Hallucigenia

Top-to-bottom: H. fortis, H. hongmeia and H. sparsa. Credit: Junnn11 on the Wikimedia Commons.

## Statistics

### Species

*H. sparsa
H. fortis
H. hongmeia*

### Diet

Detritivorous

*Hallucigenia* is an extinct genus of hallucigeniid panarthropods that lived in British Columbia and China from the Early to Middle Cambrian.

## History

It was described by Charles Walcott as a polychaete *Canadia sparsa*. Morris (1977) redescribed the fossil and coined a new genus, and since no fossil showing simultaneous sclerites and legs was yet known, he depicted it as moving with its sclerites. A dark stain near the end of the animal was also suggested to be a head, featureless, envisioning the mouth being fed by a line of limbs, since only the forward pairs would have been able to reach the head. Morris also thought that hollow tubes within each limb may have been a mouth, which led to others wondering how it may have walked on stiff legs. This was eventually accepted, being the best hypothesis. Others suggested that the fossil may have been part of a larger creature, much like what *Anomalocaris* was found to be. Ramsold and Xianguang (1991) reinterpreted it as a lobopod when working with specimens of *Microdictyon*, inverting the animal by suggesting the paired limbs were for walking and the sclerites for defense. They also postulated that the stain of many specimens was not anatomical. Further preparation of specimens would confirm their belief that the limbs were paired by peeling away overlying sediment to find that the animal was buried in a strange way that would obscure this element of its anatomy. The stain was then thought to be a decompositional artifact.

## Description

* Collection wall with a *Hallucigenia* specimen at the Royal Ontario Museum.
**Credit: Clumsystiggy on DeviantArt.**
* *Hallucigenia sparsa* bearing visible sclerites.
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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.

## Classification

Since revisions in the 1990s, it has been consistently called a lobopodian panarthropod or uncertain standing. It has been, historically, considered stem to onychophorans due to phylogenetic analysis, most notably in the cone-in-cone structure of its claws. However, it may also be basal to panarthropods outside of the velvet worm stem clade. This hypothesis sees this trait was plesiomorphous to Panarthropoda. It also has details shared with the ancestral ecdysozoan and lost in velvet worms, such as the foregut armature. Yang et al. (2015) find:

## 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.

## References

https://en.wikipedia.org/wiki/Hallucigenia