Hallucigenia

Description

Hallucigenia is a 0.5 - long tubular animal with up to ten pairs of slender legs (lobopods). The first 2 or 3 leg pairs are slender and featureless, while the remaining 7 or 8 pairs each terminate with 1 or 2 claws. Above the trunk region are 7 pairs of rigid conical sclerites (spines) corresponding to the 3rd–9th leg pairs. The trunk is either featureless (H. sparsa) or divided by heteronomous annulations (H. fortis and H. hongmeia). The "head" and "tail" end of the animal are difficult to identify; one end extends some distance beyond the legs and often droops down as if to reach the substrate. Some specimens display traces of a simple gut.

Research in the mid-2010s clarified that the longer end is a head with anteroventral mouth and at least a pair of simple eyes. The shape of head differs between species – elongated in H. sparsa, rounded in H. fortis, while those of H. hongmeia remain unknown. At least in H. sparsa, the head possesses radial teeth and pharyngeal teeth within the front of the gut.

Hallucigenia spines are made up of one to four nested elements. The spine surface of H. sparsa is covered in an ornament of minute triangular "scales", while the spine surface of Hallucigenia hongmeia is a net-like texture of microscopic circular openings, which can be interpreted as the remains of Papillae.

History of study

Hallucigenia sparsa was originally described by Charles Walcott as a species of the polychaete worm Canadia. In his 1977 redescription of the organism, Simon Conway Morris recognized the animal as something quite distinct, for which he proposed the name Hallucigenia because of the "bizarre and dream-like appearance of the animal." No specimen was available that showed both rows of legs, so Conway Morris reconstructed the animal walking on its spines, with its single row of legs interpreted as tentacles on the animal's back. A dark stain at one end of the animal was interpreted as a featureless head. Only the forward tentacles could easily reach to the "head", meaning that a mouth on the head would have to be fed by passing food along the line of tentacles. Conway Morris suggested that a hollow tube within each of the tentacles might be a mouth.

An alternative interpretation considered Hallucigenia to be an appendage of a larger, unknown animal. There had been precedent for this, as Anomalocaris had been originally identified as three separate creatures before being identified as a single huge (for its time) 1.12 ft to 1.24 ft creature.

In 1991, Lars Ramskold and Hou Xianguang, working with additional specimens of a "hallucigenid", Microdictyon, from the lower Cambrian Maotianshan shales of China, reinterpreted Hallucigenia as a lobopodian, a legged worm-like taxon which at that time were still thought to be exclusively related to onychophoran (velvet worm), carnivorous animals that resemble a caterpillar and shoot a sticky substance from their papillae to ensnare their prey. They inverted it, interpreting the tentacles, which they believe to be paired, as walking structures and the spines as protective. Further preparation of fossil specimens showed that "second legs" were buried at an angle to the plane along which the rock had split, and could be revealed by removing the overlying sediment. Ramskold and Hou also believe that the blob-like "head" is actually a stain that appears in many specimens, not a preserved portion of the anatomy. This stain may be an artifact of decomposition.

Affinity

Since the revisions around 1990s, Hallucigenia is unquestionably a lobopodian panarthropod, although the relationship with other panarthropods remains controversial. Hallucigenia has long been interpreted as a stem-group onychophoran (velvet worms) – a position that has found support from multiple phylogenetic analysis. A key character demonstrating this affinity is the cone-in-cone construction of Hallucigenia claws, a feature shared only with modern onychophorans. On the other hand, some analysis rather support the position of Hallucigenia as a basal panarthropod outside of onychophoran stem-group. Under this scenario, the cone-in-cone structure shared between Hallucigenia and onychophorans represent panarthropod plesiomorphy. Hallucigenia also exhibits certain characters inherited from the ancestral ecdysozoan, but lost in the modern onychophorans – in particular its distinctive foregut armature.

Below is a cladogram for Hallucigenia according to Yang et al., 2015:

Many studies show that the Hallucigenia genus may be paraphyletic.

Diversity

In 2002, Desmond Collins informally suggested that new Hallucigenia fossils from the Burgess Shale showed male and female forms, one with "a rigid trunk, robust neck and a globular head" and the other thinner, and with a small head.

Three species of Hallucigenia have been described. The first specimen, Hallucigenia sparsa, was discovered in Canada. Two other species, H. fortis and H. hongmeia, are represented by the Maotianshan Shales' fossils of Chengjiang.

