Diasoma is a proposed clade of

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

uniting the classes

Scaphopoda Scaphopoda (plural scaphopods , from Ancient Greek σκᾰ́φης ''skáphē'' "boat" and πούς ''poús'' "foot"), whose members are also known as tusk shells or tooth shells, are a class of shelled marine invertebrates belonging to the ph ...

and

Bivalvia Bivalvia () or bivalves, in previous centuries referred to as the Lamellibranchiata and Pelecypoda, is a class (biology), class of aquatic animal, aquatic molluscs (marine and freshwater) that have laterally compressed soft bodies enclosed b ...

. Whether scaphopods and bivalves are each other's closest living relatives among mollusks is disputed, leaving the monophyly of Diasoma in doubt. Diasoma was originally proposed on morphological grounds by Bruce Runnegar and John Pojeta Jr., in 1974. The name means "through-body", referring to the relatively straight gut with a mouth at the anterior end and anus at the posterior end, contrasting with

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 ...

and

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 ...

, in which the gut is more curved and the mouth and anus are usually much closer together. The grouping was accepted by many studies in the 1980s and 1990s, but a phylogenetic analysis of

18s rDNA 18S ribosomal RNA (abbreviated 18S rRNA) is a part of the ribosomal RNA in eukaryotes. It is a component of the Eukaryotic small ribosomal subunit (40S) and the cytosolic homologue of both the 12S rRNA in mitochondria and the 16S rRNA in plastid ...

conducted by Gerhard Steiner and Hermann Dreyer in 2003 found scaphopods to be more closely related to cephalopods than bivalves. A 2020 phylogenetic analysis by Kevin Kocot and colleagues found scaphopods to be more closely related to gastropods than bivalves. However, a molecular phylogenetic analysis published by Hao Song and colleagues in 2023 supports the monophyly of Diasoma. The extinct rostroconchs, a group possibly ancestral to the Scaphopoda, are also considered to belong to Diasoma. Song and colleagues inferred that Diasoma originated approximately 520 million years ago, during the

Cambrian period 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 Ordovici ...

, and considered the earlier fossil genera '' Anabarella'', ''

Watsonella ''Watsonella'' is an extinct genus of mollusc known from early (Terreneuvian) Cambrian strata. It has been hypothesized to be close to the origin of bivalves. It contains a single species, ''Watsonella crosbyi''. The genus is closely related to ...

'', and '' Mellopegma'' to be members of the diasome

stem group In phylogenetics, the crown group or crown assemblage is a collection of species composed of the living representatives of the collection, the most recent common ancestor of the collection, and all descendants of the most recent common ancestor. ...

References

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