Temporal range: early Cambrian–Present
Monoplacophora, meaning "bearing one plate", is a polyphyletic superclass of molluscs with a cap-like shell now living at the bottom of the deep sea. Extant representatives were not recognized as such until 1952; previously they were known only from the fossil record.
Discussion about monoplacophorans is made difficult by the slippery definition of the taxon; some authors take it to refer to all non-gastropod molluscs with a single shell, or all single-shelled molluscs with serially repeated units; whereas other workers restrict the definition to cap-shaped forms, excluding spiral and other shapes of shell. The inclusion of the gastropod-like Bellerophontoidea within the group is also contentious.
Anatomy and physiology
Monoplacophorans are univalved (though not gastropodal), limpet-shaped, and are untorted. They have a pseudometamerism of bilaterally symmetrical repeated organs and muscles. The extant members of the class live only in the deep ocean (the abyssal zone, the continental shelf, and the continental slope) at depths below 180 metres (590 ft). Cambrian forms predominately lived in shallow seas, whereas later Paleozoic forms are more commonly found in deeper waters with soft, muddy sea floors.
The repeated organs include from three to six pair of "gills" (actually ctenidia) located in a curved line along each side of the foot (though the number is not always considered definitive of a given species), and as many as six "kidneys" (actually nephridia). The tip or point of their low shells points forward rather than towards the back. The shell ranges from 3mm to 37mm in diameter depending on species. The mouth is located within the animal's undeveloped head in front of its single large foot and contains a radula, a defining characteristic of the mollusca. They also have a cone-shaped stomach with a single crystalline style though no gastric shield. The intestines are long and make between four and six loops before reaching the posteriorly-positioned anus.
The sexes are separate with any given animal having two pair of either ovaries or testes connected to either the third or fourth pair of kidneys. One genus, Micropilina, has apparently been recorded as brooding young in the distal oviduct and pallial groove, releasing the young when approximately 300 micrometers in diameter.
In 2006 a molecular study on Laevipilina antarctica suggested that extant Monoplacophora and Polyplacophora form a well-supported clade with the researched Neopilina closest to the chitons. The two classes in this new clade, with the proposed name Serialia, all show a variable number of serially repeated gills and eight sets of dorsoventral pedal retractor muscles.
This study contradicts the fossil evidence, which suggests that the Monoplacophora are the sister group to the remainder of the conchiferans, and that the cephalopods (squids, octopuses, and relatives) arose from within the monoplacophoran lineage. However, some authors dispute this view and do not necessarily see modern Monoplacophora as related to their fossil ancestors.
The concept of Serialia is supported by other molecular studies. 
The fossil record does indicate that the ancestral mollusc was monoplacophoran-like and that the Polyplacophora arose from within the Monoplacophora – not the other way round; this could be reconciled if a secondary loss of shells caused a monoplacophoran body form to reappear secondarily. This is plausible: modern monoplacophorans are not closely related to vent-dwelling representatives from the Silurian, at least.
- Tryblidiidae von Zittel, 1899
- Palaeacmaeidae † (uncertain, as the Cambrian type species is a cnidarian. It is maintained here as a receptacle for the Paleozoic genus Parmophorella)
|This section needs expansion. You can help by adding to it. (August 2009)|
The taxonomy of the Gastropoda by Bouchet & Rocroi, 2005 also contains Paleozoic molluscs of uncertain systematic position. It is not known whether these were gastropods or monoplacophorans.
- Lindberg, D. R. (2009). "Monoplacophorans and the Origin and Relationships of Mollusks". Evolution: Education and Outreach. 2: 191–203. doi:10.1007/s12052-009-0125-4.
- Gubanov, A. P.; Peel, J. S. (2001). "Latest Helcionelloid Molluscs from the Lower Ordovician of Kazakhstan". Palaeontology. 44: 681. doi:10.1111/1475-4983.00198.
- Counts, Clement L. (2006). "Chapter 17:Monoplacophora". In Sturm, C.F.; Pearce, T.A.; Valdes, Angel. The Mollusks: A Guide to Their Study, Collection, And Preservation. Universal. pp. 212–214. ISBN 9781581129304.
