Theevolution of the molluscs is the way in which theMollusca, one of the largest groups ofinvertebrate animals,evolved. Thisphylum includesgastropods,bivalves,scaphopods,cephalopods, and several other groups.Thefossil record of mollusks is relatively complete, and they are well represented in most fossil-bearing marine strata. Very early organisms which have dubiously^{[further explanation needed]} been compared to molluscs includeKimberella andOdontogriphus.

The tinyHelcionellid fossilYochelcionella is thought to be an earlymollusc^[1]

Spirally coiled shells appear in manygastropods.^[2]

Good evidence exists for the appearance ofgastropods,cephalopods andbivalves in theCambrian period538.8 to 486.85 million years ago. However, the evolutionary history both of the emergence of molluscs from the ancestral groupLophotrochozoa, and of their diversification into the well-known living andfossil forms, is still vigorously debated.

Debate occurs about whether someEdiacaran and EarlyCambrian fossils are molluscs.^[3]Kimberella, from about555 million years ago, has been described by some palaeontologists as "mollusc-like",^[4][5] but others are unwilling to go further than "probablebilaterian".^[6][7] There is an even sharper debate about whetherWiwaxia, from about505 million years ago, was a mollusc, and much of this centers on whether its feeding apparatus was a type ofradula or more similar to that of somepolychaete worms.^[6][8] Nicholas Butterfield, who opposes the idea thatWiwaxia was a mollusc, has written that earliermicrofossils from515 to 510 million years ago are fragments of a genuinely mollusc-like radula.^[9] This appears to contradict the concept that the ancestral molluscan radula was mineralized.^[10]

However, theHelcionellids, which first appear over540 million years ago in Early Cambrian rocks fromSiberia andChina,^[11][12] are thought to be early molluscs with rather snail-like shells. Shelled molluscs therefore predate the earliesttrilobites.^[1] Although most helcionellid fossils are only a few millimeters long, specimens a few centimeters long have also been found, most with morelimpet-like shapes. The tiny specimens have been suggested to be juveniles and the larger ones adults.^[13]

Some analyses of helcionellids concluded these were the earliestgastropods.^[14] However, other scientists are not convinced these Early Cambrian fossils show clear signs of thetorsion characteristic of modern gastropods, that twists the internal organs so the anus lies above the head.^[2][15][16]

  = Septa

  =Siphuncle

Septa andsiphuncle innautiloid shell

Volborthella, some fossils of which predate530 million years ago, was long thought to be a cephalopod, but discoveries of more detailed fossils showed its shell was not secreted, but built from grains of the mineralsilicon dioxide (silica), and it was not divided into a series of compartments bysepta as those of fossil shelled cephalopods and the livingNautilus are.Volborthella's classification is uncertain.^[17] The Late Cambrian fossilPlectronoceras is now thought to be the earliest clearly cephalopod fossil, as its shell had septa and asiphuncle, a strand of tissue thatNautilus uses to remove water from compartments it has vacated as it grows, and which is also visible in fossilammonite shells. However,Plectronoceras and other early cephalopods crept along the seafloor instead of swimming, as their shells contained a "ballast" of stony deposits on what is thought to be the underside, and had stripes and blotches on what is thought to be the upper surface.^[18] All cephalopods with external shells except thenautiloids became extinct by the end of theCretaceous period65 million years ago.^[19] However, the shell-lessColeoidea (squid,octopus,cuttlefish) are abundant today.^[20]

The Early Cambrian fossilsFordilla andPojetaia are regarded asbivalves.^{[21][22][23][24]} "Modern-looking" bivalves appeared in theOrdovician period,488 to 443 million years ago.^[25] One bivalve group, therudists, became majorreef-builders in the Cretaceous, but became extinct in theCretaceous–Paleogene extinction event.^[26] Even so, bivalves remain abundant and diverse.

TheHyolitha are a class of extinct animals with a shell andoperculum that may be molluscs. Authors who suggest they deserve their ownphylum do not comment on the position of this phylum in the tree of life^[27]

Thephylogeny (evolutionary "family tree") of molluscs is a controversial subject. In addition to the debates about whetherKimberella and any of the "halwaxiids" were molluscs or closely related to molluscs,^[5][6][8][9] debates arise about the relationships between the classes of living molluscs.^[7] In fact, some groups traditionally classified as molluscs may have to be redefined as distinct but related.^[30]

Molluscs are generally regarded members of theLophotrochozoa,^[28] a group defined by havingtrochophore larvae and, in the case of livingLophophorata, a feeding structure called alophophore. The other members of the Lophotrochozoa are theannelid worms and seven marinephyla.^[31] The diagram on the right summarizes a phylogeny presented in 2007.

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.^[32] For example, it is uncertain whether the ancestral mollusc wasmetameric (composed of repeating units)—if it was, that would suggest an origin from anannelid-like worm.^[33] Scientists disagree about this: Giribet and colleagues concluded, in 2006, the repetition of gills and of the foot's retractor muscles were later developments,^[34] while in 2007, Sigwart concluded the ancestral mollusc was metameric, and it had a foot used for creeping and a "shell" that was mineralized.^[7] In one particular branch of the family tree, the shell ofconchiferans is thought to have evolved from thespicules (small spines) ofaplacophorans; but this is difficult to reconcile with theembryological origins of spicules.^[32]

The molluscan shell appears to have originated from a mucus coating, which eventually stiffened into acuticle. This would have been impermeable and thus forced the development of more sophisticated respiratory apparatus in the form of gills.^[1] Eventually, the cuticle would have become mineralized,^[1] using the same genetic machinery (theengrailed gene) as most other bilaterianskeletons.^[33] The first mollusc shell almost certainly was reinforced with the mineralaragonite.^[35]

The evolutionary relationships 'within' the molluscs are also debated, and the diagrams below show two widely supported reconstructions:

Morphological analyses tend to recover a conchiferan clade that receives less support from molecular analyses,^[36] although these results also lead to unexpected paraphylies, for instance scattering the bivalves throughout all other mollusc groups.^[37]

However, an analysis in 2009 using bothmorphological andmolecular phylogenetics comparisons concluded the molluscs are notmonophyletic; in particular,Scaphopoda andBivalvia are both separate, monophyletic lineages unrelated to the remaining molluscan classes; the traditional phylum Mollusca ispolyphyletic, and it can only be made monophyletic if scaphopods and bivalves are excluded.^[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.^[39] Rather than eliminating unlikely relationships, the latest studies add new permutations of internal molluscan relationships, even bringing the conchiferan hypothesis into question.^[40]

• Ruppert, E.E., Fox, R.S., and Barnes, R.D. (2004).Invertebrate Zoology (7 ed.). Brooks / Cole.ISBN 0-03-025982-7.{{cite book}}: CS1 maint: multiple names: authors list (link)

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

• Malacology
• Evolution of protostomes

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