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Monoplacophora , meaning "bearing one plate", is a polyphyletic superclass of molluscs with a cap-like shell inhabiting deep sea environments .
Extant Extant is the opposite of the word extinct. It may refer to: * Extant hereditary titles * Extant literature, surviving literature, such as ''Beowulf'', the oldest extant manuscript written in English * Extant taxon, a taxon which is not extin ...
representatives were not recognized as such until 1952; previously they were known only from the
fossil record A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved in ...
, and were thought to have become extinct 375 million years ago. Although the shell of many monoplacophorans is
limpet Limpets are a group of aquatic snails that exhibit a conical gastropod shell, shell shape (patelliform) and a strong, muscular foot. Limpets are members of the class Gastropoda, but are polyphyletic, meaning the various groups called "limpets" ...
-like in shape, they are not gastropods, nor do they have any close relation to gastropods.


Definition

Discussion about monoplacophorans is made difficult by the slippery definition of the taxon; some authors take it to refer to all non-gastropod mollusks with a single shell, or all single-shelled mollusks with serially repeated units; whereas other workers restrict the definition to cap-shaped forms, excluding spiral and other shapes of shell. The inclusion of the gastropod-like
Bellerophontoidea Bellerophontoidea, common name "bellerophonts",Bouchet P. & Rocroi J.-P. (Ed.); Frýda J., Hausdorf B., Ponder W., Valdes A. & Warén A. 2005. ''Classification and nomenclator of gastropod families''. Malacologia: International Journal of Malaco ...
within the group is also contentious. 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.


Taxonomy

Taxonomy of Monoplacophora per Bouchet, et al. (2017): Class Monoplacophora *† Subclass Cyrtolitiones **† Order Sinuitopsida ***† Superfamily Cyrtolitoidea S. A. Miller, 1889 ****† Family
Cyrtolitidae Cyrtolitidae is an extinct family of monoplacophorans in the order Cyrtonellida. Genera * '' Cloudia'' * '' Cyclocyrtonella'' * '' Cyrtolites'' * '' Cyrtonellopsis'' * '' Kolihadiscus'' * '' Neocyrtolites'' * '' Paracyrtolites'' * '' Quasisin ...
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 Cyrtonellida is a group of "monoplacophora", representing either a sister taxon to, or a polyphyletic assemblage including, the Trybliida. Subtaxa * Carcassonnellidae * Cyrtolitidae Cyrtolitidae is an extinct family of monoplacophorans i ...
***† 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 Tryblidiida is a taxon of monoplacophoran molluscans containing the only extant representatives: 37 species are still alive today, inhabiting the ocean at depths of between . History of discoveries The first captured living monoplacophoran was ...
***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)


Anatomy and physiology

Monoplacophorans are univalved (though not gastropodal),
limpet Limpets are a group of aquatic snails that exhibit a conical gastropod shell, shell shape (patelliform) and a strong, muscular foot. Limpets are members of the class Gastropoda, but are polyphyletic, meaning the various groups called "limpets" ...
-shaped, and are
untorted Torsion is a gastropod synapomorphy which occurs in all gastropods during larval development. Torsion is the rotation of the visceral mass, mantle, and shell 180˚ with respect to the head and foot of the gastropod. This rotation brings the mantl ...
. They have a pseudometamerism of
bilaterally symmetrical Symmetry in biology refers to the symmetry observed in organisms, including plants, animals, fungi, and bacteria. External symmetry can be easily seen by just looking at an organism. For example, take the face of a human being which has a pl ...
repeated organs and muscles. The extant members of the class live only in the deep ocean (the
abyssal zone The abyssal zone or abyssopelagic zone is a layer of the pelagic zone of the ocean. "Abyss" derives from the Greek word , meaning bottomless. At depths of , this zone remains in perpetual darkness. It covers 83% of the total area of the ocean an ...
, the continental shelf, and the
continental slope A continental margin is the outer edge of continental crust abutting oceanic crust under coastal waters. It is one of the three major zones of the ocean floor, the other two being deep-ocean basins and mid-ocean ridges. The continental margin ...
) at depths below . Cambrian forms predominately lived in shallow seas, whereas later Paleozoic forms are more commonly found in deeper waters with soft, muddy sea floors. Although superficially resembling
limpet Limpets are a group of aquatic snails that exhibit a conical gastropod shell, shell shape (patelliform) and a strong, muscular foot. Limpets are members of the class Gastropoda, but are polyphyletic, meaning the various groups called "limpets" ...
s when viewed dorsally, monoplacophorans are not anatomically similar to gastropods. Some similarities are shared with the
chiton Chitons () are marine molluscs of varying size in the class Polyplacophora (), formerly known as Amphineura. About 940 extant and 430 fossil species are recognized. They are also sometimes known as gumboots or sea cradles or coat-of-mail sh ...
s, 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 nephridium (plural ''nephridia'') is an invertebrate organ, found in pairs and performing a function similar to the vertebrate kidneys (which originated from the chordate nephridia). Nephridia remove metabolic wastes from an animal's body. Neph ...
). 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
chiton Chitons () are marine molluscs of varying size in the class Polyplacophora (), formerly known as Amphineura. About 940 extant and 430 fossil species are recognized. They are also sometimes known as gumboots or sea cradles or coat-of-mail sh ...
s, 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 The radula (, ; plural radulae or radulas) is an anatomical structure used by molluscs for feeding, sometimes compared to a tongue. It is a minutely toothed, chitinous ribbon, which is typically used for scraping or cutting food before the food ...
, 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.


