Belongs within: Deuterostomia.
Contains: Cyclocystoidea, Caryocystitida, Edrioblastoida, Ctenocystoidea, Cincta, Soluta, Edrioasteroidea, Blastozoa, Stylophora, Crinoidea, Asterozoa, Echinozoa.
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 water vascular system is used to manipulate eversible tube feet arranged along radial canals that are protected by distinct rows of small plates, the ambulacra. Tube feet may be used for feeding, mobility and grooming.
Many major lineages of echinoderms are extinct and their phylogeny is still hotly debated. A primary 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. Also contentious is whether stem-bearing echinoderms such as Crinoidea and Blastozoa form a clade Pelmatozoa or are non-monophyletic to the free-living Eleutherozoa.
The Helicoplacidae are an Early Cambrian group of echinoderms with a spindle- or bulb-shaped body bearing three recumbent, spiral ambulacra. The homology of these ambulacra with those of pentaradial forms, and the location of the mouth, have been subject to debate.
As well as losing the stem, the Eleutherozoa have inverted the central body axis so that the mouth is generally ventral rather than dorsal. Eleutherozoans are divided between the sea stars and brittle stars in the Asterozoa, and the sea urchins and sea cucumbers in the Echinozoa.
Polyplacus kilmeri
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 simply 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 (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 (see below for synonymy)
|--Yanjiahella biscarpa Guo, Li et al. 2012 [incl. Y. ancarpa Guo, Li et al. 2012, Y. monocarpa Guo, Li et al. 2012]TG19
`--+--CtenocystoideaTG19
|--Ctenoimbricata Zamora, Rahman & Smith 2012ZRS12
| `--*C. spinosa Zamora, Rahman & Smith 2012ZRS12
`--+--CinctaTG19
`--+--SolutaTG19
`--+--Helicoplacidae [Helicoplacida, Helicoplacoidea, Westgardellidae]ZR14
| |--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
`--+--EdrioasteroideaGS20
|--BlastozoaGS20
|--+--Camptostroma Ruedemann 1933GS20, D67 [Camptostromatoidea, Camptostromoidea]
| | `--*C. roddyi Ruedemann 1933D67
| |--StylophoraGS20
| `--CrinoideaGS20
`--Eleutherozoa [Cryptosyringida]SW07
| i. s.: Apostichopus japonicusGAS03
|--AsterozoaB01
|--EchinozoaB01
`--Stromatocystitidae [Stromatocystitoidea]R66
|--Xenocystites Bassler 1936R66
| `--*X. carteri Bassler 1936R66
`--Stromatocystites Pompeckj 1896GS20, 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
Amphidotus sulcatusH79
Archaeotrypa Fritz 1947TE04
|--*A. prima Fritz 1947TE04
`--A. secunda Fritz 1947TE04
Caulonia Loriol 1873Z01
Larites (n. d.)K67
Ridersia watsonaeA99
Tiaracystis [Tetracystida]J18
Bockia Gekker 1938SG93
|--*B. neglecta Gekker 1938U67a
`--B. mirabilis Bockelie 1981SG93
Eocystites Billings 1868U67a [=Eocystis Bather 1900J18, U67a]
`--*E. primaevus Billings 1868 [=*Eocystis primaevus]U67a
Acanthocystites Barrande 1887 [=Acanthocystis Bather 1889 non Carter 1863]U67a
`--*A. briareus Barrande 1887 [=*Acanthocystis briareus]U67a
Cigara Barrande 1887J18, U67a
`--*C. dusli Barrande 1887U67a
Sinoeocrinus luiZR14
UbaghsicystisTG19
CyclocystoideaSW07
BalantiocystisA99
FistuliporitaSG93
|--CaryocystitidaSW07
|--Nolichuckia Sprinkle 1973SG93
| `--N. casteri Sprinkle 1973SG93
`--Polycosmitida [Polycosmites]K67
|--Stichocystis Jaekel 1899K67 [StichocystidaeSG93]
| `--*S. geometrica (Angelin 1878) [=Caryocystis geometrica]K67
`--Polycosmites Jaekel 1918 [Polycosmitidae]K67
`--*P. bohemicus Jaekel 1918K67
Edrioblastoida SG93
Echinodermata [Amphoridea, Anthodiata, Atava, Carpoidea, Crinozoa, Cystidea, Cystideae, Cystideen, Cystoidea, Cystoidia, Eocrinidae, Eocrinoidea, Gogiidea, Homalozoa, Hydrophoridea, Pelmatozoa, Rhombifera, Taxiporitidae]
*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
References
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[B01] Boczarowski, A. 2001. Isolated sclerites of Devonian non-pelmatozoan echinoderms. Palaeontologia Polonica 59: 1–219.