- Giribet, G.; Okusu, A, A.; Lindgren, A.R., A. R.; Huff, S.W., S. W.; Schrödl, M, M.; Nishiguchi, M.K., M. K. (May 2006). "Evidence for a clade composed of molluscs with serially repeated structures: monoplacophorans are related to chitons" (Free full text). Proceedings of the National Academy of Sciences of the United States of America. 103 (20): 7723–7728. Bibcode:2006PNAS..103.7723G. doi:10.1073/pnas.0602578103. PMC . PMID 16675549.
- Scheltema, A. H. (February 1, 1993). "(1993) Biol. Bull 184, 57–78 Aplacophora as Progenetic Aculiferans and the Coelomate Origin of Mollusks as the Sister Taxon of Sipuncula". The Biological Bulletin. 184 (1): 57–78. doi:10.2307/1542380. JSTOR 1542380.
- Haszprunar, G. (2000) Am. Malacol. Bull 15, 115–130.
- Salvini-Plawen, L. V. & Steiner, G. (1996) in Origin and Evolutionary Radiation of the Mollusca ed. Taylor, J. D. (Oxford Univ. Press, Oxford), pp. 29–51.
- Clarke, M.R.; Trueman, E.R., eds. (1988). "Main features of cephalopod evolution". The Mollusca. 12: Palaeontology and Neontology of Cephalopods. Orlando, Fla.: Acad. Pr. ISBN 0-12-751412-0.
- Budd, G. E.; Jensen, S. (2000). "A critical reappraisal of the fossil record of the bilaterian phyla". Biological Reviews of the Cambridge Philosophical Society. 75 (2): 253–95. doi:10.1111/j.1469-185X.1999.tb00046.x. PMID 10881389.
- Wilson, N.; Rouse, G.; Giribet, G. (2010). "Assessing the molluscan hypothesis Serialia (Monoplacophora+Polyplacophora) using novel molecular data.". Molecular Phylogenetics & Evolution. 54 (1): 187–193. doi:10.1016/j.ympev.2009.07.028. PMID 19647088.
- Runnegar, B.; Pojeta Jr, J. (Oct 1974). "Molluscan Phylogeny: the Paleontological Viewpoint". Science. 186 (4161): 311–317. Bibcode:1974Sci...186..311R. doi:10.1126/science.186.4161.311. JSTOR 1739764. PMID 17839855.
- Little, C.; Vrijenhoek, R. C. (2003). "Are hydrothermal vent animals living fossils?". Trends in Ecology & Evolution. 18 (11): 582–588. doi:10.1016/j.tree.2003.08.009.
- Bouchet P. & Rocroi J.-P. (Ed.); Frýda J., Hausdorf B., Ponder W., Valdés Á. & Warén A. 2005. Classification and nomenclator of gastropod families. Malacologia: International Journal of Malacology, 47(1-2). ConchBooks: Hackenheim, Germany. ISBN 3-925919-72-4. ISSN 0076-2997. 397 pp. http://www.vliz.be/Vmdcdata/imis2/ref.php?refid=78278
- Lemche, Henning 1957. A new living deep-sea mollusc of the Cambro-Devonian class Monoplacophora. Nature, 179: 413–416, London.
- Lemche, Henning, in Marie Jenkins. 1972. The Curious Mollusks, New York.
- Horný, Radwan 1963. On the systematic position of cyrtonelloids (Mollusca). Časopsis národního Muzea, oddil přírodovědný, 132: 90–93, Prague.
- Rozov, S. N. 1975. A new order of the Monoplacophora. Paleontological Journal, 9: 39–43, Washington.
- Michael Schrödl; Katrin Linse & Enrico Schwabe (August 2006). "Review on the distribution and biology of Antarctic Monoplacophora, with first abyssal record of Laevipilina antarctica". Polar Biology. 29 (9): 721–727. doi:10.1007/s00300-006-0132-7.
|Wikimedia Commons has media related to Monoplacophora.|