Evolution of molluscs

{{Short description|The origin and diversification of molluscs through geologic time}}

The evolution of the molluscs is the way in which the Mollusca, one of the largest groups of invertebrate animals, evolved. This phylum includes gastropods, bivalves, scaphopods, cephalopods, and several other groups.

The fossil record of mollusks is relatively complete, and they are well represented in most fossil-bearing marine strata. Very early organisms which have dubiously{{explanation needed|date=December 2021}} been compared to molluscs include Kimberella and Odontogriphus.

Fossil record

{{multiple image

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| direction = horizontal

| image1 = Yochelcionella_cyrano_svg_hariadhi.svg

| width1 = 166

| alt1 =

| caption1 = The tiny Helcionellid fossil Yochelcionella is thought to be an early mollusc

| image2 = Neptunea despecta.jpg

| width2 = 150

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| caption2 = Spirally coiled shells appear in many gastropods.Ruppert, pp. 300–343

}}

Good evidence exists for the appearance of gastropods, cephalopods and bivalves in the Cambrian period {{ma|Cambrian|{{period end|Cambrian}}}}. 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.

Debate occurs about whether some Ediacaran and Early Cambrian fossils are molluscs.{{cite book|author=Nelson R Cabej|date=2019|url=https://books.google.com/books?id=QdO1DwAAQBAJ&dq=Myoscolex&pg=PA152|title=Epigenetic Mechanisms of the Cambrian Explosion|publisher=Elsevier Science|page=152|isbn=9780128143124}} Kimberella, from about {{ma|555}}, has been described by some palaeontologists as "mollusc-like",{{Cite journal

| author = Fedonkin, M.A. |author2=Waggoner, B.M. | year = 1997

| title = The Late Precambrian fossil Kimberella is a mollusc-like bilaterian organism

|s2cid=4395089 | doi-access = free }}{{Cite journal | title=New data on Kimberella, the Vendian mollusc-like organism (White Sea region, Russia): palaeoecological and evolutionary implications | author=Fedonkin, M.A., Simonetta, A. and Ivantsov, A.Y. | journal=Geological Society, London, Special Publications | year=2007 | pages=157–179 | doi=10.1144/SP286.12 | url=http://www.geosci.monash.edu.au/precsite/docs/workshop/prato04/abstracts/fedonkin2.pdf | access-date=2008-07-10 | volume=286 | issue=1 | bibcode=2007GSLSP.286..157F | s2cid=331187 | archive-url=https://web.archive.org/web/20080721232846/http://www.geosci.monash.edu.au/precsite/docs/workshop/prato04/abstracts/fedonkin2.pdf | archive-date=2008-07-21 | url-status=dead }} but others are unwilling to go further than "probable bilaterian".{{Cite journal

| author = Butterfield, N.J. | year = 2006

| title = Hooking some stem-group "worms": fossil lophotrochozoans in the Burgess Shale

| pmid = 17120226

| s2cid = 29130876

}} There is an even sharper debate about whether Wiwaxia, from about {{ma|505}}, 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.{{Cite journal

| title = A soft-bodied mollusc with radula from the Middle Cambrian Burgess Shale

| pmid = 16838013 | bibcode=2006Natur.442..159C

|hdl=1912/1404 |s2cid=4431853 | hdl-access = free }} Nicholas Butterfield, who opposes the idea that Wiwaxia was a mollusc, has written that earlier microfossils from {{ma|515|510}} are fragments of a genuinely mollusc-like radula.{{Cite journal

| author=Butterfield, N.J. | title=An Early Cambrian Radula

| doi=10.1666/07-066.1| s2cid=86083492

}} This appears to contradict the concept that the ancestral molluscan radula was mineralized.{{Cite journal | pages = 224–230 | issue = 2 | volume = 194 | year = 1998 | pmid = 28570844| jstor = 1543051| journal = Biological Bulletin | doi = 10.2307/1543051 | last2 = Lins | first1 = R. | first2 = U. | last3 = Farina | title = Minerals of the radular apparatus of Falcidens sp. (Caudofoveata) and the evolutionary implications for the Phylum Mollusca | first3 = M. | last1 = Cruz | url = https://www.biodiversitylibrary.org/part/31306 }}

