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The first molluscs probably arose during Precambrian times because fossils attributed to Mollusca have been found in geological strata as old as the early Cambrian period. On the basis of such shared features as spiral cleavage, mesoderm from the 4d blastomere, and trochophore larva, many zoologists argue that Mollusca are protostomes, allied with the annelids in Lophotrochozoa. Opinions differ, however, as to the exact nature of the relationship among lophotrochozoans. Some characters suggest that molluscs and annelids are sister taxa, but we do not depict a branching order for these taxa.
Annelid worms have a developmental pattern very similar to that of molluscs, but the annelid body is metameric, composed of serially repeated segments, whereas there are no true segments in molluscs. Both annelids and molluscs are coelomate protostomes, but the coelom is greatly reduced in molluscs as compared with annelids. Opinions differ as to whether molluscs were derived from a wormlike ancestor independent of annelids, share an ancestor with annelids after the advent of the coelom, or share a segmented common ancestor with annelids. Until the lophotrochozoan phylogeny is better resolved, it will not be possible to determine whether molluscs and annelids shared a coelomate ancestor.
The hypothesis that annelids and molluscs shared a segmented ancestor is strengthened if the repeated body parts present in Neopilina (class Monoplacophora), and in some chitons, can be considered evidence of metamerism. However, recent morphological and developmental studies indicate that these parts are not remnants of an ancestral metameric body. A new perspective on the evolution of repeated parts (gills and muscles) comes from analysis of a new molluscan cladogram. This cladogram was based on molecular characters from a wide range of molluscs, including a monoplacophoran.
The cladogram places monoplacophorans as the sister taxon to chitons; it unites the two taxa with repeated body parts in a clade called Serialia. Further, clade Serialia does not branch from the base of the molluscan tree, as it would if the ancestral mollusc were segmented. Instead, clade Serialia branches from the molluscan lineage after the wormlike caudofoveates and solenogastres, indicating that the repeated structures are derived molluscan features, not ancestral features.
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