Evolution Mollusca

the use of love darts land snail monachoides vicinus form of sexual selection


fossil record

good evidence exists appearance of gastropods (e.g. aldanella), cephalopods (e.g. plectronoceras, ?nectocaris) , bivalves (pojetaia, fordilla) towards middle of cambrian period, c. 500 million years ago, though arguably each of these may belong stem lineage of respective classes. however, evolutionary history both of emergence of molluscs ancestral group lophotrochozoa, , of diversification well-known living , fossil forms, still vigorously debated.

debate occurs whether ediacaran , cambrian fossils molluscs. kimberella, 555 million years ago, has been described paleontologists mollusc-like , others unwilling go further probable bilaterian ., if

there sharper debate whether wiwaxia, 505 million years ago, mollusc, , of centers on whether feeding apparatus type of radula or more similar of polychaete worms. nicholas butterfield, opposes idea wiwaxia mollusc, has written earlier microfossils 515 to 510 million years ago fragments of genuinely mollusc-like radula. appears contradict concept ancestral molluscan radula mineralized.

however, helcionellids, first appear on 540 million years ago in cambrian rocks siberia , china, thought molluscs rather snail-like shells. shelled molluscs therefore predate earliest trilobites. although helcionellid fossils few millimeters long, specimens few centimeters long have been found, more limpet-like shapes. tiny specimens have been suggested juveniles , larger ones adults.

some analyses of helcionellids concluded these earliest gastropods. however, other scientists not convinced these cambrian fossils show clear signs of torsion identifies modern gastropods twists internal organs anus lies above head.

septa , siphuncle in nautiloid shell

volborthella, fossils of predate 530 million years ago, long thought cephalopod, discoveries of more detailed fossils showed shell not secreted, built grains of mineral silicon dioxide (silica), , not divided series of compartments septa of fossil shelled cephalopods , living nautilus are. volborthella s classification uncertain. late cambrian fossil plectronoceras thought earliest cephalopod fossil, shell had septa , siphuncle, strand of tissue nautilus uses remove water compartments has vacated grows, , visible in fossil ammonite shells. however, plectronoceras , other cephalopods crept along seafloor instead of swimming, shells contained ballast of stony deposits on thought underside, , had stripes , blotches on thought upper surface. cephalopods external shells except nautiloids became extinct end of cretaceous period 65 million years ago. however, shell-less coleoidea (squid, octopus, cuttlefish) abundant today.

the cambrian fossils fordilla , pojetaia regarded bivalves. modern-looking bivalves appeared in ordovician period, 488 to 443 million years ago. 1 bivalve group, rudists, became major reef-builders in cretaceous, became extinct in cretaceous–paleogene extinction event. so, bivalves remain abundant , diverse.

the hyolitha class of extinct animals shell , operculum may molluscs. authors suggest deserve own phylum not comment on position of phylum in tree of life

phylogeny

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.