Cephalopoda

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   Order [[Octopodida]] Leach 1818
   Order [[Octopodida]] Leach 1818
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==Evolutionary History==
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==[[Cephalopoda phylogeny|Evolutionary History]]==
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The cephalopods first appeared in the late [[Cambrian]].  The first forms had gently curved shells.  During the [[Ordovician]] the group underwent an astonishing evolutionary radiation, possibly due to the new ecological niches made possible by the extinction of [[Anomalocarida|anomalocarids]] at the end of the Cambrian.  Some eight new orders appeared.  There was tremendous diversity among them.  Some had long straight shells, short straight ones, curved, lightly coiled, and tightly coiled ones evolved.  The internal structure of the shell differed greatly as well, mostly in the structure of the [[siphuncle]].  Most were probably relatively slow movers, at least compared to today's forms.  The largest ones had huge straight shells that reached 3 to 5 or even 10 metres in length. Many of these early forms were [[paraphyletic]], having given rise to newer equal or higher taxa; all are classified within the [[monophyletic]] and valid Subclass Nautiloidea.
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==''References'' ==
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The nautiloid cephalopods remained unchallenged through the Ordovician and [[Silurian]], finally giving way to the large predatory fish of the late Devonian. About this time the ammonoids began to take over from the nautiloids.  The ammonoids are rare in the early Devonian, but by the end of the period and the beginning of the [[Carboniferous]] they increase greatly in diversity.  During this time, all but two the remaining nautiloid orders die out. The Coleoidea meanwhile make their first appearance in the [[Late Mississippian]] (middle Carboniferous) but remain rare.
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Teichert,Curt 1988. "Main Features of Cephalopod Evolution", pp.19-20, in ''The Mollusca'' vol.12, ''Paleontology and Neontology of Cephalopods'', ed. by M.R. Clarke & E.R. Trueman, Academic Press, Harcourt Brace Jovanovich
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The end-Permian extinction exterminated all but a single family of ammonoids.  But these adaptable mollusks recovered strongly as the Mesozoic dawned, and the [[Ceratitida|ceratite]] lineage appeared with a great evolutionary radiation during the [[Triassic]].  So successful were these creatures that the Triassic period has been called "The Age of ''[[Ceratites]]''". Over 80 families are known from this time.  Another mass extinction at the end of the Triassic saw the demise of the ceratites, along with the last remaining straight-shelled nautiloids (the [[Pseudorthocerida|pseudorthocerids]]).  At this time new groups of ammonoids with much more complex sutures ("[[ammonite]]s" in the strict sense) took over.
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As well as ammonites, the squid-like [[Belemnitida|belemnites]], representing the Coleoidea, also underwent a huge evolutionary radiation as the [[Jurassic]] dawned.  The first representatives of modern coleoid groups like octopus and squid were other groups of cephalopods that appeared during the Jurassic, but being soft-bodied and only very rarely preserved it is not certain from the fossil record how common they were. But there is no denying that ammonoids, belemnoids, and proto-modern-style coleoids all formed a very significant part of the Jurassic and Cretaceous nektonic marine ecosystems. The ammonoids and belemnoids were to remain highly successful until the end of the [[Cretaceous]], where the same extinction event that killed off the dinosaurs and the other Mesozoic [[megafauna]] also exterminated the Ammonoids. A few belemnoids straggled on until the Eocene, but they were now heavily out-competed by the modern Coleoidea (octopus, squid, cuttlefish, etc).
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The Coleoidea remain an important and remarkably successful group of marine invertebrates to this day. Meanwhile, only a few species of pearly nautilus continue as the last survivors of the once important Nautiloidea.
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===Phylogeny===
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For detailed phylogeny, see [[Cephalopoda phylogeny]].
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'''Credits'''   
'''Credits'''   
© M. Alan Kazlev 1998-2002 with revisions by John M 2010.
© M. Alan Kazlev 1998-2002 with revisions by John M 2010.

Revision as of 07:27, 30 January 2011

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