Phylum Mollusca

Tree of Life

Phylum Mollusca

Kingdom Animalia

Phylum MolluscaPhylum Mullusca is both large and diverse. Molluscs comprise the familiar animals commonly known for both their shells and fine seafood, including snails, clams, octopus and squid.

Molluscs' development suggest a close ancestral relationship to other protostomes, notably the Annelids, that includes segmented worms. Mulluscs are believed to have appeared near the base of the Cambrian and have left an extensive fossil record of several tens of thousands of species. Science believes Molluscs fully participated in the Cambrian Explosion phenomena, with the appearance and disapperance of many forms, many of which remain undiscovered in the fossil record.

Gastropods, Bivalves, and Cephalopods, the major groups of interest as fossils, diversified and specialized into the Ordovician to become ubiquitous throughout marine ecosystems. They adapted with the new selective pressures as other marine life forms similrly expanded, and they became both hunter of and hunted by new forms. The fossil record attests to the bivalves adapting to fresh water environments in the Devonian, as well as the appearance of land-based Gastropods in the Carboniferous. The ammonites particularly flourished throughout the Mesozoic before going extinct in the K-T event along with the dinosaurs.

Phylum Mollusca
Class Aplacophora Extant, deep sea wormlike animals
Class Polyplacophora Chitons, extant animals
Conchifera (those that carry shells) Class Monoplacophora (note 1) Primitive with discernable segmentation (example)
Class Bivalvia Extant, scallops, clams, oysters and mussels
Class Scaphopoda Extant, tubular in form and rare in the fossil record
Class Gastropoda (note 2) Largest and most successful group of Mollusks
Class Cephalopoda Diverse group containing ammonites, nautiluses, squid, octopus
Class Rostroconchia (note 3) An enigmatic Class that went extinct in the Permian


  1. Monoplacophora: Class thought extinct until 1952, they currently live deep in ocean trenches. Body segmentation resembles primitive forms of Phylum Annelida, suggesting a common ancestor with the annelids. Tryblidiida could be considered a living fossil, while Stenothecoida and Helcionelloida failed to survive the Cambrian.
  2. Gastropoda: This most prolific and most successful class of mollusks appeared in the Upper Cambrian and today contains more than 60,000 known living species species, of which snails and slugs are most commonly known. The taxonomy comprises a myriad of subclasses, orders and superfamilies, and seemingly undergoes constant revision.
  3. Rostroconchia: a class of extinct mollusks dating from the early Cambrian to the late Permian, initially thought to be bivalves, but were later given their own class.