Monoplacophora

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]

Taxonomy of Monoplacophora per Bouchet, et al. (2017):[4]

Class Monoplacophora

• † Subclass Cyrtolitiones
  • † Order Sinuitopsida
    • † Superfamily Cyrtolitoidea S. A. Miller, 1889
      • † Family Cyrtolitidae S. A. Miller, 1889
      • † Family Carcassonnellidae Horný, 1997
    • † Superfamily Cyclocyrtonelloidea Horný, 1962
      • † Family Cyclocyrtonellidae Horný, 1962 (= Yochelsoniidae Horný, 1962 (inv.))
      • † Family Multifariitidae Bjaly, 1973
      • † Family Sinuellidae Starobogatov & Moskalev, 1987
      • † Family Sinuitinidae Starobogatov & Moskalev, 1987
• † Subclass Cyrtonelliones
  • † Order Cyrtonellida
    • † Superfamily Cyrtonelloidea Knight & Yochelson, 1958
      • † Family Cyrtonellidae Knight & Yochelson, 1958 (= Cyrtonellopsinae Horný, 1965)
• † Subclass Eomonoplacophora
  • (Unassigned to Order)
    • † Superfamily Maikhanelloidea Missarzhevsky, 1989
      • † Family Maikhanellidae Missarzhevsky, 1989 (= Purellidae Vassiljeva, 1990)
• Subclass Tergomya (= Pilinea)
  • † Order Kirengellida (= Romaniellida)
    • † Superfamily Archaeophialoidea Knight & Yochelson, 1958
      • † Family Archaeophialidae Knight & Yochelson, 1958
      • † Family Peelipilinidae Horný, 2006
      • † Family Pygmaeoconidae Horný, 2006
    • † Superfamily Kirengelloidea Starobogatov, 1970
      • † Family Kirengellidae Starobogatov, 1970
      • † Family Romaniellidae Rozov, 1975
      • † Family Nyuellidae Starobogatov & Moskalev, 1987
    • † Superfamily Hypseloconoidea Knight, 1952
      • † Family Hypseloconidae Knight, 1952
  • Order Tryblidiida
    • Superfamily Tryblidioidea Pilsbry, 1899
      • † Family Tryblidiidae Pilsbry, 1899
      • † Family Proplinidae Knight & Yochelson, 1958
      • † Family Drahomiridae Knight & Yochelson, 1958
      • † Family Bipulvinidae Starobogatov, 1970
    • Superfamily Neopilinoidea Knight & Yochelson, 1958
      • Family Neopilinidae Knight & Yochelson, 1958
        • Subfamily Neopilininae Knight & Yochelson, 1958 (= Vemidae Moskalev, Starobogatov & Filatova, 1983; = Laevipilinidae Moskalev, Starobogatov & Filatova, 1983; = Monoplacophoridae Moskalev, Starobogatov & Filatova, 1983)
        • Subfamily Veleropilininae Starobogatov & Moskalev, 1987 (= Rokopellidae Starobogatov & Moskalev, 1987; = Micropilinidae Haszprunar & Schaefer, 1997)

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]

Although superficially resembling limpets when viewed dorsally, monoplacophorans are not anatomically similar to gastropods. Some similarities are shared with the chitons, such as having segmented anatomy (organs arranged in series). There are eight pairs of dorso-ventral muscles (shell muscles). The nervous system is relatively simple, with no true ganglion present.

The repeated organs include from three to six pairs 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. Like in chitons, the head is poorly defined, and there are no eyes. 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. Tentacles are situated behind the mouth. 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. Monoplacophorans also have oesophageal pouches.

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.[5]

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.[6] 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,[7][8][9] and that the cephalopods (squids, octopuses, and relatives) arose from within the monoplacophoran lineage.[10] However, some authors dispute this view and do not necessarily see modern Monoplacophora as related to their presumed fossil ancestors.[11]

The concept of Serialia is supported by other molecular studies.[12]

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 around.[13] 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.[14]

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

Living families:

• Tryblidiida
  • Laevipilinidae
  • Micropilinidae
  • Monoplacophoridae
  • Neopilinidae

Extinct 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)[16][15] also contains Paleozoic molluscs of uncertain systematic position. It is not known whether these were gastropods or monoplacophorans.

• Lemche, Henning (1957). "A new living deep-sea mollusc of the Cambro-Devonian class Monoplacophora". Nature. London, UK. 179 (4556): 413–416. Bibcode:1957Natur.179..413L. doi:10.1038/179413a0. S2CID 4173823.
• Lemche, Henning (1972). Jenkins, Marie (ed.). The Curious Mollusks. New York, NY.{{cite book}}: CS1 maint: location missing publisher (link)

• 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. S2CID 23753587.

Wikimedia Commons has media related to Monoplacophora.

• "monoplacophora". Bio 136: Anatomy. University of California – Santa Cruz (UCSC). Archived from the original on 20 November 2007.
• "Taxonomy of Monoplacophora". DiscoverLife.
• "Monoplacophora". Animal Diversity Web. University of Michigan.
• "Monoplacophora". Nature Mapping Program. Seattle, WA: University of Washington. Archived from the original on 15 July 2007.
• "Tryblidiida (Monoplacophora)". Palaeos.com. Includes pictures and thorough discussion of cladistic issues.
• "images of Monoplacophora" (PDF). Archived from the original (PDF) on 16 June 2011.