Phylogeny Mollusca

the phylogeny (evolutionary family tree ) of molluscs controversial subject. in addition debates whether kimberella , of halwaxiids molluscs or closely related molluscs, debates arise relationships between classes of living molluscs. in fact, groups traditionally classified molluscs may have redefined distinct related.

molluscs regarded members of lophotrochozoa, group defined having trochophore larvae and, in case of living lophophorata, feeding structure called lophophore. other members of lophotrochozoa annelid worms , 7 marine phyla. diagram on right summarizes phylogeny presented in 2007.

because relationships between members of family tree uncertain, difficult identify features inherited last common ancestor of molluscs. example, uncertain whether ancestral mollusc metameric (composed of repeating units)—if was, suggest origin annelid-like worm. scientists disagree this: giribet , colleagues concluded, in 2006, repetition of gills , of foot s retractor muscles later developments, while in 2007, sigwart concluded ancestral mollusc metameric, , had foot used creeping , shell mineralized. in 1 particular branch of family tree, shell of conchiferans thought have evolved spicules (small spines) of aplacophorans; difficult reconcile embryological origins of spicules.

the molluscan shell appears have originated mucus coating, stiffened cuticle. have been impermeable , forced development of more sophisticated respiratory apparatus in form of gills. eventually, cuticle have become mineralized, using same genetic machinery (engrailed) other bilaterian skeletons. first mollusc shell reinforced mineral aragonite.

the evolutionary relationships within molluscs debated, , diagrams below show 2 supported reconstructions:

morphological analyses tend recover conchiferan clade receives less support molecular analyses, although these results lead unexpected paraphylies, instance scattering bivalves throughout other mollusc groups.

however, analysis in 2009 using both morphological , molecular phylogenetics comparisons concluded molluscs not monophyletic; in particular, scaphopoda , bivalvia both separate, monophyletic lineages unrelated remaining molluscan classes; traditional phylum mollusca polyphyletic, , can made monophyletic if scaphopods , bivalves excluded. 2010 analysis recovered traditional conchiferan , aculiferan groups, , showed molluscs monophyletic, demonstrating available data solenogastres contaminated. current molecular data insufficient constrain molluscan phylogeny, , since methods used determine confidence in clades prone overestimation, risky place emphasis on areas of different studies agree. rather eliminating unlikely relationships, latest studies add new permutations of internal molluscan relationships, bringing conchiferan hypothesis question.