Cephalopoda

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m (Octopods have arms only, squid and cuttlefish have both arms ''and'' tentacles (the tentacles are modified arms))
(Systematics: revised links Endoceratoidea and Actinoceratoidea)
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The trouble with this scheme is that it is impossible to know from the fossil shells alone whether all the extinct forms had two or four gills, or anything really much about their soft body structure.  Moreover just from shell structure alone it became obvious that the simplistic three-fold classification was wanting.  Certainly the Nautiloidea appear to be not a single subclass but a very amorphous, paraphyletic group, so much so that the term Nautiloidea now really means "all cephalopods that are not ammonoids or coleoids".
The trouble with this scheme is that it is impossible to know from the fossil shells alone whether all the extinct forms had two or four gills, or anything really much about their soft body structure.  Moreover just from shell structure alone it became obvious that the simplistic three-fold classification was wanting.  Certainly the Nautiloidea appear to be not a single subclass but a very amorphous, paraphyletic group, so much so that the term Nautiloidea now really means "all cephalopods that are not ammonoids or coleoids".
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Thus, while there is some agreement regarding recent cephalopods, the classification of the various extinct forms is very uncertain, precisely due to this fact that (with one or two rare exceptions known from the Devonian, and a Jurassic ammonite that preserved muscle attachment scars) the soft body parts are not known.  The following list basically follows Curt Teichert "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, 1988, except that (a) Teichert's two Subclasses [[Endoceratoidea]] and [[Actinoceratoidea]] have been discarded, since the two orders [[Endocerida]] and the [[Actinocerida]] are probably not so distinct from their contemporaries as to justify such a high taxonomic ranking; and (b) I have incorporated it with the [[Palcephalopoda]]/[[Neocephalopoda]] Hypothesis.
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Thus, while there is some agreement regarding recent cephalopods, the classification of the various extinct forms is very uncertain, precisely due to this fact that (with one or two rare exceptions known from the Devonian, and a Jurassic ammonite that preserved muscle attachment scars) the soft body parts are not known.  The following list basically follows Curt Teichert "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, 1988, except that (a) Teichert's two Subclasses [[Endocerida|Endoceratoidea]] and [[Actinocerida|Actinoceratoidea]] have been discarded, since the two orders [[Endocerida]] and the [[Actinocerida]] are probably not so distinct from their contemporaries as to justify such a high taxonomic ranking; and (b) I have incorporated it with the [[Palcephalopoda]]/[[Neocephalopoda]] Hypothesis.
===Class Cephalopoda===
===Class Cephalopoda===

Revision as of 00:57, 17 August 2009

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