Shell of Monoplacophora Monoplacophora , meaning "bearing one plate", is a

polyphyletic A polyphyletic group is an assemblage that includes organisms with mixed evolutionary origin but does not include their most recent common ancestor. The term is often applied to groups that share similar features known as Homoplasy, homoplasies ...

class Class, Classes, or The Class may refer to: Common uses not otherwise categorized * Class (biology), a taxonomic rank * Class (knowledge representation), a collection of individuals or objects * Class (philosophy), an analytical concept used d ...

mollusc Mollusca is a phylum of protostome, protostomic invertebrate animals, whose members are known as molluscs or mollusks (). Around 76,000 extant taxon, extant species of molluscs are recognized, making it the second-largest animal phylum ...

s with a cap-like shell, inhabiting

deep sea The deep sea is broadly defined as the ocean depth where light begins to fade, at an approximate depth of or the point of transition from continental shelves to continental slopes. Conditions within the deep sea are a combination of low tempe ...

environments.

Extant Extant or Least-concern species, least concern 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 * Exta ...

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 ...

, 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 with a conical gastropod shell, shell shape (patelliform) and a strong, muscular foot. This general category of conical shell is known as "patelliform" (dish-shaped). Existing within the class Gastropoda, ...

-like in shape, they are not

gastropods Gastropods (), commonly known as slugs and snails, belong to a large taxonomic class of invertebrates within the phylum Mollusca called Gastropoda (). This class comprises snails and slugs from saltwater, freshwater, and from the land. Ther ...

, 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 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. 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 Helcionellid or Helcionelliformes is an order of small fossil shells that are universally interpreted as molluscs, though no sources spell out why this taxonomic interpretation is preferred. These animals are first found about in the late Nema ...

from the traditional, cap-like tergomyans, this latter group containing extant Tryblidiids.

Taxonomy

Taxonomy of Monoplacophora per Bouchet, et al. (2017): Class Monoplacophora * subclass

Cephalopoda 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, ...

? *† 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 ***† 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),

-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 mant ...

. 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, the face of a human being has a plane of symme ...

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. The word ''abyss'' comes from the Greek word (), meaning "bottomless". At depths of , this zone remains in perpetual darkness. It covers 83% of the total area ...

, the

continental shelf A continental shelf is a portion of a continent that is submerged under an area of relatively shallow water, known as a shelf sea. Much of these shelves were exposed by drops in sea level during glacial periods. The shelf surrounding an islan ...

, 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 margi ...

) 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

s when viewed dorsally, monoplacophorans are not anatomically similar to

gastropod Gastropods (), commonly known as slugs and snails, belong to a large Taxonomy (biology), taxonomic class of invertebrates within the phylum Mollusca called Gastropoda (). This class comprises snails and slugs from saltwater, freshwater, and fro ...

s. 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 sea cradles or coat-of-mail shells or suck ...

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 A ganglion (: ganglia) is a group of neuron cell bodies in the peripheral nervous system. In the somatic nervous system, this includes dorsal root ganglia and trigeminal ganglia among a few others. In the autonomic nervous system, there are ...

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 (: 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. Nephridia co ...

). 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

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 (; : radulae or radulas) is an anatomical structure used by mollusks 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 enters ...

, 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 pairs 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 In biology, a clade (), also known as a Monophyly, monophyletic group or natural group, is a group of organisms that is composed of a common ancestor and all of its descendants. Clades are the fundamental unit of cladistics, a modern approach t ...

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

s. The two classes in this new clade, with the proposed name Serialia, all show a variable number of serially repeated

gill A gill () is a respiration organ, respiratory organ that many aquatic ecosystem, aquatic organisms use to extract dissolved oxygen from water and to excrete carbon dioxide. The gills of some species, such as hermit crabs, have adapted to allow r ...

s 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, containing five extant classes: Monoplacophora, Cephalopoda, Gastropoda, Bivalvia, and Scaphopoda. Conchiferans can bear a single shell as in snails and ammonites, a single pair of shell ...

ns, and that the

cephalopods A cephalopod is any member of the molluscan Taxonomic rank, class Cephalopoda (Greek language, Greek plural , ; "head-feet") such as a squid, octopus, cuttlefish, or nautilus. These exclusively marine animals are characterized by bilateral symm ...

(

squid A squid (: squid) is a mollusc with an elongated soft body, large eyes, eight cephalopod limb, arms, and two tentacles in the orders Myopsida, Oegopsida, and Bathyteuthida (though many other molluscs within the broader Neocoleoidea are also ...

octopus An octopus (: octopuses or octopodes) 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, cuttlefish, and nautiloids. Like oth ...

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

Fossil species

Living families: *

** Laevipilinidae ** Micropilinidae ** Monoplacophoridae ** Neopilinidae Extinct families: *

** † Tryblidiidae von Zittel, 1899 * † Palaeacmaeidae (uncertain, as the

Cambrian The Cambrian ( ) is the first geological period of the Paleozoic Era, and the Phanerozoic Eon. The Cambrian lasted 51.95 million years from the end of the preceding Ediacaran period 538.8 Ma (million years ago) to the beginning of the Ordov ...

type species is a

cnidarian Cnidaria ( ) is a phylum under kingdom Animalia containing over 11,000 species of aquatic invertebrates found both in fresh water, freshwater and marine environments (predominantly the latter), including jellyfish, hydroid (zoology), hydroids, ...

. It is maintained here as a receptacle for the

Paleozoic The Paleozoic ( , , ; or Palaeozoic) Era is the first of three Era (geology), geological eras of the Phanerozoic Eon. Beginning 538.8 million years ago (Ma), it succeeds the Neoproterozoic (the last era of the Proterozoic Eon) and ends 251.9 Ma a ...

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) The taxonomy of the Gastropoda as it was revised in 2005 by Philippe Bouchet and Jean-Pierre Rocroi is a system for the scientific classification of gastropod mollusks (Gastropods are a taxonomic class of animals which consists of snails and sl ...

also contains Paleozoic molluscs of uncertain systematic position. It is not known whether these were

s or monoplacophorans.

References

* *

External links

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