Probably the most bizarre Burgess Shale fossil is Hallucigenia. This fossil has been completely reinterpreted since the description presented by Gould(24) This reinterpretation has resulted both from more detailed study of existing fossil specimens and the discovery of exceptionally well-preserved fossils of similar organisms in China.(25) Hallucigenia is now recognized as a member of a diverse and widespread group of Cambrian organisms called lobopods. They are very similar to, and may belong to, an obscure living phylum called the Onychophora. These caterpillar-like organisms walked on fleshy legs and bore plate-like or spine-like mineralized structures on their dorsal sides. Although these small plates and spines were previously recognized as part of the Early Cambrian "small shelly fauna," their biological affinities were unknown until these recent discoveries.
The Cambrian lobopods occupy a transitional morphological position between several living phyla. The oldest known lobopod from the Early Cambrian is Xenusion. This organism bears similarities to both palaeoscolecid worms and to living onychophorans and tardigrads.(26) Furthermore, lobo-pods also have morphological features in common with the arthropods, particularly with peculiar Cambrian forms such as Opabinia and Anomalocaris.(27) Recent redescription of Opabinia has also disclosed the presence of lobopod limbs strongly suggesting a lobopod to arthropod transition.(28) The discovery of a Cambrian gill-bearing lobopod reinforces this conclusion.(29) These forms fall nicely into a transitional position between extant phyla.
Another very important group of Early Cambrian fossils is represented by a wide variety of tiny cap-shaped and scalelike skeletal elements. It is now known that many of these belonged to slug-like animals that bore these hollow mineralized structures like a dermal armor. Two well-known, and well-preserved, examples of this group of organisms are Wiwaxia and Halkieria. Called the Machaeridia or the Coelosceritophora, these organisms are mosaics of phylum-level characteristics, and their taxonomic affinity is a matter of present debate. A strong case can be made for the assignment of at least some of these taxa to the Mollusca.(30) However, a relationship to the polychaete annelid worms is also strongly suggested by some workers, as with Wiwaxia.(31) The taxonomic confusion associated with these scale-bearing slug-like animals, and with the lobopods, is consistent with their stratigraphic position at the base of the Cambrian metazoan radiation.
The above discussion shows that the presentation of the Precambrian to Cambrian fossil record given by Battson does not reflect our present understanding of the history of life.(32) Many metazoan groups appeared before the Cambrian, including representatives of several living phyla. Furthermore, the many small scale, plate, and spine-bearing organisms of the earliest Cambrian, while sharing characteristics with several living phyla, are also similar enough to each other to be classified by some workers into a single phylum.(33) Even when the metazoan fossil record for the entire Cambrian is considered, the morphological disparity cannot be equated with that of living organisms, unless the subsequent appearance of all vertebrate and insect life be ignored. In addition, many living phyla, including most worm phyla, are unknown from the fossil record until well into the Phanerozoic.(34) Thus, to claim the near simultaneous appearance of virtually all living phlya in the Cambrian is not an objective statement of the fossil evidence but a highly speculative, and I believe unsupported, interpretation of it(35)
Finally, there is a question of whether the rapid diversification of metazoans in the Late Precambrian and Early Cambrian reflects an equally rapid increase in complexity. An interesting study by Valentine and others uses the number of cell types as a useful measure of morphological complexity. They plot the estimated times of origin of major body plans against their cell type numbers. The resulting plot shows that the upper bound of complexity has increased steadily and nearly linearly from the origin of the metazoa to the present. Furthermore, they conclude that "...the metazoan `explosion' near the Precambrian/Cambrian transition was not associated with any important increase in complexity of body plans...(36) This suggests that the appearance of new higher taxa in the Cambrian did not involve the sudden appearance of major new levels of complexity.