However, the Helcionellids, which first appear over {{ma|540}} in Early Cambrian rocks from Siberia and China,{{Cite journal|author= Parkhaev, P. Yu. |year=2007 |title=The Cambrian 'basement' of gastropod evolution |journal=Geological Society, London, Special Publications |volume=286 |issue=1 |isbn= 978-1-86239-233-5 |pages=415–421 |url= https://books.google.com/books?id=GA7-8JIh9IwC&pg=PA415 |access-date=2009-11-01 |doi= 10.1144/SP286.31|bibcode = 2007GSLSP.286..415P |s2cid=130979274 }}{{Cite journal | doi = 10.1016/j.palaeo.2007.03.046| last1 = Steiner | first1 = M. | last2 = Li | first2 = G. | last3 = Qian | first3 = Y. | last4 = Zhu | first4 = M. | last5 = Erdtmann | first5 = B. D. | title = Neoproterozoic to Early Cambrian small shelly fossil assemblages and a revised biostratigraphic correlation of the Yangtze Platform (China) | journal = Palaeogeography, Palaeoclimatology, Palaeoecology | volume = 254 | issue = 1–2 | page = 67 | year = 2007 | bibcode = 2007PPP...254...67S}} are thought to be early molluscs with rather snail-like shells. Shelled molluscs therefore predate the earliest trilobites.{{Cite journal | first1=B. | first2= J.| last2= Pojeta Jr| title = Molluscan Phylogeny: the Paleontological Viewpoint| volume = 186| last1= Runnegar| journal = Science| issue = 4161| pages = 311–317| date=Oct 1974 | jstor = 1739764| pmid = 17839855| doi = 10.1126/science.186.4161.311|bibcode = 1974Sci...186..311R | s2cid= 46429653}} Although most helcionellid fossils are only a few millimeters long, specimens a few centimeters long have also been found, most with more limpet-like shapes. The tiny specimens have been suggested to be juveniles and the larger ones adults.{{Cite journal

| author = Mus, M. M.|author2= Palacios, T.|author3= Jensen, S. | year = 2008

| title = Size of the earliest molluscs: Did small helcionellids grow to become large adults?

}}

Some analyses of helcionellids concluded these were the earliest gastropods.{{cite journal| journal=Journal of Paleontology| title=Latest Early Cambrian Small Shelly Fossils, Trilobites, and Hatch Hill Dysaerobic Interval on the Quebec Continental Slope | pages=287–305 | volume=76| last1 = Landing| first1 = E. | issue=2| jstor=1307143 | date=2002| doi=10.1666/0022-3360(2002)076<0287:LECSSF>2.0.CO;2| last2 = Geyer | first2 = G.| first3 = K. E.| last3 = Bartowski }} 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.{{Cite book|last=Frýda|first=J.|author2=Nützel, A. and Wagner, P.J.|title=Phylogeny and evolution of the Mollusca|editor=Ponder, W.F. |editor2=Lindberg, D.R.|pages=239–264|chapter=Paleozoic Gastropoda|publisher=California Press|year=2008|isbn=978-0-520-25092-5|chapter-url=https://books.google.com/books?id=nm0IZAQQ6S0C&pg=RA1-PA239}}{{Cite journal|last=Kouchinsky|first=A.|year=2000|title=Shell microstructures in Early Cambrian molluscs|journal=Acta Palaeontologica Polonica|volume=45|issue=2|pages=119–150|url=http://app.pan.pl/archive/published/app45/app45-119.pdf|access-date=4 Nov 2009}}

{{Annotated image | float=right | caption=Septa and siphuncle in nautiloid shell

| image=Nautiloid septa n siphuncle 01.png | width=215 | image-width=150 | height=150

| annotations=

{{Annotation|120|5|{{legend2|blue|border=1px blue solid|{{=}} Septa}}}}

{{Annotation|120|35|{{legend2|yellow|border=1px silver solid|{{=}} Siphuncle}}}}

}}

Volborthella, some fossils of which predate {{ma|530}}, 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 mineral silicon dioxide (silica), and it was not divided into a series of compartments by septa as those of fossil shelled cephalopods and the living Nautilus are. Volborthella's classification is uncertain.{{Cite book

|author1 = Hagadorn, J.W.

