Evolution of molluscs

Diagram of primitive mollusc viewed from left side. Hypothetical mollusc ancestor. From Encyclopædia Britannica Eleventh Edition.

The evolution of the Mollusca is the way in which one of the largest groups of invertebrate animals evolved. This phylum of animals includes gastropods, bivalves, scaphopods, cephalopods, and several other groups. The fossil record of mollusks is relatively complete, and they are well represented in most fossil-bearing marine strata. Very early organisms which have dubiously been compared to molluscs include Kimberella and Odontogriphus.

Cladogram

An understanding of molluscan phylogeny dating from 2006 is shown below, as a horizontal cladogram (source:^[1]). This cladogram is entirely out of date; most studies now agree on an Aculiferan topology, with aplacophorans derived from polyplacophorans.^[2]

Mollusca   Aplacophora  Solenogastres Caudofoveata  Testaria  Polyplacophora  Palaeoloricata Neoloricata Conchifera  Monoplacophora  (unnamed)   Cyrtosoma  Gastropoda Cephalopoda  Diasoma  Rostroconchia  (unnamed)  Scaphopoda Bivalvia Basal Mollusca phylogeny

References

1. Zoology 250 Phylogenetic Trees (2006) http://www.biology.ualberta.ca/courses.hp/zool250/Clades/clade08-Mollusca.htm
2. Sigwart, J. D.; Lindberg, D. R. (2014). "Consensus and Confusion in Molluscan Trees: Evaluating Morphological and Molecular Phylogenies". Systematic Biology 64 (3): 384. doi:10.1093/sysbio/syu105. PMID 25472575.

Further reading

• From sea to land and beyond – New insights into the evolution of euthyneuran Gastropoda (Mollusca)