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Hallucigenia sparsa
3D animation of Hallucigenia sparsa.
Animation by Phlesch Bubble © Royal Ontario Museum
Taxonomy:
Hallucigenia is one of a variety of lobopodian taxa from the Cambrian, which are early members of the lineage that gave rise to arthropods, and whose only modern survivors are onychophorans (velvet worms) and tardigrades (water bears). Lobopodians characteristically have annulated, unjointed bodies and bear soft limbs after which they are called: the lobopods. Hallucigenia defines the family Hallucigeniidae, which includes other “tentacular” lobopodians with dorsal plates or spines. Hallucigeniidae are found to be basal lobopodians by some (Caron & Aria 2017, 2020), or allied with onychophorans by others (Smith & Ortega-Hernandez 2014; Yang et al. 2015201; Howard et al. 2020).
Genus – from the Latin hallucinatus, “wandering of the mind,” after the bizarreness of the animal. The origin of the suffix is unclear, but perhaps -genus, “sprung from”.
species – from the Latin sparsus, “rare, or scattered,” reflecting the rarity of the specimens available in the original study.
Burgess Shale and vicinity: None.
Other deposits: H. fortis from the early Cambrian Stage 3 Chengjiang biota (Hou & Bergström 1995) and H. hongmeia from the early Cambrian Stage 4 Guanshan biota (Steiner et al. 2012).
Age & Localities:
The Walcott and Raymond Quarries on Fossil Ridge. The Tulip Beds (S7) on Mount Stephen.
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 & 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). It was later pointed out that hallucigeniid spines were geographically widespread, and were found from the Terreneuvian (early Cambrian) to through the Wuliuan (middle Cambrian) (Caron et al. 2013). These spines display a “cone-in-cone” growth structure comparable to the sclerotic elements of modern onychophorans (Smith & Ortega-Hernandez 2014), a condition now shown to be ancestral in lobopodians (Caron & Aria 2020). Hallucigenia sparsa was finally redescribed in full by Smith and Caron (2015), who documented the presence of eyes and of a ring of plates inside the mouth.
Description:
Hallucigenia has a worm-like body with a small head at the end of a long neck, which bears three pairs of very thin, “tentacle”-like lobopods; the trunk bears seven pairs of long dorsal spines covering the last pair of tentacles and 6 pairs of somewhat stouter but still elongate limbs, ending in strong claws. The rear-most pair of legs does not have a corresponding pair of spines above. The spines are embedded inside skin “sheaths” on the dorsal side of the animal. The head is slightly swollen and bears a pair of simple dorsal eyes as well as a terminal mouth. Around the pharyngeal opening is a ring of many small plates, and the pharyngeal wall itself is lined with a multitude of small teeth.
The redescription by Conway Morris (1977) was based on about 30 specimens, but 132 were later studied by Smith and Caron (Smith & Caron 2015). Overall, Hallucigenia is rare, and in the Walcott Quarry it represents 0.19% of the specimens counted in the community (Caron and Jackson, 2008).
Ecology:
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. Recently, morphological characteristics—including the frontal tentacles, as well as long, slender lobopods in association with stout claws—have prompted the proposition that Hallucigenia and its relatives were suspension feeders (Caron & Aria 2017), which would have then used sponges as substrate to stand erect in the water column.
References:
- CARON, J. and ARIA, C. 2020. The Collins’ monster, a spinous suspension‐feeding lobopodian from the Cambrian Burgess Shale of British Columbia. Palaeontology, 63, 979–994.
- CARON, J. B., SMITH, M. and HARVEY, T. H. P. 2013. Beyond the Burgess Shale: Cambrian microfossils track the rise and fall of hallucigeniid lobopodians. Proceedings of the Royal Society B: Biological Sciences, 280, 20131613.
- CARON, J.-B. and ARIA, C. 2017. Cambrian suspension-feeding lobopodians and the early radiation of panarthropods. BMC Evolutionary Biology, 17, 29.
- CONWAY MORRIS, S. 1977. A new metazoan from the Cambrian Burgess Shale of British Columbia. Palaeontology, 20, 623–640.
- HOU, X. and BERGSTRÖM, J. 1995. Cambrian lobopodians–ancestors of extant onychophorans? Zoological Journal of the Linnean Society, 114, 3–19.
- HOWARD, R. J., HOU, X., EDGECOMBE, G. D., SALGE, T., SHI, X. and MA, X. 2020. A tube-dwelling early Cambrian lobopodian. Current Biology, 30, 1529-1536.e2.
- RAMSKÖLD, L. 1992. The second leg row of Hallucigenia discovered. Lethaia, 25, 221–224.
- RAMSKÖLD, L. and HOU, X. G. 1991. New early Cambrian animal and onychophoran affinities of enigmatic metazoans. Nature, 351, 225–228.
- SMITH, M. R. and ORTEGA-HERNANDEZ, J. 2014. Hallucigenia’s onychophoran-like claws and the case for Tactopoda. Nature, 514, 363–366.
- SMITH, M. R. and CARON, J.-B. 2015. Hallucigenia’s head and the pharyngeal armature of early ecdysozoans. Nature, 523, 75–78.
- STEINER, M., HU, S. X., LIU, J. and KEUPP, H. 2012. A new species of Hallucigenia from the Cambrian Stage 4 Wulongqing Formation of Yunnan (South China) and the structure of sclerites in lobopodians. Bulletin of Geosciences, 87, 107–124.
- WALCOTT, C. 1911. Cambrian Geology and Paleontology II. Middle Cambrian annelids. Smithsonian Miscellaneous Collections, 57(5), 109–145.
- ———. 1931. Addenda to descriptions of Burgess Shale fossils. Smithsonian Miscellaneous Collections, 85(3), 1–46.
- YANG, J., ORTEGA-HERNANDEZ, J., GERBER, S., BUTTERFIELD, N. J., HOU, J., LAN, T. and ZHANG, X. 2015. A superarmored lobopodian from the Cambrian of China and early disparity in the evolution of Onychophora. Proceedings of the National Academy of Sciences of the United States of America, 112, 8678–8683.