|author2 = Waggoner, B.M.

|name-list-style = amp

|year = 2002

|pages = 135–150

|contribution = The Early Cambrian problematic fossil Volborthella: New insights from the Basin and Range

|editor = Corsetti, F.A.

|title = Proterozoic-Cambrian of the Great Basin and Beyond, Pacific Section SEPM Book 93

|publisher = SEPM (Society for Sedimentary Geology)

|url = http://www.amherst.edu/~jwhagadorn/publications/volb.pdf

|url-status = bot: unknown

|archive-url = https://web.archive.org/web/20060911152548/http://www.amherst.edu/~jwhagadorn/publications/volb.pdf

|archive-date = 2006-09-11

}} The Late Cambrian fossil Plectronoceras is now thought to be the earliest clearly cephalopod fossil, as its shell had septa and a siphuncle, a strand of tissue that Nautilus uses to remove water from compartments it has vacated as it grows, and which is also visible in fossil ammonite 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.{{Cite book

| author=Vickers-Rich, P., Fenton, C.L., Fenton, M.A. and Rich, T.H.

| title=The Fossil Book: A Record of Prehistoric Life

| publisher=Courier Dover Publications

| year=1997

| isbn=0-486-29371-8

| pages=[https://archive.org/details/fossilbookrecor00rich/page/269 269–272]

| url=https://archive.org/details/fossilbookrecor00rich/page/269

}} All cephalopods with external shells except the nautiloids became extinct by the end of the Cretaceous period {{ma|65}}.{{Cite journal|author=Marshall C.R.|author2= Ward P.D. |year=1996 |title=Sudden and Gradual Molluscan Extinctions in the Latest Cretaceous of Western European Tethys |journal=Science |volume=274 |issue=5291 |pages=1360–1363 |doi=10.1126/science.274.5291.1360 |pmid=8910273

|bibcode = 1996Sci...274.1360M |s2cid= 1837900 }} However, the shell-less Coleoidea (squid, octopus, cuttlefish) are abundant today.{{cite web|url=http://www.thecephalopodpage.org/evolution.php|title=A Broad Brush History of the Cephalopoda|last=Monks|first=N.|access-date=2009-03-21}}

The Early Cambrian fossils Fordilla and Pojetaia are regarded as bivalves.{{Cite journal

| author=Pojeta, J. | year=2000 | title=Cambrian Pelecypoda (Mollusca)

| journal=American Malacological Bulletin | volume=15 | pages=157–166

}}{{Cite journal

| author=Schneider, J.A. | title=Bivalve systematics during the 20th century

| journal=Journal of Paleontology |date = 2001| volume=75

| issue=6 | pages=1119–1127

| doi=10.1666/0022-3360(2001)075<1119:BSDTC>2.0.CO;2

}}{{Cite journal | doi = 10.1111/j.1502-3931.1999.tb00534.x| title = The first evolutionary-adaptive lineage within fossil molluscs| journal = Lethaia| volume = 32| issue = 2| pages = 155| year = 2007| last1 = Gubanov | first1 = A. P. | last2 = Kouchinsky | first2 = A. V. | last3 = Peel | first3 = J. S. }}{{Cite journal

| author=Gubanov, A.P.|author2= Peel, J.S.

| title=The early Cambrian helcionelloid mollusc Anabarella Vostokova

| year=2003

|s2cid= 84893338

| doi-access=free}} "Modern-looking" bivalves appeared in the Ordovician period, {{ma|488|443}}.{{Cite journal

| author=Zong-Jie, F.

| title=An introduction to Ordovician bivalves of southern China, with a discussion of the early evolution of the Bivalvia

| journal=Geological Journal | volume=41 | issue=3–4 | doi=10.1002/gj.1048

| pages=303–328

| year=2006

| s2cid=129430674

}} One bivalve group, the rudists, became major reef-builders in the Cretaceous, but became extinct in the Cretaceous–Paleogene extinction event.{{Cite journal

| author=Raup, D.M.|author2=Jablonski, D.

| title=Geography of end-Cretaceous marine bivalve extinctions | journal=Science | year=1993

|bibcode = 1993Sci...260..971R }} Even so, bivalves remain abundant and diverse.

