PART 2 of the Burgess Shale Fossils:

One of the most unusual-looking Burgess Shale fossils is Hallucigenia sparsa (Walcott, 1911). Hallucigenia is found in the Burgess Shale in British Columbia, and it is found in slightly earlier rocks in China. It is also found as isolated spines elsewhere in the world. Its geologic age is approximately a half a billion years old.

Hallucigenia sparsa (length between 1 and 5 cm long) does not have jointed legs, nor a segmented body, nor a hard exoskeleton. This fossil has been interpreted as being a predecessor to arthropods. It branched off in an evolutionary sense prior to the arrival of arthropods in the fossil record. In other words, Hallucigenia is “an almost arthropod.” It has many sharp, long spines, which Conway-Morris (1977) believed were used as walking legs. 

Hallucigenia sparsa was originally described as a species of a polychaete worm belonging to genus Canadia Walcott, 1911. Walcott did not figure this fossil until 1931. Conway-Morris (1977) redescribed this fossil and assigned to his new genus Hallucigenia, which was commonly referred to as “the worm with the missing head” because its “head” turned out to be just a stain of decomposed organic matter. The name Hallucigenia is Latin, meaning “wandering of the mind.

Ramsköld and Xianguang (1991) reinterpreted Hallucigenia as belonging to a stem group of onychophorans (or velvet worms), which are rare, caterpillar-like animals alive today in rain forests. There are about 100 living species). Ramsköld and Xianguang (1991) discovered also that Hallucigenia actually has legs, thus they inverted the life-orientation of this fossil and reported that the spines were used for protection, not for walking.

Cited References:

Conway-Morris, S. 1977. A new metazoan from the Cambrian Burgess Shale of British Columbia. Paleontology 20:623-640. [requires a subscription or pay-per-view fee].

Ramsköld, L. and H. Xianguang. 1991. New Early Cambrian animal and onychophoran affinities of enigmatic metazoans. Nature 351 (6323):225-228. [requires a subscription or pay-per-view fee].

Walcott, C.D. 1911. Cambrian geology and paleontology. II. Middle Cambrian annelids. Smithsonian Miscellaneous Collections 57(5):109-144.