Monoplacophora

Monoplacophorans Temporal range: early Cambrian–mid Devonian PreЄ Є O S D C P T J K Pg N
Neopilina sp. Head region is on the right.
Scientific classification
Kingdom:	Animalia
Phylum:	Mollusca
(unranked):	Monoplacophora Odhner, 1940
Included taxa
†Archinacelloidea †Maikhanellidae †Pelagiellida Tergomya

Monoplacophora, meaning "bearing one plate", is a polyphyletic class of mollusks with a cap-like shell, living on the bottom of deep sea. Extant representatives were unknown until 1952; previously they were known only from the fossil record.

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.^[1] The inclusion of the gastropod-like Bellerophontoidea within the group is also contentious.^[2]

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]Neopilina is considered as a living fossil. they have a limpet like a shell

Habitat

Extant monoplacophorans live in deeper waters (175 m & deeper). Cambrian forms predominately lived in shallow seas, whereas later Paleozoic forms are more commonly found and in deeper waters with soft, muddy sea floors.^[4]

Phylogenetic position

In 2006 a molecular study on Laevipilina antarctica suggested that 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 (squid and octopus) arose from within the monoplacophoran lineage.^[9] However, some authors dispute this view and do not necessarily see modern monoplacophora as related to their fossil ancestors.^[10]

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;^[11] 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.^[12]

Cambrian monoplacophoran Knightoconus antarcticus is thought 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 †

ordo ?

• family ?
  • Knightoconus
    • Knightoconus antarcticus

The taxonomy of the Gastropoda by Bouchet & Rocroi, 2005^[13] also contains Paleozoic molluscs of uncertain systematic position. It is not known whether these were gastropods or monoplacophorans.

References

1. ^ 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.  edit
2. ^ 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.  edit
3. ^ 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.  edit
4. ^ 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.  edit
5. ^ Giribet; 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" (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 1472512. PMID 16675549. http://www.pnas.org/cgi/pmidlookup?view=long&pmid=16675549.  edit
6. ^ 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. doi:10.2307/1542380. JSTOR 1542380. http://www.biolbull.org/cgi/content/abstract/184/1/57?ijkey=d45e9547a760e1cb9921f81243d05ae05773204c&keytype2=tf_ipsecsh. 
7. ^ Haszprunar, G. (2000) Am. Malacol. Bull 15, 115–130.
8. ^ 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.
9. ^ Clarke, M.R.; Trueman, E.R., ed (1988). "Main features of cephalopod evolution". The Mollusca. 12: Palaeontology and Neontology of Cephalopods. Orlando, Fla.: Acad. Pr.. ISBN 0127514120. 
10. ^ Budd, G. E.; Jensen, S. �R. (2000). "A critical reappraisal of the fossil record of the bilaterian phyla". Biological Reviews of the Cambridge Philosophical Society 75 (2): 253–295. doi:10.1017/S000632310000548X. PMID 10881389.  edit
11. ^ Runnegar, B.; Pojeta J, 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.  edit
12. ^ "Are hydrothermal vent animals living fossils?". Trends in Ecology & Evolution 18 (11): 582–588. 2003. doi:10.1016/j.tree.2003.08.009.  edit
13. ^ 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.

Further reading

• 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 and 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. http://www.springerlink.com/content/2280180506873217/. 

External links

• Anatomy
• DiscoverLife: Taxonomy of Monoplacophora
• Animal Diversity Web: Monoplacophora
• Washington Nature Mapping Program: Monoplacophora
• Palaeos.com, Monoplacophora (Tryblidiida) Includes pictures and thorough discussion of cladistic issues.
• images of Monoplacophora