View source for Evolution of molluscs

{{Short description|The origin and diversification of molluscs through geologic time}}
{{See also|Evolution of cephalopods}}
[[File:Archimollusc-en.svg|thumb|upright=1.5|right|{{center|Anatomical diagram of a hypothetical ancestral mollusc}}]]

The '''evolution of the molluscs''' is the way in which the [[Mollusca]], one of the largest groups of [[invertebrate]] animals, [[Evolution|evolved]]. This [[phylum]] includes [[gastropod]]s, [[bivalves]], [[scaphopod]]s, [[cephalopod]]s, 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
 | align = right
 | direction = horizontal
 | image1    = Yochelcionella_cyrano_svg_hariadhi.svg
 | width1    = 166
 | alt1      = 
 | caption1  =  The tiny [[Helcionellid]] fossil ''[[Yochelcionella]]'' is thought to be an early [[mollusc]]<ref name="RunnegarPojeta1974" />
 | image2    = Neptunea despecta.jpg
 | width2    = 150
 | alt2      = 
 | caption2  = Spirally coiled shells appear in many [[gastropod]]s.<ref name="RuppertFoxBarnes2004MolluscaGastropoda">[[#Ruppert|Ruppert]], pp. 300–343</ref>
}}

Good evidence exists for the appearance of [[gastropod]]s, [[cephalopod]]s and [[bivalve]]s 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.<ref name=Cabej>{{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}}</ref> ''[[Kimberella]]'', from about {{ma|555}}, has been described by some palaeontologists as "mollusc-like",<ref name="Fedonkin1997">{{Cite journal
 | author = Fedonkin, M.A. |author2=Waggoner, B.M. | year = 1997
 | title = The Late Precambrian fossil Kimberella is a mollusc-like bilaterian organism
 | journal = Nature | volume = 388 | issue = 6645 | page = 868 | doi = 10.1038/42242 | bibcode=1997Natur.388..868F
|s2cid=4395089 | doi-access = free }}</ref><ref name="FedonkinEtAl2007NewDataOnKimberella">{{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 }}</ref> but others are unwilling to go further than "probable [[bilateria]]n".<ref name="Butterfield2006">{{Cite journal
 | author = Butterfield, N.J. | year = 2006
 | title = Hooking some stem-group "worms": fossil lophotrochozoans in the Burgess Shale
 | journal = BioEssays | volume = 28 | issue = 12 | pages = 1161–6 | doi = 10.1002/bies.20507
 | pmid = 17120226
| s2cid = 29130876
 }}</ref><ref name=Sigwart2007/> 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.<ref name="Butterfield2006" /><ref name="CaronScheltemaSchanderRudkin2006">{{Cite journal
 | author = Caron, J.B. |author2=Scheltema, A. |author3=Schander, C. |author4=Rudkin, D.  | date=2006-07-13
 | title = A soft-bodied mollusc with radula from the Middle Cambrian Burgess Shale
 | journal = Nature | volume = 442 | issue = 7099 | pages = 159–163| doi = 10.1038/nature04894
 | pmid = 16838013 | bibcode=2006Natur.442..159C
 |hdl=1912/1404 |s2cid=4431853 | hdl-access = free }}</ref> Nicholas Butterfield, who opposes the idea that ''Wiwaxia'' was a mollusc, has written that earlier [[microfossil]]s from {{ma|515|510}} are fragments of a genuinely mollusc-like radula.<ref name="Butterfield2008EarlyCambrianRadula">{{Cite journal
| author=Butterfield, N.J. | title=An Early Cambrian Radula
| journal=Journal of Paleontology |date = May 2008| volume=82 | issue=3 | pages=543–554
| doi=10.1666/07-066.1| s2cid=86083492
}}</ref> This appears to contradict the concept that the ancestral molluscan radula was mineralized.<ref name="Cruz1998">{{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 }}</ref>

However, the [[Helcionellid]]s, which first appear over {{ma|540}} in Early Cambrian rocks from [[Siberia]] and [[China]],<ref name="Parkhaev 2007">{{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 |url-access=subscription }}</ref><ref name="Steiner2007">{{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}}</ref> are thought to be early molluscs with rather snail-like shells. Shelled molluscs therefore predate the earliest [[trilobite]]s.<ref name="RunnegarPojeta1974">{{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}}</ref> 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.<ref name="Mus2008">{{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?
 | journal = Geology | volume = 36 | issue = 2 | page = 175 | doi = 10.1130/G24218A.1|bibcode=2008Geo....36..175M
}}</ref>

Some analyses of helcionellids concluded these were the earliest [[gastropod]]s.<ref>{{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 }}</ref> However, other scientists are not convinced these Early Cambrian fossils show clear signs of the [[Torsion (gastropod)|torsion]] characteristic of modern gastropods, that twists the internal organs so the anus lies above the head.<ref name="RuppertFoxBarnes2004MolluscaGastropoda" /><ref name="FrýdaNützelWagner2008">{{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}}</ref><ref>{{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}}</ref>