The Hyolitha are a class of extinct animals with a shell and operculum that may be molluscs. Authors who suggest they deserve their own phylum do not comment on the position of this phylum in the tree of life{{Cite journal | last1 = Malinky | first1 = J. M. | doi = 10.1666/08-094R.1 | title = Permian Hyolithida from Australia: The Last of the Hyoliths? | journal = Journal of Paleontology | volume = 83 | pages = 147–152 | year = 2009 | s2cid = 85924056 }}

Phylogeny

{{thumb|width=400|content=

{{clade|style=font-size:80%;

|label1=Lophotrochozoa

|1={{clade

|1=Brachiopods

|2={{clade

|1={{clade

|1=Bivalves

|2=Monoplacophorans
("limpet-like", "living fossils")

|3={{clade

|1=Gastropods
(snails, slugs, limpets, sea hares)

|2={{clade

|1=Cephalopods
(nautiloids, ammonites, squid, etc.)

|2=Scaphopods (tusk shells)

}}

|2={{clade

|1=Aplacophorans
(spicule-covered, worm-like)

|2=Polyplacophorans (chitons)

}}

|2={{clade

|label1=Halwaxiids

|1={{clade

|1=Wiwaxia

|2=Halkieria

}}

|2=Orthrozanclus

}}

|3=Odontogriphus

}}

|caption=A possible "family tree" of molluscs (2007).{{Cite journal

| author=Sigwart, J.D.|author2= Sutton, M.D. |date = October 2007| title=Deep molluscan phylogeny: synthesis of palaeontological and neontological data

| pmid=17652065

| pmc = 2274978

| journal=Proceedings of the Royal Society B | volume=274 | issue=1624 | pages=2413–2419

| doi=10.1098/rspb.2007.0701

}} For a summary, see {{cite web

| title=The Mollusca | publisher=University of California Museum of Paleontology

| url=http://www.ucmp.berkeley.edu/taxa/inverts/mollusca/mollusca.php | access-date=2008-10-02

}}{{cite web

| title=The Mollusca | publisher=University of California Museum of Paleontology

| url=http://www.ucmp.berkeley.edu/taxa/inverts/mollusca/mollusca.php | access-date=2008-10-02

}} Does not include annelid worms as the analysis concentrated on fossilizable "hard" features.

}}

The phylogeny (evolutionary "family tree") of molluscs is a controversial subject. In addition to the debates about whether Kimberella and any of the "halwaxiids" were molluscs or closely related to molluscs, debates arise about the relationships between the classes of living molluscs. In fact, some groups traditionally classified as molluscs may have to be redefined as distinct but related.

Molluscs are generally regarded members of the Lophotrochozoa, a group defined by having trochophore larvae and, in the case of living Lophophorata, a feeding structure called a lophophore. The other members of the Lophotrochozoa are the annelid worms and seven marine phyla.{{cite web

| title=Introduction to the Lophotrochozoa

| publisher=University of California Museum of Paleontology

| url=http://www.ucmp.berkeley.edu/phyla/lophotrochozoa.html | access-date=2008-10-02

