Monoplacophora
Encyclopedia
Monoplacophora, meaning "bearing one plate", is a polyphyletic class
of mollusks with a cap-like shell, living on the bottom of deep sea
. Extant representatives were unknown until 1952; previously they were known only from the fossil record.
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.Neopilina
is considered as a living fossil. they have a limpet like a shell
with the researched Neopilina closest to the chiton
s. The two classes in this new clade, with the proposed name Serialia, all show a variable number of serially repeated gill
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
ns, and that the cephalopods (squid and octopus
) arose from within the monoplacophoran lineage. However, some authors dispute this view and do not necessarily see modern monoplacophora as related to their fossil ancestors.
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 round; this could be reconciled if a secondary loss of shells caused a monoplacophoran body form to reappear secondarily. This is plausible: modern monoplacophorans are not closely related to vent-dwelling representatives from the Silurian, at least.
Cambrian monoplacophoran Knightoconus
antarcticus is thought to be an ancestor to the cephalopods.
ordo ?
The taxonomy of the Gastropoda by Bouchet & Rocroi, 2005
also contains Paleozoic molluscs of uncertain systematic position. It is not known whether these were gastropods or monoplacophorans.
Class (biology)
In biological classification, class is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, order, family, genus, and species, with class fitting between phylum and order...
of mollusks with a cap-like shell, living on the bottom of deep sea
Deep sea
The deep sea, or deep layer, is the lowest layer in the ocean, existing below the thermocline and above the seabed, at a depth of 1000 fathoms or more. Little or no light penetrates this part of the ocean and most of the organisms that live there rely for subsistence on falling organic matter...
. Extant representatives were unknown until 1952; previously they were known only from the fossil record.
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 BellerophontoideaBellerophontoidea
The Bellerophontoidea, , common name "bellerophonts", is a superfamily of extinct planospirally coiled globose molluscs, generally included in the Gastropoda that first appeared late in the Cambrian and continued until late in the Triassic.-Biology:Unlike normal gastropods, the shells of...
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.Neopilina
Neopilina
Neopilina is a highly derived genus of modern monoplacophoran.However, molecular methods show that they fall within the polyplacophoran clade, and fossil and morphological data show that they are rather derived and bear very little resemblance to an 'ancestral mollusc'.- Anatomy :Its anatomy led...
is considered as a living fossil. they have a limpet like a shell
Habitat
Extant monoplacophorans live in deeper waters (175 m & deeper). Cambrian forms predominately lived in shallow seas, whereas later Paleozoic forms are more commonly found and in deeper waters with soft, muddy sea floors.Phylogenetic position
In 2006 a molecular study on Laevipilina antarctica suggested that Monoplacophora and Polyplacophora form a well-supported cladeClade
A clade is a group consisting of a species and all its descendants. In the terms of biological systematics, a clade is a single "branch" on the "tree of life". The idea that such a "natural group" of organisms should be grouped together and given a taxonomic name is central to biological...
with the researched Neopilina closest to the chiton
Chiton
Chitons are small to large, primitive marine molluscs in the class Polyplacophora.There are 900 to 1,000 extant species of chitons in the class, which was formerly known as Amphineura....
s. The two classes in this new clade, with the proposed name Serialia, all show a variable number of serially repeated gill
Gill
A gill is a respiratory organ found in many aquatic organisms that extracts dissolved oxygen from water, afterward excreting carbon dioxide. The gills of some species such as hermit crabs have adapted to allow respiration on land provided they are kept moist...
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
Conchifera is a taxonomic term. It is a subphylum of the phylum Mollusca. It comprises all of the shell-bearing classes of molluscs, including the snails, clams, tusk shells, chitons, ammonites, monoplacophorans, and so on....
ns, and that the cephalopods (squid and octopus
Octopus
The octopus is a cephalopod mollusc of the order Octopoda. Octopuses have two eyes and four pairs of arms, and like other cephalopods they are bilaterally symmetric. An octopus has a hard beak, with its mouth at the center point of the arms...
) arose from within the monoplacophoran lineage. However, some authors dispute this view and do not necessarily see modern monoplacophora as related to their fossil ancestors.
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 round; this could be reconciled if a secondary loss of shells caused a monoplacophoran body form to reappear secondarily. This is plausible: modern monoplacophorans are not closely related to vent-dwelling representatives from the Silurian, at least.
Cambrian monoplacophoran Knightoconus
Knightoconus
Knightoconus antarcticus is a Cambrian monoplacophoran thought to represent an ancestor to the cephalopods. It had a chambered, conical shell, but lacked a siphuncle...
antarcticus is thought to be an ancestor to the cephalopods.
Fossil species
Families:- TryblidiidaTryblidiidaThe Tryblidiida are a group of monoplacophora containing the only extant representatives: a total of 29 species are alive today, inhabiting the ocean at depths of between 175 and 6500 metres .- History of discoveries :...
- TryblidiidaeTryblidiidaePilina is an extinct family of paleozoic monoplacophorans in the superfamily Tryblidioidea.- Genera :Genera in the family Tryblidiidae include:* Tryblidium Lindström, 1880...
von Zittel, 1899 - Palaeacmaeidae † (uncertain, as the CambrianCambrianThe Cambrian is the first geological period of the Paleozoic Era, lasting from Mya ; it is succeeded by the Ordovician. Its subdivisions, and indeed its base, are somewhat in flux. The period was established by Adam Sedgwick, who named it after Cambria, the Latin name for Wales, where Britain's...
type species is a cnidarian. It is maintained here as a receptacle for the PaleozoicPaleozoicThe Paleozoic era is the earliest of three geologic eras of the Phanerozoic eon, spanning from roughly...
genus Parmophorella)- Palaeacmaea Hall & Whitfield, 1872 †
- Parmophorella Matthew, 1886 †
ordo ?
- family ?
- KnightoconusKnightoconusKnightoconus antarcticus is a Cambrian monoplacophoran thought to represent an ancestor to the cephalopods. It had a chambered, conical shell, but lacked a siphuncle...
- Knightoconus antarcticus
- Knightoconus
The taxonomy of the Gastropoda by Bouchet & Rocroi, 2005
Taxonomy of the Gastropoda (Bouchet & Rocroi, 2005)
The taxonomy of the Gastropoda as it was revised by Philippe Bouchet and Jean-Pierre Rocroi is currently the most up-to-date overall system for classifying gastropod mollusks...
also contains Paleozoic molluscs of uncertain systematic position. It is not known whether these were gastropods or monoplacophorans.
Further reading
- Horný, Radwan 1963. On the systematic position of cyrtonelloids (Mollusca). Časopsis národního Muzea, oddil přírodovědný, 132: 90–93, Prague.
- Rozov, S. N. 1975. A new order of the Monoplacophora. Paleontological Journal, 9: 39–43, Washington.
External links
- Anatomy
- DiscoverLife: Taxonomy of Monoplacophora
- Animal Diversity Web: Monoplacophora
- Washington Nature Mapping Program: Monoplacophora
- Palaeos.com, Monoplacophora (Tryblidiida) Includes pictures and thorough discussion of cladistic issues.
- images of Monoplacophora