Phylogenetic position

In 2006 a molecular study on '' Laevipilina antarctica'' suggested that extant Monoplacophora and Polyplacophora form a well-supported clade with the researched ''
Neopilina ''Neopilina'' is a highly derived genus of modern monoplacophoran.Organisms, Genes and Evolution: Evolutionary Theory at the Crossroads ; Proceedings of the 7th International Senckenberg Conference. By Dieter Stefan Peters, Michael Weingarten. C ...
'' closest to the
chiton Chitons () are marine molluscs of varying size in the class Polyplacophora (), formerly known as Amphineura. About 940 extant and 430 fossil species are recognized. They are also sometimes known as gumboots or sea cradles or coat-of-mail sh ...
s. 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
conchifera Conchifera is a subphylum of the phylum Mollusca. It comprises all of the shell-bearing classes of molluscs, such as clams, tusk shells, ammonites, and monoplacophorans. The other subphylum is Aculifera. Non-monoplacophoran conchiferans emerg ...
ns, and that the
cephalopods A cephalopod is any member of the molluscan class Cephalopoda (Greek plural , ; "head-feet") such as a squid, octopus, cuttlefish, or nautilus. These exclusively marine animals are characterized by bilateral body symmetry, a prominent head, a ...
( squids,
octopus An octopus ( : octopuses or octopodes, see below for variants) is a soft-bodied, eight- limbed mollusc of the order Octopoda (, ). The order consists of some 300 species and is grouped within the class Cephalopoda with squids, cuttlef ...
es, and relatives) arose from within the monoplacophoran lineage. However, some authors dispute this view and do not necessarily see modern Monoplacophora as related to their presumed fossil ancestors. The concept of Serialia is supported by other molecular studies. 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. 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. Cambrian monoplacophoran '' Knightoconus antarcticus'' is hypothesised to be an ancestor to the
cephalopods A cephalopod is any member of the molluscan class Cephalopoda (Greek plural , ; "head-feet") such as a squid, octopus, cuttlefish, or nautilus. These exclusively marine animals are characterized by bilateral body symmetry, a prominent head, a ...
.


Fossil species

Living families: *
Tryblidiida Tryblidiida is a taxon of monoplacophoran molluscans containing the only extant representatives: 37 species are still alive today, inhabiting the ocean at depths of between . History of discoveries The first captured living monoplacophoran was ...
** Laevipilinidae ** Micropilinidae ** Monoplacophoridae ** Neopilinidae Extinct families: *
Tryblidiida Tryblidiida is a taxon of monoplacophoran molluscans containing the only extant representatives: 37 species are still alive today, inhabiting the ocean at depths of between . History of discoveries The first captured living monoplacophoran was ...
** † Tryblidiidae von Zittel, 1899 * † Palaeacmaeidae (uncertain, as the
Cambrian The Cambrian Period ( ; sometimes symbolized Ꞓ) was the first geological period of the Paleozoic Era, and of the Phanerozoic Eon. The Cambrian lasted 53.4 million years from the end of the preceding Ediacaran Period 538.8 million years ago ( ...
type species is a
cnidarian Cnidaria () is a phylum under kingdom Animalia containing over 11,000 species of aquatic animals found both in freshwater and marine environments, predominantly the latter. Their distinguishing feature is cnidocytes, specialized cells that th ...
. 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. The Taxonomy of the Gastropoda (Bouchet & Rocroi, 2005) also contains Paleozoic molluscs of uncertain systematic position. It is not known whether these were gastropods or monoplacophorans.


References

* *


Further reading

* * *


External links

* * * * * Includes pictures and thorough discussion of cladistic issues. * {{Taxonbar, from=Q194308 Extant Cambrian first appearances