}} 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.{{Cite journal| doi = 10.1016/j.ydbio.2004.04.027| pmid = 15242797| year = 2004| last1 = Henry | first1 = J.| last2 = Okusu | first2 = A.| last3 = Martindale | first3 = M.| title = The cell lineage of the polyplacophoran, Chaetopleura apiculata: variation in the spiralian program and implications for molluscan evolution| volume = 272| issue = 1| pages = 145–160| journal = Developmental Biology | doi-access = free}} For example, it is uncertain whether the ancestral mollusc was metameric (composed of repeating units)—if it was, that would suggest an origin from an annelid-like worm.{{Cite journal| last2 = Wray| last3 = Wedeen| doi = 10.1046/j.1525-142x.2000.00077.x| year = 2000| pages = 340–347| last4 = Kostriken| last5 = Desalle| pmid=11256378| last8 = Lindberg| last7 = Gates| last6 = Staton| issue = 6| volume = 2| first5 = R.| first4 = R.| first3 = C. J.| first2 = C. G.| last1 = Jacobs | first6 = J. L.| journal = Evolution & Development| title = Molluscan engrailed expression, serial organization, and shell evolution| first8 = D. R.| first7 = R. D.| first1 = D. K.| s2cid = 25274057}} Scientists disagree about this: Giribet and colleagues concluded, in 2006, the repetition of gills and of the foot's retractor muscles were later developments,{{Cite journal| last3=Lindgren, A.R.| last1=Giribet | first3=A. R. | first1=G.| last4=Huff, S.W. | first4=S. W. | first2=A.| last5 = Schrödl, M | first5=M.| last6 = Nishiguchi, M.K. | first6=M. K.| title = Evidence for a clade composed of molluscs with serially repeated structures: monoplacophorans are related to chitons| last2= Okusu, A| journal = Proceedings of the National Academy of Sciences of the United States of America| volume = 103| issue = 20| pages = 7723–7728| date=May 2006 | pmid = 16675549| pmc = 1472512| doi = 10.1073/pnas.0602578103|bibcode = 2006PNAS..103.7723G | doi-access=free }} while in 2007, Sigwart concluded the ancestral mollusc was metameric, and it had a foot used for creeping and a "shell" that was mineralized.{{Cite journal| last1 = Sigwart | first1 = J. D.| last2 = Sutton | first2= M. D.| title = Deep molluscan phylogeny: synthesis of palaeontological and neontological data| journal = Proceedings of the Royal Society B: Biological Sciences| issue = 1624| volume = 274| pages = 2413–2419| date =Oct 2007| pmid = 17652065| pmc = 2274978| doi = 10.1098/rspb.2007.0701}} For a summary, see {{cite web| title=The Mollusca | publisher=University of California Museum of Paleontology| url=http://www.ucmp.berkeley.edu/taxa/inverts/mollusca/mollusca.php| access-date=2 October 2008}} In one particular branch of the family tree, the shell of conchiferans is thought to have evolved from the spicules (small spines) of aplacophorans; but this is difficult to reconcile with the embryological origins of spicules.

The molluscan shell appears to have originated from a mucus coating, which eventually stiffened into a cuticle. This would have been impermeable and thus forced the development of more sophisticated respiratory apparatus in the form of gills. Eventually, the cuticle would have become mineralized, using the same genetic machinery (the engrailed gene) as most other bilaterian skeletons. The first mollusc shell almost certainly was reinforced with the mineral aragonite.{{Cite journal| first1 = S. M.| title = Seawater chemistry and early carbonate biomineralization| journal = Science| volume = 316| issue = 5829| last1 = Porter| page = 1302| date=Jun 2007 | issn = 0036-8075| pmid = 17540895| doi = 10.1126/science.1137284|bibcode = 2007Sci...316.1302P | s2cid = 27418253}}

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

{{thumb|align=left|content=

{{clade|style=font-size:80%

|label1=Molluscs

|1={{clade

|label1=Aculifera

|1={{clade

|1=Solenogastres

|2=Caudofoveata

}}

|2=Polyplacophorans

}}

|label2=Conchifera

|2={{clade

|1=Monoplacophorans

|2={{clade

|1=Bivalves

|2=Scaphopods

|3=Gastropods

|4=Cephalopods

}}

|caption=The "Aculifera" hypothesis}}

{{thumb|align=left|content=

{{clade|style=font-size:80%

|label1=Molluscs

|1={{clade

|1=Solenogastres

|2=Caudofoveata

|label3=Testaria

|3={{clade

|1=Polyplacophorans

|2={{clade

|1=Monoplacophorans

|2={{clade

|1=Bivalves

|2=Scaphopods

|3=Gastropods

|4=Cephalopods

}}

|caption=The "Testaria" hypothesis}}

Morphological analyses tend to recover a conchiferan clade that receives less support from molecular analyses,{{cite journal

| author1 = Winnepenninckx, B

| author2 = Backeljau, T

| author3 = De Wachter, R

| title = Investigation of molluscan phylogeny on the basis of 18S rRNA sequences