[D67] Durham, J. W. 1967a. Camptostromatoids. In: Moore, R. C. (ed.) Treatise on Invertebrate Paleontology pt S. Echinodermata 1. General characters. Homalozoa—Crinozoa (except Crinoidea) vol. 2 pp. S627–S631. The Geological Society of America, Inc., and The University of Kansas: Lawrence (Kansas).
Durham, J. W. 1967b. Notes on the Helicoplacoidea and early echinoderms. Journal of Paleontology 41 (1): 97–102.
[D93] Durham, J. W. 1993. Observations on the Early Cambrian helicoplacoid echinoderms. Journal of Paleontology 67 (4): 590–604.
[GS20] Guensburg, T. E., J. Sprinkle, R. Mooi, B. Lefebvre, B. David, M. Roux & K. Derstler. 2020. Athenacrinus n. gen. and other early echinoderm taxa inform crinoid origin and arm evolution. Journal of Paleontology 94 (2): 311–333.
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[H79] Haast, J. von. 1879. Geology of the Provinces of Canterbury and Westland, New Zealand. A report comprising the results of official explorations. “Times” Office: Christchurch.
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[SG93] Simms, M. J., A. S. Gale, P. Gilliland, E. P. F. Rose & G. D. Sevastopulo. 1993. Echinodermata. In: Benton, M. J. (ed.) The Fossil Record 2 pp. 491–528. Chapman & Hall: London.
Smith, A. B. 1988. Patterns of diversification and extinction in early Palaeozoic echinoderms. Palaeontology 31 (3): 799–828.
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[SW07] Sumrall, C. D., & G. A. Wray. 2007. Ontogeny in the fossil record: diversification of body plans and the evolution of “aberrant” symmetry in Paleozoic echinoderms. Paleobiology 33 (1): 149–163.
[TE04] Taylor, P. D., & A. Ernst. 2004. Bryozoans. In: Webby, B. D., F. Paris, M. L. Droser & I. G. Percival (eds) The Great Ordovician Biodiversification Event pp. 147–156. Columbia University Press.
[T64] Teichert, C. 1964. Doubtful taxa. In: Moore, R. C. (ed.) Treatise on Invertebrate Paleontology pt K. Mollusca 3. Cephalopoda—General Features—Endoceratoidea—Actinoceratoidea—Nautiloidea—Bactritoidea pp. K484–K490. The Geological Society of America and the University of Kansas Press.
[TG19] Topper, T. P., J. Guo, S. Clausen, C. B. Skovsted & Z. Zhang. 2019. A stem group echinoderm from the basal Cambrian of China and the origins of Ambulacraria. Nature Communications 10: 1366.
[U67a] Ubaghs, G. 1967a. Eocrinoids. In: Moore, R. C. (ed.) Treatise on Invertebrate Paleontology pt S. Echinodermata 1. General characters. Homalozoa—Crinozoa (except Crinoidea) vol. 2 pp. S455–S495. The Geological Society of America, Inc., and The University of Kansas: Lawrence (Kansas).
[U67b] Ubaghs, G. 1967b. Cymbionites and Peridionites—unclassified Middle Cambrian echinoderms. In: Moore, R. C. (ed.) Treatise on Invertebrate Paleontology pt S. Echinodermata 1. General characters. Homalozoa—Crinozoa (except Crinoidea) vol. 2 pp. S634–S637. The Geological Society of America, Inc., and The University of Kansas: Lawrence (Kansas).
[U78] Ubaghs, G. 1978. Classification of the echinoderms. In: Moore, R. C., & C. Teichert (eds) Treatise on Invertebrate Paleontology pt T. Echinodermata 2. Crinoidea vol. 1 pp. T359–T367. The Geological Society of America, Inc.: Boulder (Colorado), and The University of Kansas: Lawrence (Kansas).
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[ZR14] Zamora, S., & I. A. Rahman. 2014. Deciphering the early evolution of echinoderms with Cambrian fossils. Palaeontology 57 (6): 1105–1119.
[ZRS12] Zamora, S., I. A. Rahman & A. B. Smith. 2012. Plated Cambrian bilaterians reveal the earliest stages of echinoderm evolution. PLoS One 7 (6): e38296.
[Z01] Zompro, O. 2001. A generic revision of the insect order Phasmatodea: the New World genera of the stick insect subfamily Diapheromeridae: Diapheromerinae=Heteronemiidae: Heteronemiinae sensu Bradley & Galil, 1977. Revue Suisse de Zoologie 108 (1): 189–255.