Evolution of molluscs
The evolution of the molluscs is the way in which the
Fossil record
Good evidence exists for the appearance of
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
The Early Cambrian fossils
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]
Phylogeny
Lophotrochozoa |
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The
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:
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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
References
- ^ S2CID 46429653.
- ^ a b Ruppert, pp. 300–343
- ISBN 9780128143124.
- S2CID 4395089.
- ^ S2CID 331187. Archived from the original (PDF) on 2008-07-21. Retrieved 2008-07-10.)
{{cite journal}}
: CS1 maint: multiple names: authors list (link - ^ S2CID 29130876.
- ^ PMID 17652065. For a summary, see "The Mollusca". University of California Museum of Paleontology. Retrieved 2 October 2008.
- ^ S2CID 4431853.
- ^ S2CID 86083492.
- PMID 28570844.
- S2CID 130979274. Retrieved 2009-11-01.
- .
- .
- JSTOR 1307143.
- ISBN 978-0-520-25092-5.)
{{cite book}}
: CS1 maint: multiple names: authors list (link - ^ Kouchinsky, A. (2000). "Shell microstructures in Early Cambrian molluscs" (PDF). Acta Palaeontologica Polonica. 45 (2): 119–150. Retrieved 4 Nov 2009.
- ^ 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) - ISBN 0-486-29371-8.)
{{cite book}}
: CS1 maint: multiple names: authors list (link - S2CID 1837900.
- ^ Monks, N. "A Broad Brush History of the Cephalopoda". Retrieved 2009-03-21.
- ^ Pojeta, J. (2000). "Cambrian Pelecypoda (Mollusca)". American Malacological Bulletin. 15: 157–166.
- .
- .
- S2CID 84893338.
- S2CID 129430674.
- PMID 11537491.
- S2CID 85924056.
- ^ PMID 17652065. For a summary, see "The Mollusca". University of California Museum of Paleontology. Retrieved 2008-10-02.
- ^ "The Mollusca". University of California Museum of Paleontology. Retrieved 2008-10-02.
- ^ S2CID 84401375.
- ^ "Introduction to the Lophotrochozoa". University of California Museum of Paleontology. Retrieved 2008-10-02.
- ^ PMID 15242797.
- ^ S2CID 25274057.
- PMID 16675549.
- S2CID 27418253.
- PMID 8952075.
- PMID 15186794.
- PMID 19647088.
- PMID 19555513.
- PMID 21976685.
Further references
- Ruppert, E.E., Fox, R.S., and Barnes, R.D. (2004). Invertebrate Zoology (7 ed.). Brooks / Cole. ISBN 0-03-025982-7.)
{{cite book}}
: CS1 maint: multiple names: authors list (link