Hallucigenia - Description and Investigation

Description and Investigation

109 specimens of Hallucigenia are known from the Greater Phyllopod bed, where they comprise 0.3% of the community. The 0.5 to 3 cm-long animal is wormlike — that is, long and narrow — with a poorly defined blob, or stain, on one end. This "blob" was arbitrarily designated the 'head' even though it had none of the features generally associated with heads: mouth, eyes, or other sensory organs. According to Morris' original interpretation, the animal has seven pincer-tipped tentacles lined up on one side and seven pairs of jointed spines on the other. Six of the tentacles were paired with spines, with one in front of the spines. There were also six smaller tentacles which may be configured in three pairs behind the seven larger ones. In addition, the body continued with a flexible, tube-like, tail-like extension behind the tentacles.

Faced with an animal that had no obvious head and two types of appendages, neither of which seemed appropriate for any reasonable form of locomotion, Morris assigned the blob as the head and hypothesized that the spines were legs and that the tentacles were feeding appendages. Morris was able to demonstrate a workable, if improbable, method of walking on the spines. 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. 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 at Yvonne Navarro's website.

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 Onychophore (velvet worm). They inverted it, interpreting the tentacles, which they believe to be paired, as walking structures and the spines as protective. Interestingly, none of the 30 or so known Burgess Shale specimens shows any sign of pairing in the large tentacles; nor do their Chinese counterparts. The pairing is based on a dissection of the actual fossil, which revealed what is probably a second tentacle structure. 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.

Though Ramskold and Hou's is the accepted modern interpretation, it is far from problem-free. Unlike its contemporary Aysheaia, Hallucigenia has very little resemblance to modern Onychophora. The elongated, and clawed legs bear little resemblance to the paired annulated legs of the Onychophora. It is unknown what the spines were made of and how much 'protection' they offered. They do not seem to be preserved independent of the soft-shelled animals as carbonate or chitinous shells would probably be. It is not easy to explain why 30 or more specimens — each hypothesized to have seven pairs of rather long, flexible legs — do not show even one example of paired legs. But at least this reconstruction of the animal can plausibly walk, and the spines serve a reasonable purpose. A picture of this reconstruction as well as a photograph of an actual fossil can be seen on the Geological Survey of Canada's website.

Some paleontologists accept Ramskold and Hou's interpretation of the animal's legs, spines, and head, but also believe that Hallucigenia might be an "armored lobopod" related to Anomalocaris. This does not rule out this bizarre creature also being related to the Onychophora, but rather may point to it coming from some time during or near the split of the two closely related groups.

In 2002, Desmond Collins suggested that new Hallucigenia fossils 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.