{{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 [[Septum (marine biology)|septa]] as those of fossil shelled cephalopods and the living ''[[Nautilus]]'' are. ''Volborthella'''s classification is uncertain.<ref>{{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
}}</ref> 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.<ref>{{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
}}</ref> All cephalopods with external shells except the [[nautiloid]]s became extinct by the end of the [[Cretaceous]] period {{ma|65}}.<ref name="MarshallWard1996">{{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 }}</ref> However, the shell-less [[Coleoidea]] ([[squid]], [[octopus]], [[cuttlefish]]) are abundant today.<ref>{{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}}</ref>

The Early Cambrian fossils ''[[Fordilla]]'' and ''[[Pojetaia]]'' are regarded as [[bivalve]]s.<ref>{{Cite journal
| author=Pojeta, J. | year=2000 | title=Cambrian Pelecypoda (Mollusca)
| journal=American Malacological Bulletin | volume=15 | pages=157–166
}}</ref><ref>{{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
}}</ref><ref>{{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. }}</ref><ref>{{Cite journal
| author=Gubanov, A.P.|author2= Peel, J.S.
| title=The early Cambrian helcionelloid mollusc ''Anabarella'' Vostokova
| journal=Palaeontology | volume=46 |issue=5 | pages=1073–1087 | doi=10.1111/1475-4983.00334
| year=2003
|s2cid= 84893338
| doi-access=free}}</ref> "Modern-looking" bivalves appeared in the [[Ordovician]] period, {{ma|488|443}}.<ref>{{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
}}</ref> One bivalve group, the [[rudist]]s, became major [[reef]]-builders in the Cretaceous, but became extinct in the [[Cretaceous–Paleogene extinction event]].<ref>{{Cite journal
| author=Raup, D.M.|author2=Jablonski, D.
| title=Geography of end-Cretaceous marine bivalve extinctions | journal=Science | year=1993
| volume=260 | issue=5110 | pages=971–973 |doi=10.1126/science.11537491 | pmid=11537491
|bibcode = 1993Sci...260..971R }}</ref> Even so, bivalves remain abundant and diverse.

The [[Hyolitha]] are a class of extinct animals with a shell and [[operculum (gastropod)|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<ref name=Sumrall2009>{{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 }}</ref>

==Phylogeny==
{{thumb|width=400|content=<div style="text-align:left;">
{{clade|style=font-size:80%;
 |label1=[[Lophotrochozoa]]
 |1={{clade
 |1=[[Brachiopod]]s
 |2={{clade
 |1={{clade
 |1={{clade
 |1={{clade
 |1=[[Bivalve]]s
 |2=[[Monoplacophora]]ns<br />("limpet-like", "living fossils")
 |3={{clade
 |1=[[Gastropod]]s<br />([[snail]]s, [[slug]]s, [[limpet]]s, [[sea hare]]s)
 |2={{clade
 |1=[[Cephalopod]]s<br />([[nautiloid]]s, [[ammonites]], [[squid]], etc.)
 |2=[[Scaphopod]]s (tusk shells)
 }}
 }}
 }}
 }}
 |2={{clade
 |1=[[Aplacophora]]ns<br />(spicule-covered, worm-like)
 |2=[[Polyplacophora]]ns (chitons)
 }}
 }}
 |2={{clade
 |label1=[[Halwaxiid]]s
 |1={{clade
 |1=''[[Wiwaxia]]''
 |2=''[[Halkieria]]''
 }}
 |2=''[[Orthrozanclus]]''
 }}
 |3=''[[Odontogriphus]]''
 }}
 }}
}}
</div>|caption=A possible "family tree" of molluscs (2007).<ref name="SigwartSutton2007DeepMolluscanPhylogeny">{{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
}}</ref><ref>{{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
}}</ref> Does not include [[annelid]] worms as the analysis concentrated on fossilizable "hard" features.<ref name="SigwartSutton2007DeepMolluscanPhylogeny" />
}}
The [[phylogeny]] (evolutionary "family tree") of molluscs is a controversial subject. In addition to the debates about whether ''[[Kimberella]]'' and any of the "[[halwaxiid]]s" were molluscs or closely related to molluscs,<ref name="FedonkinEtAl2007NewDataOnKimberella" /><ref name="Butterfield2006" /><ref name="CaronScheltemaSchanderRudkin2006" /><ref name="Butterfield2008EarlyCambrianRadula" /> debates arise about the relationships between the classes of living molluscs.<ref name="Sigwart2007"/> In fact, some groups traditionally classified as molluscs may have to be redefined as distinct but related.<ref name="GoloboffEtAl2009" />

