Monoplacophora

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Monoplacophora
Temporal range:
Early Cambrian – Recent[1][2]
The holotype of
Zoological Museum, Copenhagen
Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Mollusca
Subphylum: Conchifera
Class: Monoplacophora
Odhner, 1940
Shell of Monoplacophora

Monoplacophora

fossil record, and were thought to have become extinct 375 million years ago.[2]

Although the shell of many monoplacophorans is

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

tergomyans, this latter group containing extant Tryblidiids.[3]

Taxonomy

For a taxonomy of living monoplacophoran species, see Tryblidiida.

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

Class Monoplacophora

Anatomy and physiology

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

Monoplacophorans are

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

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]

Phylogenetic position

In 2006 a molecular study on

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

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

cephalopods
.

Fossil species

Living families:

  • Tryblidiida
    • Laevipilinidae
    • Micropilinidae
    • Monoplacophoridae
    • Neopilinidae

Extinct families:

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

The

gastropods
or monoplacophorans.

References

  1. ^
    doi:10.1111/j.1502-3931.1990.tb01361.x
    .
  2. ^
    S2CID 26108547
    .
  3. ^
    doi:10.1111/1475-4983.00198
    .
  4. doi:10.4002/040.061.0201
  5. ISBN 9781581129304
    – via Google Books.
  6. PMID 16675549
    .
  7. PMID 29300613. Archived from the original
    on 16 June 2011. Retrieved 12 November 2007.
  8. ^ Haszprunar, G. (2000). "[no title cited]". Am. Malacol. Bull. 15: 115–130.
  9. ^ 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.
  10. ISBN 0-12-751412-0
    .
  11. S2CID 39772232
    .
  12. PMID 19647088
    .
  13. S2CID 46429653
    .
  14. doi:10.1016/j.tree.2003.08.009
    .
  15. ^
    ISSN 0076-2997
    . e-.
  16. ISBN 3-925919-72-4. Bouchet, P.; Rocroi, J. -P (2005). "(online data ref)". Malacologia: International Journal of Malacology (1–2). reprinted from Bouchet & Rocroi (2005a)[15]

Further reading

  • Horný, Radvan (1963). "On the systematic position of cyrtonelloids (Mollusca)". Časopsis národního Muzea, oddil přírodovědný. 132 (2). Prague, CZ: 90–94.
  • Rozov, S.N. (1975). "A new order of the Monoplacophora". Paleontological Journal. 9. Washington, DC: 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.
    S2CID 23753587
    .

External links

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