Even among the generally bizarre world of Palaeozoic echinoderms, helicoplacoids stand out as particularly wierd (see this page by Chris Mah for an overview of their wierdness). But if there was to be such a thing as a helicoplacoid family reunion then there would be one family member that even the other helicoplacoids would be looking sideways at and muttering that they were a little odd; that member would be Polyplacus.
The Helicoplacoidea were a short-lived group of animals from very early in the Cambrian period. Their overall body shape was similar to a football, or a spindle, or a sort of armour-plated turd. The ambulacral (feeding groove) arrangement was essentially Y-shaped with two upper branches and one lower branch, but this 'Y' was then wrapped around the body in a left-handed spiral. One of the upper branches stopped further away from the uppermost point than the other while the lower branch stopped some distance short of the body's lower point. Most authors regard helicoplacoids as having been sessile in life with the ambulacrum-free lower part buried into soft sediment to hold the animal upright (like a rugby ball sitting in a kickstand). The body wall was made up of a large number of small plates held together by soft tissue; the plates were not anchored to each other directly, so the animal would have been able to expand or contract as it required. However, because of this lack of direct articulation, the plates in even well-preserved fossils have invariably shifted somewhat relative to each other so that any fine-scale features, such as were the body openings were located, are obscured. A few different reconstructions of helicoplacoid anatomy have been suggested, none of which (it must be said) make a huge deal of sense. For instance, Sprinkle & Wilbur (2005) (among others) locate the mouth at the junction of the three ambulacral branches; this is the most reasonable position in comparison to the anatomy of other echinoderms but implies a lateral position for the mouth in the living animal when pretty much all other sessile animals have their mouths positioned dorsally. In contrast, Durham (1993) suggested that the mouth might be located at the upper apex which seems more sensible from a functional perspective, but implies a branching and reversal of direction in the ambulacrum that is completely unlike anything seen in any other echinoderm (and, I can't help suspecting, may be developmentally impossible).
Durham (1993) recognised nine species of helicoplacoid in four genera but Wilbur (2006) recently reduced the number of species to three, regarding the diagnostic features of the remaining 'species' as due to ontogeny and/or the degree of expansion of the specimen when preserved. Two of those species, Helicoplacus gilberti and Waucobdella nelsoni, have the whorls of the ambulacra (with biserial floor-plates and flanking cover plates) divided by distinct interambulacral zones, similar to the arrangement in other echinoderms. In Polyplacus kilmeri, however, while the overall arrangement in plates is spiral as in other helicoplacoids, there are no distinguishable ambulacra. Or, to put it another way, the skeleton appears to be all ambulacra, as the interambulacral zones have been replaced by arrays of small plates identical to the ambulacra of Helicoplacus gilberti (Wilbur, 2006). The true ambulacra of Polyplacus kilmeri have not yet been identified on either of the two specimens of this species known (Wilbur, 2006, seems to allude to the possibility that Polyplacus may be a pathological monstrosity rather than a true species but unfortunately there is simple not enough material available to establish this).
The phylogenetic position of helicoplacoids relative to other echinoderms remains highly debatable. Many authors have suggested a very basal position for helicoplacoids on the basis of their overall distinctiveness and early appearance in the fossil record, suggesting that they represent a trimerous stage in echinoderm evolution that preceeded the pentamerous stage more characteristic of the phylum. Others (e.g., Sprinkle & Wilbur, 2005) regard helicoplacoid trimery as derived rather than ancestral, perhaps from the pentamerous edrioasteroids. The suggestion of Smith (1988) that helicoplacoids might even be para- or polyphyletic, with Polyplacus closer to other echinoderms than to Helicoplacus, is based on a very speculative interpretation of Polyplacus and seems highly unlikely. The unique spiral morphology of helicoplacoids seems unlikely to have arisen twice, nor does it seem likely to have given rise to more orthodox echinoderms.
REFERENCES
Durham, J. W. 1967. Notes on the Helicoplacoidea and early echinoderms. Journal of Paleontology 41 (1): 97-102.
Durham, J. W. 1993. Observations on the early Cambrian helicoplacoid echinoderms. Journal of Paleontology 67 (4): 590-604.
Smith, A. B. 1988. Patterns of diversification and extinction in early Palaeozoic echinoderms. Palaeontology 31 (3): 799-828.
Sprinkle, J., & B. C. Wilbur. 2005. Deconstructing helicoplacoids: reinterpreting the most enigmatic Cambrian echinoderms. Geological Journal 40: 281-293.
Wilbur, B. C. 2006. Reduction in the number of Early Cambrian helicoplacoid species. Palaeoworld 15 (3-4): 283-293.