Metazoa Contents


Abbreviated Dendrogram
Spiralia ├─Brachiopoda └─┬─Annelida │ └─Mollusca ├─Aculifera │ ╞═Mattheviidae │ │ └─Aplacophora └─Conchifera ╞═Tryblidiida (Monoplacophora) │ └─Cephalopoda ╘═Helcionelloida ╞═Rostroconchia │ └─Scaphopoda ├─Bivalvia └─Gastropoda


Sepia officinalis The European Common Cuttlefish, Sepia officinalis (Cephalopoda, Coleoidea, Sepiida, Sepiidae)
Photo © Hans Hillewaert, Creative Commons Attribution Share Alike, WoRMS image

Mollusca: Contents

Part 1: Alphabetical Listings

A. Glossaries of terms and abbreviations.

Overall Glossary
      Bivalve Glossary
      Cephalopod Glossary
      Gastropod Glossary

B. Taxon Index: alphabetical list of taxa.

Very incomplete taxon list

C. References: literature citations by author.

Overall References
      Cephalopod References

Part 2: Phylogenetic Listings

A. Dendrograms ("Cladograms")

Summary Dendrogram
Molluscan Phylogeny
      Bivalve Dendrogram
      Cephalopod Dendrogram
      Helcionelloida Dendrogram       Gastropod Dendrogram

B. Descriptions

Molluscan phylogeny is a highly controversial subject. The introduction of both molecular phylogeny and cladistic methodology has if anything only obscured things, with a molecular consensus emerging totally at odds with both morphology and paleontology (e.g. bivalves and gastropods as sister taxa). The revised units also need to be uploaded

Solenogastres Solenogastres (= Neomeniomorpha; Aplacophora in part) no fossil record very small creeping worm-like soft bodied shell-less forms
Caudofoveata Caudofoveata (= Chaetodermomorpha; Aplacophora in part) ?Silurian to Recent very small deep sea worm-like soft bodied burrowing shell-less forms. The Silurian Acaenoplax, which resembles both Solenogastres, Caudofoveata and Polyplacophora, may be an early form of this lineage.
Polyplacophora Polyplacophora (= Amphineura in part) Furongian to Recent The chitons, which have eight plates or "valves" which are overlapping in post-Mesozoic forms. The foot is broad; the head reduced. The soft parts show some segmentation. The animal generally lives attached to rocks in the inter-tidal zone. Appear to be distantly related to the Solenogastres and Caudofoveata (as ancestors/cousins), and the Tryblidiida
Tryblidiida Tryblidiida (= Monoplacophora sensu stricto = Tryblidiida = Tergomya in part) Furongian to Recent Creeping cap-shelled forms with paired muscle attachments. The most primitive conchiferan group. A few dozen deep water recent species remain as "living fossils". As with the Polyplacophora the soft parts show some segmentation. Probably evolved from a soft-bodied ancestor, possibly Polyplacophora relationships.
Stenothecoida Stenothecidae (= Monoplacophora in part) Early to Middle Cambrian Small group of little known bivalved forms. There relationship with other molluscan groups is unclear, and it is not agreed whether they qualify as class status. No descendents, ancestors unknown.
Heliconelloida Helcionelloida (= Monoplacophora in part; paraphyletic) Earliest to Late Cambrian an ancestral lineage or grade. Consist of creeping or weakly infaunal forms with cap-shaped or spiral shell. Probably evolved from a soft-bodied ancestor, as the direction of shell coiling is opposite that of the Tryblidiida, which they predate in any case. Generally considered directly or indirectly the ancestors of all conchiferan mollusks other than the Tryblidiida.
Bivalvia Bivalvia (= Pelycopoda) Early Cambrian to Recent mostly sedentary or burrowing forms with a shell of two valves hinged dorsally. The foot is generally hatchet-shaped (hence the name "Pelycopod"); the head lacking; a very diverse group, includes epifaunal or infaunal forms, mostly marine but some freshwater species. Clams oysters, mussels, etc. Generally considered to have evolved from rostroconches , although this would relate them to helcionelloids , if not actually Scaphopods and Cephalopods, and the weight of current molecular and cladistic evidence indicates the bivalves diverged from the main Conchiferan lineage quite early.
Paragastropoda Paragastropoda Early Cambrian to Devonian Small group of superficially snail-like forms, and perhaps snail-like habits, distinguished by lack of torsion. Evolved from helcionelloids , no descendents. It is not even certain if this is a natural or a polyphyletic group.
Tergomya "Tergomya" in part (= Monoplacophora in part; polyphyletic) Furongian to Devonian A probably artificial assemblage of creeping cap- or partly spiral shelled forms with single or paired muscle attachments. The Tryblidiida are usually included here but they are certainly a more basal group. Non- tryblidiidan Tergomya may have evolved probably several times from helcionelloids , although some may be related to the Tryblidiida. Some of the helcionelloid -descended forms would seem to be related to the Gastropods
Gastropoda Gastropoda Furongian to Recent Diverse group of shelled or shell-less forms, evolved from a spiral-shelled helcionelloid or helcionelloid -descended "tergomyan" ancestor that underwent torsion. The body is usually asymmetrical, with a distinct head, pair of eyes, and one or two pairs of tentacles. The shell is usually spirally-coiled, but may (in the case of limpets) be cap shaped, and in some species is vestigial or completely absent. The foot is broad and used to creep along the substrate. Includes marine, freshwater, and terrestrial species (the only mollusks to invade the land). Snails, slugs and their relatives
Rostroconchia Rostroconchia Early Cambrian to Late Permian A Paleozoic group of infaunal forms. The shell consists of two valves joined in adults by an enclosed hinge. Evolved from helcionelloids . The posterior of the shell is usually produced into an elongate tube, presumably for inhalant and exhalent siphons. Originally considered to have given rise to both Scaphopods and Bivalves, as they share many similarities with both those groups. But relationships among early molluscan classes remain controversial
Scaphopoda Scaphopoda Devonian to Recent small semi-infaunal forms, the tubular shell open at both ends and resembles a miniature elephant's tusk, with one end larger than the other. The foot is conical; there are no gills. Probably evolved from rostroconches . Shown to be related to the Cephalopods.
Cricoconarida Cricoconarida (= Tentaculitoidea) Middle Cambrian to Devonian very small pelagic forms with tubular shells. They are thought to be related to the Cephalopods, and seem to have born a number of arms.
Cephalopoda Cephalopoda Latest Cambrian to Recent diverse group of large intelligent predaceous forms, with a large external shell (most Paleozoic and Mesozoic forms, and current Nautilus), a vestigial internal shell, or (in the case of the octopus) the shell is lost altogether. The head is large with well-developed eyes, and armed with horny chitinous jaws and many arms or tentacles for grasping prey; the head fused to the foot (hence the name). Evolved from helcionelloids .
Hyolitha Hyolitha Earliest Cambrian to Permian Enigmatic animals with small conical shells, shell- microstructure indicates close to mollusks but experts do not agree whether they actually were mollusks .

Part 3: Specific terminology

The Shell
The Radula

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