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monoplacophorans
Scientific classification
Kingdom: Animalia Phylum: Mollusca Class: Monoplacophora Odhner, 1940
Orders
Cyrtonellida Tryblidiida Pelagiellida

Monoplacophora is a class of mollusks thought to be extinct until April 1952, when a living animal was dredged up from deep marine sediments in the Middle America Trench off the Pacific coast of Costa Rica, and named Neopilina by its discoverer, Danish biologist Dr. Henning Lemche ^[1]. Malacology experiences few shattering moments, but in this case "their discovery has been described as 'the most dramatic one in the history of malacology.'" (Clausen). So far, more than a dozen living species of Monoplacofora have been discovered. The name "Monoplacophora" means 'bearing one plate'. All the present species live deep down in ocean trenches. An attempt at a common name, gastroverm, has proved unsuccessful.^[2]

Contents 1 Description 2 Notes 3 Taxonomy 4 References 5 External links

Little is known about the monoplacophorans. They have a single, flat, rounded bilateral shell that is often thin and fragile. The apex of the shell is forward. The fossil shells resemble chitons (Class Polyplacophora) or more precisely, limpets (Class Gastropoda); however, fossils showed a series of scars on the inner side of the shell that had served as muscle attachments: it was enough to determine that the fossil mollusks were in fact neither chiton-like nor limpet-like in the arrangement of their soft parts.

Now with living Monoplacophora to study, it can be seen that their body segments exhibit a serial repetition of kidneys, gills and reproductive structure. This used to be interpreted as a true segmentation, which suggested a "missing link" between mollusks and annelids. More recent studies have shown that the repetition of these organs is secondary. All known mollusks are thus non-segmented, and a derivation from annelids, that are always segmented, is very unlikely. The ancestors of mollusks were maybe flatworms.

Monoplacophorans move on a rounded foot. Respiration is through five or six pairs of gills on either side of the body. Their reduced head lacks eyes or tentacles. They seem to feed on microscopic organisms in mud or bottom detritus.

Previous specimens dated from the Paleozoic. Neopilina galatheae was the first known living specimen. These "living fossils" are about 0.5 to 3.0 cm in length.

Order Tryblidioidea

• Superfamily Tryblidiacea
  • Family Laevipilinidae
    • Genus Laevipilina. H. McLean, 1979
  • Family Monoplacophoridae
    • Genus Monoplacophorus L. I. Moskaslev, Y. A. Starobogatov & Z. A. Filatova, 1983
  • Family Neopilinidae
    • Genus Neopilina H. Lemche, 1957
      • Subgenus Lemchephila L. I. Moskalev, Y. A. Starobogatov & Z. A. Filetove, 1983
      • Subgenus Lemchephyala (= Lemchephila) L. I. Moskalev, Y. A. Starobogatov & Z. A. Filetove, 1983
    • Genus Micropilina
    • Genus Rokopella
    • Genus Veleropilina
  • Family Tryblidiidae
    • Subfamiliy Neopilininae
      • Genus Neopilina H. Lemche, 1957
        • Subgenus Neopilina
          • Species Neopilina galathea H. Lemche, 1957
          • Species Neopilina veleronis R. J. Menzies & W. Layton, 1963
    • Subfamily Tryblidiinae
  • Family Vemidae
    • • Genus Vema. H. Clarke & R. J. Menzies, 1959
        • Species Vema ewingi. H. Clarke & R. J. Menzies, 1959

• Lemche, Henning. 1957. "A new living deep-sea mollusc of the Cambro-Devonian class Monoplacophora." Nature 179:413-416.
• Lemche, Henning, in Marie Jenkins, The Curious Mollusks (New York, 1972).

• Anatomy
• Conrad D. Clausen, "Neopilina"; a living fossil"