Monoplacophora

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, and were previously thought to have become extinct over 380 million years ago.

Monoplacophora
Temporal range: Middle Pleistocene–Recent[1][2]
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The holotype of Neopilina galatheae at the Zoological Museum, Copenhagen
Scientific classification
Kingdom: Animalia
Phylum: Mollusca
(unranked): Monoplacophora
Odhner, 1940

Although the shell of many monoplacophorans is limpet-like in shape, they are not gastropods, nor do they have any close relation to gastropods.[1]

Definition

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.[2] The inclusion of the gastropod-like Bellerophontoidea within the group is also contentious.[3]

One attempt to resolve this confusion was to separate out the predominantly coiled helcionelloids from the traditional, cap-like tergomyans, this latter group containing extant Tryblidiids.[3]

Anatomy and physiology
Internal anatomy of Micropilina. The head region is on the left by the mouth.

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.[2]

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 3 mm to 37 mm 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.[4]

Phylogenetic position

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.[5] 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,[6][7][8] and that the cephalopods (squids, octopuses, and relatives) arose from within the monoplacophoran lineage.[9] However, some authors dispute this view and do not necessarily see modern Monoplacophora as related to their presumed fossil ancestors.[10]

The concept of Serialia is supported by other molecular studies.[11]

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.[12] This could be reconciled if a secondary loss of shells caused a monoplacophoran body form to re-appear secondarily, which is plausible: At the very least, modern monoplacophorans are not closely related to vent-dwelling representatives from the Silurian.[13]

Cambrian monoplacophoran Knightoconus antarcticus is hypothesised to be an ancestor to the cephalopods.

Fossil species

Families:

  • † Tryblidiida
  • † 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)
    • † Palaeacmaea Hall & Whitfield, 1872
    • † Parmophorella Matthew, 1886
    • † Knightoconus
      • † Knightoconus antarcticus

Many Cambrian-Devonian species have been described as "monoplacophorans", but the only fossil members of the crown group date to the Pleistocene.[1]

The Taxonomy of the Gastropoda (Bouchet & Rocroi, 2005)[15][14] also contains Paleozoic molluscs of uncertain systematic position. It is not known whether these were gastropods or monoplacophorans.

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References
  1. Taviani, M.; Sabelli, B.; Candini, F. (1990). "A fossil Cenozoic monoplacophoran". Lethaia. 23: 213–216.
  2. Lindberg, D.R. (2009). "Monoplacophorans and the origin and relationships of mollusks" (PDF). Evolution: Education and Outreach. 2 (2): 191–203. doi:10.1007/s12052-009-0125-4.
  3. Gubanov, A.P.; Peel, J.S. (2001). "Latest helcionelloid molluscs from the Lower Ordovician of Kazakhstan". Palaeontology. 44 (4): 681–694. doi:10.1111/1475-4983.00198.
  4. Counts, Clement L. (2006). "Chapter 17: Monoplacophora". In Sturm, C.F.; Pearce, T.A.; Valdes, Angel (eds.). The Mollusks: A guide to their study, collection, and preservation. Universal. pp. 212–214. ISBN 9781581129304 – via Google Books.
  5. Giribet, G.; Okusu, A.; Lindgren, A.R.; Huff, S.W.; Schrödl, M.; Nishiguchi, M.K. (May 2006). "Evidence for a clade composed of molluscs with serially repeated structures: Monoplacophorans are related to chitons". 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 1472512. PMID 16675549.
  6. Scheltema, A.H. (1 February 1993). "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. PMID 29300613. Archived from the original on 16 June 2011. Retrieved 12 November 2007.CS1 maint: date and year (link)
  7. Haszprunar, G. (2000). "[no title cited]". Am. Malacol. Bull. 15: 115–130.
  8. Salvini-Plawen, L.V.; Steiner, G. (1996). Taylor, J.D. (ed.). Origin and Evolutionary Radiation of the Mollusca. Oxford, UK: Oxford University Press. pp. 29–51.
  9. Clarke, M.R.; Trueman, E.R., eds. (1988). "Main features of cephalopod evolution". Palaeontology and Neontology of Cephalopods. The Mollusca. 12. Orlando, FL: Academic Press. ISBN 0-12-751412-0.
  10. Budd, G.E.; Jensen, S. (2000). "A critical re-appraisal 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.
  11. 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.
  12. Runnegar, B.; Pojeta, J. Jr. (October 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.
  13. 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.
  14. Frýda, J.; Hausdorf, B.; Ponder, W.; Valdés, Á.; Warén, A. (2005a). Bouchet, P.; Rocroi, J.-P. (eds.). "Classification and nomenclator of gastropod families". Malacologia: International Journal of Malacology. Ann Arbor: Institute of Malacology. 47 (1–2). ISSN 0076-2997. e-ISSN 2168-9075.
  15. Bouchet, P.; Rocroi, J.-P., eds. (2005b). Classification and nomenclator of gastropod families. Hackenheim, Germany: ConchBooks. ISBN 3-925919-72-4. "(online data ref)". reprinted from Bouchet & Rocroi (2005a)[14]
  • Lemche, Henning (1957). "A new living deep-sea mollusc of the Cambro-Devonian class Monoplacophora". Nature. London, UK. 179: 413–416.
  • Lemche, Henning (1972). Jenkins, Marie (ed.). The Curious Mollusks. New York, NY.

Further reading
  • Horný, Radvan (1963). "On the systematic position of cyrtonelloids (Mollusca)". Časopsis národního Muzea, oddil přírodovědný. Prague, CZ. 132 (2): 90–94.
  • Rozov, S.N. (1975). "A new order of the Monoplacophora". Paleontological Journal. Washington, DC. 9: 39–43.
  • Schrödl, Michael; Linse, Katrin & Schwabe, Enrico (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.
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