Cephalopoda

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(Phylogeny: removed incertae sedis part for separate page)
(Systematics: refined classificaions at top)
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==Systematics==
==Systematics==
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The class Cephalopoda has been traditionally divided into three subclasses on the basis  of shell structure, or two subclasses on the basis of gills and other soft parts. As shown here:
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The class Cephalopoda has been traditionally divided into three subclasses on the basis  of shell structure, or two subclasses on the basis of gills and other soft parts. Both shown here:
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'''Tetrabranchia''': four gills, primitive forms, external shell - Nautiloidea
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Shell based classification, used exclusively in paleontology, with three basic subclasses.
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'''Ammonoidea'''
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'''Nautiloidea''' -- includes primitive forms, all have an external shell and retrosiphonate siphuncle. Modern representatives, Nautilus and  Allonautilus are tetrabranchiate.
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'''Dibranchiata''': two gills, advanced forms, reduced or absent shell - Coleoidea
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'''Ammonoidea''' -- evolved forms with an external shell and prosiphonate siphuncle derived from the Nautilioidea. Biologically closely related to the dibranchiates. Became extinct at the end of the Cretaceous.
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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".''Never the less, all nautiloids are united by the fact they have simple, adorally concave septa, retrosiphonate septal necks, sutures that are never elaborate,  distinguishing them from subsequent groups, and are descended from a common ancestor, Plectronoceras''  
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'''Coleodia''' -- shell-less forms and those with an internal vestigial shell represented by the dibranchiate squid, octopods, cuttlefish and spirula that live today. Includes the extinct belemnites.
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Older, anatomical (gill) based classification.
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'''Tetrabranchiata'''-- forms with four gills (2 pairs), represented by Nautilus, thought to be primitive.
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May be equivalent to the Nautiloidea.
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'''Dibranchiata'''-- forms with two gills, considered advanced. Equivalent to the Coleoidea, possibly the Ammonoidea and mayby even some Nautilodea
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The trouble with the anatomical 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".''Never the less, all nautiloids are united by the fact they have simple, adorally concave septa, retrosiphonate septal necks, sutures that are never elaborate,  distinguishing them from subsequent groups, and are descended from a common ancestor, Plectronoceras''  
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.
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.

Revision as of 13:17, 2 September 2009

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