the organism’s uncertain affinities. It was later shown that the supposed tentacles represented just one row of paired “legs” – the others were buried under a layer of rock and the paired spines were on the dorsal surface (Ramsköld and Hou, 1991, Ramsköld, 1992). The anteroposterior orientation was also reversed, with the former head interpreted as possible decay fluids seeping from the body (Ramsköld, 1992).
Hallucigenia has a worm-like body with a small head at the end of a long neck; the trunk bears seven pairs of long dorsal spines and seven pairs of slender leg-like lobes. The spacing between lobes and spines is relatively constant. The spine pairs are shifted forward so that the posterior pair of legs does not have a corresponding pair of spines above. Each leg terminates in a pair of claws and the rigid spines have inflexible basal plates. The neck area bears two or three pairs of very fine anterior “appendages” lacking terminal claws. The head is indistinct but the mouth is anterior; a straight gut ends in a posterior anus. It is possible the posterior end is in fact more bulbous than previously thought.
About thirty specimens were studied by Conway Morris (1977). Overall, Hallucigenia is rare, and in the Walcott Quarry it represents 0.19% of the specimens counted in the community (Caron and Jackson, 2008).
Hallucigenia is often found in association with the sponge Vauxia and other organic debris. This co-occurrence has led to suggestions that Hallucigenia fed on sponges, using its clawed legs to hang on, with its spines protecting it from predation. It is also possible that Hallucigenia scavenged on decaying animal remains.