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Hallucigenia is a genus of Cambrian animals known from articulated fossils in exceptional Burgess Shale-type deposits in Canada and China, and from isolated spines around the world. Its quirky name reflects its unusual appearance and eccentric history of study; when it was erected as a genus, the animal was reconstructed upside down and back to front. Hallucigenia is now recognized as a "lobopodian worm" and is considered to represent an early ancestor of the living velvet worms, close relatives of arthropods.

Discovery[]

The genus name was coined by Simon Conway Morris when he re-examined the various specimens of Charles Walcott's Burgess Shale worm genus Canadia in 1979. Conway Morris found that what Walcott had called one genus in fact included several quite different animals. One of them was so unusual that nothing about it made sense. Since the species wasn't a worm, Conway Morris had to come up with a new name to replace Canadia. He named the species Hallucigenia sparsa because of its "bizarre and dream-like quality" (like a hallucination). Hallucigenia was initially considered by Stephen Jay Gould to be unrelated to any living species, but most paleontologists now believe that the species was a relative of modern arthropods along with Anomalocaris and Opabinia. Other Lobopods from the Burgess Shale includes Anomalocaris, Opabinia, and Aysheaia. It's closely related to regular onychophorans and even Aysheaia. A another animal that also appeared in the Burgess Shale Burgessochaeta was even mistaken as a species of Canadia. When first discovered, it was often displayed upside-down.

Description[]

20210000 Hallucigenia diagrammatic reconstruction

Reconstructions of H. fortis, H. hongmeia, and H. sparsa in scale.

Hallucigenia is a 0.5—3.5 cm long tubular organism with seven or eight pairs of slender legs, each terminating with a pair of claws. Above each leg is a rigid conical spine. The 'head' and 'tail' end of the organism are difficult to identify; one end extends some distance beyond the legs and often droops down as if to reach the floor. Although some specimens display traces of a gut, the internal anatomy has not been formally described.

Hallucigenia's spines are made up of one to four nested elements. Their surface is covered in an ornament of minute triangular 'scales'.

Hallucigenia is unlike nearly any living animal today, although it was likely the ancestor to modern day arthropods. There were over 109 specimens of these strange aquatic creatures, and they ranged in size from 0.5 to 3 cm long. It had a round, worm-like body that was likely squishy to the touch.

Like arthropods and worms, it was an invertebrate, so it didn't have a backbone, however it did have hard, sharp spikes that stuck out of its back and likely kept potential predators away. It also had clawed, tentacle-like appendages that helped it move around the ocean floor. It had two tentacles out in front that were likely adapted to feeling its way around the bottom of the seas it lived in, and making sure it was going in the right direction, as Hallucigenia had only simple eyes on its face. Hallucigenia sparsa was likely a prey of Hurdia and Opabinia.

In 2015, the head of Hallucigenia was found showing it had a very different head than previously thought. What was originally thought to be the head was really just a stain of fluids that got out of the creature when it fossilized. The real head had a pair of simple eyes, and weirdly, what seemed to be a grin on its face.

It was a tubular animal with ten pairs of lobopods, the first two or three being slender and basic in form, with the remaining pairs ending in one or two claws. Seven pairs of conical sclerites were organized above the trunk and corresponding with the third through ninth pairs. The trunk is also either simple in form in H. sparsa or heteronomous annulations, as in H. fortis and H. hongmeia. One end sits away from the limbs and often reaches towards the substrate, making identification of each end difficult. Some fossils preserve what once was a gut. In the 2010's, the longer end was determined to be the head, possessing an anteroventral mouth and what may have been a pair of primitive eyes. H. sparsa has an elongate head whereas H. fortis is rounded and H. hongmeia is to be determined. In H. sparsa, the head has radial and pharyngeal teeth in the front of the gut. The dorsal sclerites comprise four nested parts, with the surface of H. sparsa being ornamented by small triangular 'scales', and H. hongmeia being net-like with circular, microscopic openings that may be the remnants of papillae.

History of study[]

20210917 Hallucigenia sparsa interpretations

Various interpretations of Hallucigenia sparsa throughout the history of study

Hallucigenia was originally described by 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, establishing the new genus. No specimen was available that showed both rows of legs, and as such 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. This raised questions such as how it would walk on the stiff legs, but it was accepted as the best available interpretation. A picture of the animal as reconstructed by Morris can be found here.

HallucigeniaSparsa-ROM-June11-10

Specimen with obvious spines

An alternative interpretation considered Hallucigenia to be an appendage of a larger, unknown animal. There had been precedent for this, as the species Anomalocaris had been originally identified as three separate creatures before being identified as a single huge (for its time) 3-foot-long (0.91 m) creature. Given the uncertainty of its taxonomy, Hallucigenia was tentatively placed within the phylum Lobopodia, a catch-all clade containing numerous odd "worms with legs."

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 an Onychophoran (velvet worm). 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 the '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.[5] 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.

Affinity[]

H

Restoration of H. sparsa

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 Hallucigeniaclaws, 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 Hallucigeniaaccording to Yang et al., 2015:

Microdictyon
Cardiodictyon
Hallucigenia sparsa
Hallucigenia fortis
Hallucigenia hongmeia
Luolishaniidae
Acinocricus
Collinsovermis
Luolishania
Collinsium

Paleobiology[]

Collins (2002), informally, postulated that new fossils from the Burgess shale represent male and female forms; "a rigid trunk, robust neck and a globular head" and the other being more slender and smaller in the head. Three species are known: H. sparsa, the type from the Burgess Shale of British Columbia, comprising 109 specimens (0.3% of the overall community) of the Greater Phyllopod bed; H. fortis and H. hongmeia of the Maotianshan Shales, Chenjiang, China. The Chinese lagerstätten specimens are a minor element of the fauna. Unlike those, isolate sclerites are widely known in Cambrian faunas, known from carbonaceous and mineralized specimens.

JPInstitute.com Description[]

This was a small, strange creature that lived about 550 million years ago in the Paleozoic era. It is so unique that there is almost nothing in the modern world to which it can be compared. It was only one inch long and for some years there was confusion about which of its appendages were legs and which were spines (or some other type of projection). It was only after other creatures with soft legs were identified that scientists think they know which end was up. They still aren't able to determine, however, which was the front end and which was the back end of this little sea-dweller.

References[]

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