Spirally coiled shells appear in many
gastropods.[2]
Good evidence exists for the appearance of
gastropods,
cephalopods and
bivalves in the
Cambrian period 538.8 to 485.4 million years ago. 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 really are molluscs.[3]Kimberella, from about 555 million years ago, has been described by some paleontologists as "mollusc-like",[4][5] but others are unwilling to go further than "probable
bilaterian".[6][7] There is an even sharper debate about whether Wiwaxia, from about 505 million years ago, 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.[6][8] Nicholas Butterfield, who opposes the idea that Wiwaxia was a mollusc, has written that earlier
microfossils from 515 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, the
Helcionellids, which first appear over 540 million years ago in Early Cambrian rocks from
Siberia and
China,[11][12] are thought to be early molluscs with rather snail-like shells. Shelled molluscs therefore predate the earliest
trilobites.[1] 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.[13]
Some analyses of helcionellids concluded these were the earliest
gastropods.[14] 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.[2][15][16]
Volborthella, some fossils of which predate 530 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 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.[17] 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.[18] All cephalopods with external shells except the
nautiloids became extinct by the end of the
Cretaceous period 65 million years ago.[19] However, the shell-less
Coleoidea (
squid,
octopus,
cuttlefish) are abundant today.[20]
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[27]
A possible "family tree" of molluscs (2007).[28][29] Does not include
annelid worms as the analysis concentrated on fossilizable "hard" features.[28]
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,[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 the
Lophotrochozoa,[28] 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.[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 was
metameric (composed of repeating units)—if it was, that would suggest an origin from an
annelid-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 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.[32]
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.[1] Eventually, the cuticle would have become mineralized,[1] using the same genetic machinery (the
engrailed gene) as most other bilaterian
skeletons.[33] The first mollusc shell almost certainly was reinforced with the mineral
aragonite.[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 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.[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]
^Steiner, M.; Li, G.; Qian, Y.; Zhu, M.; Erdtmann, B. D. (2007). "Neoproterozoic to Early Cambrian small shelly fossil assemblages and a revised biostratigraphic correlation of the Yangtze Platform (China)". Palaeogeography, Palaeoclimatology, Palaeoecology. 254 (1–2): 67.
Bibcode:
2007PPP...254...67S.
doi:
10.1016/j.palaeo.2007.03.046.
^Mus, M. M.; Palacios, T.; Jensen, S. (2008). "Size of the earliest mollusks: Did small helcionellids grow to become large adults?". Geology. 36 (2): 175.
Bibcode:
2008Geo....36..175M.
doi:
10.1130/G24218A.1.
^Landing, E.; Geyer, G.; Bartowski, K. E. (2002). "Latest Early Cambrian Small Shelly Fossils, Trilobites, and Hatch Hill Dysaerobic Interval on the Quebec Continental Slope". Journal of Paleontology. 76 (2): 287–305.
doi:
10.1666/0022-3360(2002)076<0287:LECSSF>2.0.CO;2.
JSTOR1307143.
^Frýda, J.; Nützel, A. and Wagner, P.J. (2008).
"Paleozoic Gastropoda". In Ponder, W.F.; Lindberg, D.R. (eds.). Phylogeny and evolution of the Mollusca. California Press. pp. 239–264.
ISBN978-0-520-25092-5.{{
cite book}}: CS1 maint: multiple names: authors list (
link)
^Hagadorn, J.W. & Waggoner, B.M. (2002). "The Early Cambrian problematic fossil Volborthella: New insights from the Basin and Range". In Corsetti, F.A. (ed.).
Proterozoic-Cambrian of the Great Basin and Beyond, Pacific Section SEPM Book 93(PDF). SEPM (Society for Sedimentary Geology). pp. 135–150. Archived from the original on 2006-09-11.{{
cite book}}: CS1 maint: bot: original URL status unknown (
link)
^Gubanov, A. P.; Kouchinsky, A. V.; Peel, J. S. (2007). "The first evolutionary-adaptive lineage within fossil molluscs". Lethaia. 32 (2): 155.
doi:
10.1111/j.1502-3931.1999.tb00534.x.
^Zong-Jie, F. (2006). "An introduction to Ordovician bivalves of southern China, with a discussion of the early evolution of the Bivalvia". Geological Journal. 41 (3–4): 303–328.
doi:
10.1002/gj.1048.
S2CID129430674.
^Malinky, J. M. (2009). "Permian Hyolithida from Australia: The Last of the Hyoliths?". Journal of Paleontology. 83: 147–152.
doi:
10.1666/08-094R.1.
S2CID85924056.
^
abJacobs, D. K.; Wray, C. G.; Wedeen, C. J.; Kostriken, R.; Desalle, R.; Staton, J. L.; Gates, R. D.; Lindberg, D. R. (2000). "Molluscan engrailed expression, serial organization, and shell evolution". Evolution & Development. 2 (6): 340–347.
doi:
10.1046/j.1525-142x.2000.00077.x.
PMID11256378.
S2CID25274057.