Cephalopoda

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(Class Cephalopoda: modifications)
(Palcephalopoda-Neocephalopoda: modified)
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While there is general agreement regarding classification of recent cephalopods, mostly coleoids, based on soft part anatomical features, classification of extinct fossil forms based on such criteria is impossible other than for a few inferences that night be drawn from preserved muscle attachment scars, preserved radula, tentacle imprints, and isolated body impressions. Shell based features on the other hand provide the basis for a reliable, phylogenetically plausible  and generally accepted classification of extinct forms.   
While there is general agreement regarding classification of recent cephalopods, mostly coleoids, based on soft part anatomical features, classification of extinct fossil forms based on such criteria is impossible other than for a few inferences that night be drawn from preserved muscle attachment scars, preserved radula, tentacle imprints, and isolated body impressions. Shell based features on the other hand provide the basis for a reliable, phylogenetically plausible  and generally accepted classification of extinct forms.   
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====Palcephalopoda-Neocephalopoda ====
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'''Palcephalopoda-Neocephalopoda'''
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The Palcephalopoda-Neocephalopoda distinction is based on essential differences between the living nautiloid genera, ''Nautilus'' and ''Allonautilus'', and the huge variety of living coleoids. Living nautiloides are tetrabranchiate (have 4 gills in 2 pairs), are lateradulate (13 elements per radular row), and produce multiple batches of well develovoped offspring from large yolk-rich eggs. The have an external shell, retained from their ancestors but have mutliple finger-like protractable tentacles, probably a derived character. Also their eyes have no lens but operate like a pin-hole camera, again apparently primitive,  Coleoids are dibranchiate (have 2 gills), are angusteradulate (9 elements per radular row) and in general produce a single batch of numerous planctonic offspring before dying. They lack an external shell, a obvious derived character but have 8 or 10 muscular suckered and/or hooked tentacles which seems to be a retained primitive trait. On the other hand coleoids have well developed eyes with lenses, sometimes covered by a corneal membrane.
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The <u>Palcephalopoda-Neocephalopoda</u> distinction is based on essential differences between the living nautiloid genera, ''Nautilus'' and ''Allonautilus'', and the huge variety of living coleoids. Living nautiloides are tetrabranchiate (have 4 gills in 2 pairs), are lateradulate (13 elements per radular row), and produce multiple batches of well develovoped offspring from large yolk-rich eggs. The have an external shell, retained from their ancestors but have mutliple finger-like protractable tentacles, probably a derived character. Also their eyes have no lens but operate like a pin-hole camera, again apparently primitive,  Coleoids are dibranchiate (have 2 gills), are angusteradulate (9 elements per radular row) and in general produce a single batch of numerous planctonic offspring before dying. They lack an external shell, a obvious derived character but have 8 or 10 muscular suckered and/or hooked tentacles which seems to be a retained primitive trait. On the other hand coleoids have well developed eyes with lenses, sometimes covered by a corneal membrane.
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The extent to which these distinctions can be extrapolated into fossil forms is questionable and subject to individual interpretation. Ammonoids for example are thought to be more closely related to coleoids than to nautiloids on the basis of being angueratulate (with 9 radular elements like coleoids) from the few radula found, in spite of having an external shell, and are therefor considered neocephalopods. Paleozoic orthocerids are considered neocephalopods because their protoconch is like that of ammonoids in spite of having an obviously nautiloid phragmocone.
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The extent to which these distinctions can be extrapolated into fossil forms is questionable and subject to individual interpretation. Ammonoids for example are thought to be more closely related to coleoids than to nautiloids on the basis of being angueradulate (with 9 radular elements like coleoids) from the few radula found, in spite of having an external shell, and are therefor considered neocephalopods. Paleozoic orthocerids are considered neocephalopods because their protoconch is like that of ammonoids in spite of having an obviously nautiloid phragmocone.
==== Alternative Taxonomies====
==== Alternative Taxonomies====

Revision as of 13:37, 8 December 2011

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