| date = 1996

| journal = Molecular Biology and Evolution

| volume = 13

| issue = 10

| pages = 1306–1317

| pmid = 8952075

| doi = 10.1093/oxfordjournals.molbev.a025577

| doi-access = free

}} although these results also lead to unexpected paraphylies, for instance scattering the bivalves throughout all other mollusc groups.{{Cite journal| last1 = Passamaneck | first1 = Y.| last2 = Schander | first2 = C.| last3 = Halanych | first3 = K.| title = Investigation of molluscan phylogeny using large-subunit and small-subunit nuclear rRNA sequences.| journal = Molecular Phylogenetics & Evolution| volume = 32| issue = 1| pages = 25–38| year = 2004| pmid = 15186794| doi = 10.1016/j.ympev.2003.12.016}}

However, an analysis in 2009 using both morphological and molecular phylogenetics comparisons concluded the molluscs are not monophyletic; in particular, Scaphopoda and Bivalvia are both separate, monophyletic lineages unrelated to the remaining molluscan classes; the traditional phylum Mollusca is polyphyletic, and it can only be made monophyletic if scaphopods and bivalves are excluded.{{Cite journal| doi = 10.1111/j.1096-0031.2009.00255.x| title = Phylogenetic analysis of 73 060 taxa corroborates major eukaryotic groups| year = 2009| last1 = Goloboff | first1 = Pablo A.| last2 = Catalano | first2 = Santiago A.| last3 = Mirande | first3 = J. Marcos| last4 = Szumik | first4 = Claudia A.| last5 = Arias | first5 = J. Salvador| last6 = Källersjö | first6 = Mari| last7 = Farris | first7 = James S.| journal = Cladistics| volume = 25| issue = 3| pages = 211–230 | pmid = 34879616| s2cid = 84401375| doi-access = free| hdl = 11336/78055| hdl-access = free}} A 2010 analysis recovered the traditional conchiferan and aculiferan groups, and showed molluscs were monophyletic, demonstrating that available data for solenogastres was contaminated.{{Cite journal| doi = 10.1016/j.ympev.2009.07.028| pmid = 19647088| year = 2010| last1 = Wilson | first1 = N.| last2 = Rouse | first2 = G.| last3 = Giribet | first3 = G.| title = Assessing the molluscan hypothesis Serialia (Monoplacophora+Polyplacophora) using novel molecular data.| volume = 54| issue = 1| pages = 187–193| journal = Molecular Phylogenetics & Evolution }} 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.{{Cite journal| issue = 1| volume = 6| journal = Frontiers in Zoology| title = Phylogenetic support values are not necessarily informative: the case of the Serialia hypothesis (a mollusk phylogeny)| pages = 12| year = 2009| doi = 10.1186/1742-9994-6-12| pmc = 2710323| pmid = 19555513 | first6 = H.| last6 = Wägele| last3 = Klussmann-Kolb | first2 = H.| last2 = Letsch | first1 = J. | first3 = A.| last4 = Mayer | first5 = B.| last5 = Misof | first4 = C.| last1 = Wägele| doi-access = free}} Rather than eliminating unlikely relationships, the latest studies add new permutations of internal molluscan relationships, even bringing the conchiferan hypothesis into question.{{Cite journal | last1 = Vinther | first1 = J. | last2 = Sperling | first2 = E. A. | last3 = Briggs | first3 = D. E. G. | author-link3 = Derek Briggs | last4 = Peterson | first4 = K. J. | title = A molecular palaeobiological hypothesis for the origin of aplacophoran molluscs and their derivation from chiton-like ancestors | journal = Proceedings of the Royal Society B: Biological Sciences | volume = 279 | issue = 1732 | pages = 1259–68 | year = 2011 | doi = 10.1098/rspb.2011.1773 | pmid=21976685 | pmc=3282371}}

References

Further references

{{Cite book | ref=Ruppert | author=Ruppert, E.E., Fox, R.S., and Barnes, R.D. | title=Invertebrate Zoology | publisher=Brooks / Cole | edition=7 | isbn=0-03-025982-7 | year=2004 | url=https://archive.org/details/isbn_9780030259821 }}

Evolution of molluscs

Fossil record

Phylogeny

References

Further references

Further reading