Evolution of molluscs

Good evidence exists for the appearance of gastropods, cephalopods and bivalves in the Cambrian period 538.8 to 486.85 million years ago.

However, the evolutionary history both of the emergence of molluscs from the ancestral group Lophotrochozoa, and of their diversification into the well-known living and fossil forms, is still vigorously debated.

[6][7] There is an even sharper debate about whether Wiwaxia, from about 505 million years ago, was a mollusc, and much of this centers on whether its feeding apparatus was a type of radula or more similar to that of some polychaete worms.

[6][8] Nicholas Butterfield, who opposes the idea that Wiwaxia was a mollusc, has written that earlier microfossils from 515 to 510 million years ago are fragments of a genuinely mollusc-like radula.

[14] However, other scientists are not convinced these Early Cambrian fossils show clear signs of the torsion characteristic of modern gastropods, that twists the internal organs so the anus lies above the head.

Scaphopods (tusk shells) Aplacophorans(spicule-covered, worm-like) Polyplacophorans (chitons) Wiwaxia Halkieria Orthrozanclus Odontogriphus The phylogeny (evolutionary "family tree") of molluscs is a controversial subject.

[30] Molluscs are generally regarded members of the Lophotrochozoa,[28] a group defined by having trochophore larvae and, in the case of living Lophophorata, a feeding structure called a lophophore.

Because the relationships between the members of the family tree are uncertain, it is difficult to identify the features inherited from the last common ancestor of all molluscs.

[30] A 2010 analysis recovered the traditional conchiferan and aculiferan groups, and showed molluscs were monophyletic, demonstrating that available data for solenogastres was contaminated.

[38] Current molecular data are insufficient to constrain the molluscan phylogeny, and since the methods used to determine the confidence in clades are prone to overestimation, it is risky to place too much emphasis even on the areas of which different studies agree.