Monoplacophora

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 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 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 • 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==

''. The head region is on the left by the mouth. Monoplacophorans are univalved (though not gastropodal), limpet-shaped, and are untorted. They have a pseudometamerism of bilaterally symmetrical repeated organs and muscles. The extant members of the class live only in the deep ocean (the abyssal zone, the continental shelf, and the continental slope) 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. == 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 closest to the chitons. 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 conchiferans, and that the cephalopods (squids, octopuses, 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. == Fossil species ==

Living families: • Tryblidiida • Laevipilinidae • Micropilinidae • Monoplacophoridae • Neopilinidae Extinct families: • Tryblidiida • † Tryblidiidae von Zittel, 1899 • † Palaeacmaeidae (uncertain, as the Cambrian type species is a cnidarian. 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. also contains Paleozoic molluscs of uncertain systematic position. It is not known whether these were gastropods or monoplacophorans. == References ==