The Echinodermata are a diverse group of marine invertebrates characterised by the possession of an ultrastructurally distinct calcite exoskeleton (the stereom), a water vascular system (at least in living taxa) and, in most species, a secondary radial (usually pentamerous) symmetry. The phylogeny of echinoderms, of which many major lineages are extinct, is still hotly debated, and the (probably unwise) tree given below is drawn from a number of conflicting topologies. A primary contentious issue is the status of Palaeozoic taxa lacking a pentamerous organisation, such as the Stylophora and Ctenocystoidea. Some authors regard these taxa as diverging before the evolution of pentamery; others regard them as derived taxa that have secondarily lost this character.
The clade Eleutherozoa includes the free-living echinoderms, which have lost the stem of more basal lineages, as well as inverting the central body axis so that the mouth is generally ventral rather than dorsal. In the Asterozoa, sea stars and brittle stars, the post-larval body is comprised of a central disc with radiating appendages (arms). The Echinozoa lack such appendages and have an somewhat globular test (albeit with the overall shape modified in many subgroups) with the ambulacra arranged meridionally, radiating from the the mouth more or less towards the anus.
The Blastozoa are a group of mostly stalked Palaeozoic echinoderms, distinguished by the possession of brachioles, axial appendages extending from branches in ambulacral rays (Mooi 2001). The relationships of blastozoans to other echinoderms remain contentious: some regard them as an extinct clade, while others regard the crinoids as a derived subgroup. Many early blastozoans have been included in the Lower Cambrian to Silurian ‘Eocrinoidea’, with sutural pores often present between the plates of the theca (Ubaghs 1967), but most authors agree that the ‘eocrinoids’ are probably paraphyletic with regard to other blastozoans.
Published 24 March 2010
Even among the generally bizarre world of Palaeozoic echinoderms, helicoplacoids stand out as particularly weird (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.
Systematics of Echinodermata
Characters (from Ubaghs 1967, ‘General characters of Echinodermata’, Treatise on Invertebrate Paleontology pt S vol. 1): Marine, benthonic (or exceptionally pelagic) animals, living in an attached position or free, but never colonial. Enterocoelic, nonsegmented, coelomate, no differentiated head or brain; fundamentally bilaterally symmetrical, but modified by asymmetry introduced by atrophy of some organs of the right anterior side of the body and corresponding overdevelopment of organs of the left side; radial symmetry (typically pentamerous), secondarily imposed on larval asymmetry; no differentiated excretory apparatus. Endoskeleton formed of plates or distinct ossicles, composed of crystalline calcite deposited in organic mesenchymatous network displaying a reticulate microstructure and distinctive crystallographic properties. Water-vascular system of sacs and canals of coelomic nature opening outward in a pore and giving rise to numerous small projections on the surface of the body.
<==Echinodermata [Amphoridea, Blastozoa, Carpoidea, Crinozoa, Eocrinoidea, Homalozoa, Taxiporitidae] |--Yanjiahella biscarpa Guo, Li et al. 2012 [incl. Y. ancarpa Guo, Li et al. 2012, Y. monocarpa Guo, Li et al. 2012]TG19 `--+--+--StylophoraSW07 | `--+--CtenocystoideaTG19 | `--Ctenoimbricata Zamora, Rahman & Smith 2012ZR14, ZRS12 | `--*C. spinosa Zamora, Rahman & Smith 2012ZRS12 `--+--CinctaTG19 `--+--SolutaTG19 `--+--Helicoplacidae [Helicoplacida, Helicoplacoidea, Westgardellidae]SW07 | |--Helicoplacus Durham & Caster 1963TG19, D93 [incl. Westgardella Durham 1993D93; Helicoplacidae] | | `--*H. gilberti Durham & Caster 1963W06 (see below for synonymy) | |--Waucobdella Durham 1967D93 | | `--*W. nelsoni Durham 1967D93 [=Helicoplacus nelsoniSG93] | `--Polyplacus Durham 1967 [Polyplacida]W06 | `--*P. kilmeri Durham 1967D93 `--+--Camptostroma Ruedemann 1933TG19, D67a [Camptostromatoidea, Camptostromoidea] | `--*C. roddyi Ruedemann 1933D67a `--+--+--Lepidocystidae [Imbricata, Lepidocystoidea]SW07 | | |--Kinzercystis durhami Sprinkle 1973TG19, SG93 | | `--Lepidocystis Foerste 1938K67 | | `--*L. wanneri Foerste 1938D67b | `--+--EdrioasteroideaSW07 | `--+--CyclocystoideaSW07 | `--PelmatozoaSW07 `--Eleutherozoa [Cryptosyringida]SW07 | i. s.: Apostichopus japonicusGAS03 |--AsterozoaB01 |--EchinozoaB01 `--Stromatocystitidae [Stromatocystitoidea]R66 |--Xenocystites Bassler 1936R66 | `--*X. carteri Bassler 1936R66 `--Stromatocystites Pompeckj 1896TG19, R66 [=Stromatocystis Bather 1900R66] |--*S. pentangularis Pompeckj 1896R66 |--S. balticus Jaekel 1899R66 `--S. walcotti Schuchert 1919SG93 Echinodermata incertae sedis: Xyloplax [Concentricycloidea, Concentricyclomorpha]B01 `--X. medusiformesM06 HaplozoaU78 |--Cymbionites Whitehouse 1941 [Cymbionitidae]U67b | `--*C. craticula Whitehouse 1941U67b `--Peridionites Whitehouse 1941 [Peridionitidae]U67b `--*P. navicula Whitehouse 1941U67b Pichyceras Rusconi 1955T64 `--*P. jorusconii Rusconi 1955T64 Capsulina Seguenza 1880LT64 `--*C. loculicida Seguenza 1880LT64 Protocyclina Paalzow 1922LT64 `--*P. liassina Paalzow 1922LT64 Dibrachicystis purujoensisZR14 Helicocystis moroccoensisZR14 Archaeocystites Barrande 1887 [=Archaeocystis Haeckel 1896]U67a Cardiocystites Barrande 1887 [=Cardiocystis Bather 1900]U67a `--*C. bohemicus Barrande 1887 [=*Cardiocystis bohemicus]U67a Lapillocystites Barrande 1887 [=Lapillocystis Bather 1889]U67a `--*L. fragilis Barrande 1887 [=*Lapillocystis fragilis]U67a Pilocystites Barrande 1887U67a `--*P. primitivus Barrande 1887U67a Rhopalocystis Ubaghs 1963 non Grove 1911 (ICBN) [Rhopalocystidae]U67a `--*R. destombesi Ubaghs 1963U67a Cheirocystis antiqua Paul 1972WA17 Amphidotus sulcatusH79 Archaeotrypa Fritz 1947TE04 |--*A. prima Fritz 1947TE04 `--A. secunda Fritz 1947TE04 Caulonia Loriol 1873Z01
*Helicoplacus gilberti Durham & Caster 1963W06 [incl. Westgardella blancoensis Durham 1993W06, H. casteri Durham 1993W06, H. curtisi Durham & Caster 1963W06, *Westgardella curtisiD93, H. evernderni Durham 1967W06, H. firbyi Durham 1967W06, Westgardella firbyiD93, H. guthi Durham 1993W06]
*Type species of generic name indicated
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