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]. The study of mollusks experiences few shattering moments, but in this case the "discovery [of living Monoplacophora] has been described as 'the most dramatic one in the history of malacology.'" (Clausen). So far, more than a dozen living species of Monoplacophora 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] For other uses, see Scientific classification (disambiguation). ... For other uses, see Animal (disambiguation). ... Classes Caudofoveata Aplacophora Polyplacophora Monoplacophora Bivalvia Scaphopoda Gastropoda Cephalopoda † Rostroconchia † Helcionelloida † ?Bellerophontidae The molluscs (British spelling) or mollusks (American spelling) are members of the large and diverse phylum Mollusca, which includes a variety of familiar animals well-known for their decorative shells or as seafood. ... Nils Hjalmar Odhner (1884-1973) was a scandinavian zoologist who studied mollusks. ... Year 1940 (MCMXL) was a leap year starting on Monday (link will display the full 1940 calendar) of the Gregorian calendar. ... Classes Caudofoveata Aplacophora Polyplacophora - Chitons Monoplacophora Bivalvia - Bivalves Scaphopoda - Tusk shells Gastropoda - Snails and Slugs Cephalopoda - Squids, Octopuses, etc. ... For other uses, see Animal (disambiguation). ... The trench lies at the convergence of the Pacific, Cocos, Nazca, North American, and Caribbean plates The Middle America Trench is an oceanic trench in the eastern Pacific Ocean, stretching from central Mexico to Costa Rica. ... Classes Caudofoveata Aplacophora Polyplacophora - Chitons Monoplacophora Bivalvia - Bivalves Scaphopoda - Tusk shells Gastropoda - Snails and Slugs Cephalopoda - Squids, Octopuses, etc. ...

Description

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. Apex is the, usually pointed, tip of the spire of the shell of a gastropod. ... Lined Chiton (Tonicella lineata) Chitons, also called polyplacophorans and rarely polyplacophores, are 860 species of molluscs of the Class Polyplacophora. ... Subclasses Eogastropoda (True Limpets and relatives) Orthogastropoda The gastropods, also previously known as gasteropods, or univalves, and more commonly known as snails and slugs, are the largest and most successful class of mollusks, with 60,000-75,000 known living species. ...

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. Vertebrates have a segmented vertebral column and brain. ... Look up missing link in Wiktionary, the free dictionary. ... Classes Caudofoveata Aplacophora Polyplacophora - Chitons Monoplacophora Bivalvia - Bivalves Scaphopoda - Tusk shells Gastropoda - Snails and Slugs Cephalopoda - Squids, Octopuses, etc. ... Classes and subclasses Class Polychaeta (paraphyletic?) Class Clitellata    Oligochaeta - Earthworms and others    Acanthobdellida    Branchiobdellida    Hirudinea - Leeches Class Myzostomida Class Archiannelida (polyphyletic) Class Echiura *Some authors consider the subclasses under Clitellata to be classes The annelids, collectively called Annelida, are a large phylum of animals, comprising the segmented worms, with about... Classes Monogenea Trematoda Cestoda Turbellaria The flatworms (Platyhelminthes, Greek platy: flat; helminth: worm) are a phylum of relatively simple soft-bodied invertebrate animals. ...

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. The Paleozoic Era (from the Greek palaio, old and zoion, animals, meaning ancient life) is the earliest of three geologic eras of the Phanerozoic eon. ... This article or section does not adequately cite its references or sources. ...

Notes

The Silurian is a major division of the geologic timescale that extends from the end of the Ordovician period, about 443. ...

Taxonomy

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

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... 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...

References

• 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).

External links

• Anatomy
• Conrad D. Clausen, "Neopilina"; a living fossil" Creationist perspective on the discovery