Molluscs are generally regarded members of the [[Lophotrochozoa]],<ref name="SigwartSutton2007DeepMolluscanPhylogeny" /> 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 [[phylum|phyla]].<ref>{{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
}}</ref> 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.<ref name="Henry2004">{{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}}</ref> For example, it is uncertain whether the ancestral mollusc was [[metamerism (biology)|metameric]] (composed of repeating units)—if it was, that would suggest an origin from an [[annelid]]-like worm.<ref name=Jacobs2000>{{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}}</ref> Scientists disagree about this: Giribet and colleagues concluded, in 2006, the repetition of gills and of the foot's retractor muscles were later developments,<ref name="GiribetOkusuEtAlMolluscsWithSeriallyRepeatedStructures">{{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 }}</ref> while in 2007, Sigwart concluded the ancestral mollusc was metameric, and it had a foot used for creeping and a "shell" that was mineralized.<ref name=Sigwart2007>{{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}}</ref> In one particular branch of the family tree, the shell of [[conchifera]]ns is thought to have evolved from the [[wikt:spicule|spicule]]s (small spines) of [[aplacophora]]ns; but this is difficult to reconcile with the [[embryology|embryological]] origins of spicules.<ref name=Henry2004/>

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.<ref name=RunnegarPojeta1974/> Eventually, the cuticle would have become mineralized,<ref name=RunnegarPojeta1974/> <!-- this mineralization may have happened one or many times,{{Verify source|date=March 2009}} but uses -->using the same genetic machinery (the [[engrailed (gene)|engrailed]] gene) as most other bilaterian [[skeleton]]s.<ref name=Jacobs2000/> The first mollusc shell almost certainly was reinforced with the mineral [[aragonite]].<ref>{{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}}</ref>

The evolutionary relationships 'within' the molluscs are also debated, and the diagrams below show two widely supported reconstructions:
{{col-begin|width=auto}}
{{col-break}}
{{thumb|align=left|content=<div style="text-align:left">
 {{clade|style=font-size:80%
 |label1=Molluscs
 |1={{clade
 |label1=[[Aculifera]]
 |1={{clade
 |1={{clade
 |1=[[Solenogastres]]
 |2=[[Caudofoveata]]
 }}
 |2=[[Polyplacophora]]ns
 }}
 |label2=[[Conchifera]]
 |2={{clade
 |1=[[Monoplacophora]]ns
 |2={{clade
 |1=[[Bivalve]]s
 |2=[[Scaphopod]]s
 |3=[[Gastropod]]s
 |4=[[Cephalopod]]s
 }}
 }}
 }}
 }}
 </div>|caption=The "[[Aculifera]]" hypothesis<ref name="SigwartSutton2007DeepMolluscanPhylogeny"/>}}
{{col-break}}
{{thumb|align=left|content=<div style="text-align:left">
 {{clade|style=font-size:80%
 |label1=Molluscs
 |1={{clade
 |1={{clade
 |1=[[Solenogastres]]
 |2=[[Caudofoveata]]
 |label3=[[Testaria]]
 |3={{clade
 |1=[[Polyplacophora]]ns
 |2={{clade
 |1=[[Monoplacophora]]ns
 |2={{clade
 |1=[[Bivalve]]s
 |2=[[Scaphopod]]s
 |3=[[Gastropod]]s
 |4=[[Cephalopod]]s
 }}
 }}
 }}
 }}
 }}
 }}
 </div>|caption=The "[[Testaria]]" hypothesis<ref name="SigwartSutton2007DeepMolluscanPhylogeny"/>}}
{{col-end}}

Morphological analyses tend to recover a conchiferan clade that receives less support from molecular analyses,<ref name=Winnepenninckx>{{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
 }}</ref> although these results also lead to unexpected paraphylies, for instance scattering the bivalves throughout all other mollusc groups.<ref name=Passamaneck2004>{{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}}</ref>

However, an analysis in 2009 using both [[morphology (biology)|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.<ref name="GoloboffEtAl2009">{{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 (journal)|Cladistics]]| volume = 25| issue = 3| pages = 211–230 | pmid = 34879616| s2cid = 84401375| doi-access = free| hdl = 11336/78055| hdl-access = free}}</ref> A 2010 analysis recovered the traditional conchiferan and aculiferan groups, and showed molluscs were monophyletic, demonstrating that available data for solenogastres was contaminated.<ref>{{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 }}</ref> 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.<ref name=Wagele2009>{{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}}</ref> Rather than eliminating unlikely relationships, the latest studies add new permutations of internal molluscan relationships, even bringing the conchiferan hypothesis into question.<ref name="Vinther2011">{{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}}</ref>

==References==
{{Reflist|32em}}

==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 }}

== Further reading ==
* [http://www.biomedcentral.com/1471-2148/8/57 ''From sea to land and beyond – New insights into the evolution of euthyneuran Gastropoda (Mollusca)'']

{{Evolution}}

{{Authority control}}

{{DEFAULTSORT:Evolution Of Mollusca}}
[[Category:Malacology]]
[[Category:Evolution of protostomes|Molluscs]]

[[cs:Měkkýši#Fylogenetický strom]]