Distribution

Hallucigenia was first described from the Burgess Shale in southeastern British Columbia, Canada. 109 specimens of Hallucigenia are known from the Greater Phyllopod bed, where they comprise 0.3% of the community. Hallucigenia also forms a minor component of Chinese lagerstätten. Isolated hallucigeniid spines, however, are widely distributed in a range of Cambrian deposits, preserved both as carbonaceous and mineralized fossils.

In popular culture

The bizarre appearance of Hallucigenia has inspired a number of science fiction creators when designing otherworldly or primordial creatures. Some examples include:

• Attack on Titan: The biological common ancestor of all life, which acts as a supernatural mediator of biological history on Earth, takes heavy inspiration in its design from Hallucigenia, reflecting its ancient origins. When Hallucigenia parasitically enters Ymir, the Titan race is created.
• The Imago Sequence: One of the short stories in this horror sci-fi anthology by Laird Barron is named after Hallucigenia, where it is the evolutionary ancestor and conclusion of humanity. The antagonist, Kaleb Choate, supercharges his evolution using novel quantum technology to turn into an Eldritch abomination that resembles Hallucigenia.

References

References

1. Conway Morris, S.. (1977). "A new metazoan from the Cambrian Burgess Shale of British Columbia". Palaeontology.
2. (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.
3. Zimmer, Carl. (2 July 2015). "The Cambrian Explosion's Strange-Looking Poster Child". [[The New York Times]].
4. Ortega-Hernández, Javier. (2015-10-05). "Lobopodians". Current Biology.
5. (2014-03-15). "Cambrian lobopodians: A review of recent progress in our understanding of their morphology and evolution". Palaeogeography, Palaeoclimatology, Palaeoecology.
6. (2018). "A three-dimensionally preserved lobopodian from the Herefordshire (Silurian) Lagerstätte, UK". Royal Society Open Science.
7. (1995-05-01). "Cambrian lobopodians-ancestors of extant onychophorans?". Zoological Journal of the Linnean Society.
8. (2012-09-01). "Morphology of Cambrian lobopodian eyes from the Chengjiang Lagerstätte and their evolutionary significance". Arthropod Structure & Development.
9. (2015). "''Hallucigenia''{{'}}s head and the pharyngeal armature of early ecdysozoans". Nature.
10. [[Rebecca Morelle. Morelle, Rebecca]], [https://www.bbc.com/news/science-environment-33262884 Face of bizarre sea creature Hallucigenia revealed], BBCNews, Science and Environment, 2015.06.25
11. (September 2013). "Beyond the Burgess Shale: Cambrian microfossils track the rise and fall of hallucigeniid lobopodians". Proceedings of the Royal Society B: Biological Sciences.
12. (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.
13. WALCOTT, C. 1911. [https://repository.si.edu/handle/10088/34820 Cambrian Geology and Paleontology II. Middle Cambrian annelids]. Smithsonian Miscellaneous Collections, 57(5): 109–145.
14. Gould, Stephen, Jay. (1992). "The Reversal of Hallucigenia.". Natural History.
15. (April 1992). "The second leg row of Hallucigenia discovered". Lethaia.
16. (1991). "New early Cambrian animal and onychophoran affinities of enigmatic metazoans". Nature.
17. (2016). "Onychophoran-like musculature in a phosphatized Cambrian lobopodian". Biology Letters.
18. (2014). "''Hallucigenia''{{'}}s onychophoran-like claws and the case for Tactopoda". Nature.
19. (2017-01-31). "Cambrian suspension-feeding lobopodians and the early radiation of panarthropods". BMC Evolutionary Biology.
20. (2020). "The Collins' monster, a spinous suspension-feeding lobopodian from the Cambrian Burgess Shale of British Columbia". Palaeontology.
21. (June 2015). "A superarmored lobopodian from the Cambrian of China and early disparity in the evolution of Onychophora". Proceedings of the National Academy of Sciences.
22. Collins, Desmond. (2002). "Hallucigenia unveiled. Abstracts, Palaeontological Association, 46th annual meeting". Palaeontology Newsletter.
23. (October 2006). "Taphonomy of the Greater Phyllopod Bed community, Burgess Shale". PALAIOS.
24. (2021-02-19). "Hallucigenia – the Primordial Spinal Creature from Attack on Titan". In Asian Spaces.
25. "Hallucigenia". Laird Barron Mapping Project.