Cephalopoda

From Palaeos.org

Jump to: navigation, search
(added Cephalopod based References, changed Dentrogram sources to further non-cpehalopod dendrogram sources, moved exobilolgy to the bottom)
(Systematics: Removed Teichert 1988 ref in text, removed to References.)
Line 63: Line 63:
The Subclass Nautiloidea forms a large and diverse, all be it paraphyletic, group, divisable into phyllogenetically defined superorders (or infraclasses). All nautiloids are united by the fact they have simple, adorally concave septa, retrosiphonate septal necks, sutures that are never elaborate,and are descended from a common ancestor, ''Plectronoceras'' They are more than simply cephalopods that are neither ammonoids or coleoids.  
The Subclass Nautiloidea forms a large and diverse, all be it paraphyletic, group, divisable into phyllogenetically defined superorders (or infraclasses). All nautiloids are united by the fact they have simple, adorally concave septa, retrosiphonate septal necks, sutures that are never elaborate,and are descended from a common ancestor, ''Plectronoceras'' They are more than simply cephalopods that are neither ammonoids or coleoids.  
-
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 Teichert (1988) with the Palcephalopoda/Neocephalopoda hypothesis incorporated; except that Teichert's two Subclasses '''Endoceratoidea''' and '''Actinoceratoidea''' have been discarded, since the two orders [[Endocerida]] and [[Actinocerida]] are probably not so distinct from their contemporaries as to justify such a high taxonomic ranking.
===Class Cephalopoda===
===Class Cephalopoda===
Line 71: Line 71:
The palcephalopoda includes most of the Nautiloidea as originally defined but leaves out the Orthocerida which are combined with the Bactritida, Ammonoidea, and coleoids in the Neocephalopoda. Palcephalopods characteristically have cap-shaped apeces with a cicatrix ore something similar and are retrosiphonte, meaning the septal necks in the siphuncle point to the rear. The apex in neocephalopoda is bulbous and lacks a cicatrix. The only living palcephalopods are ''Nautilus'' and ''Allonautilus''. Both are tetrabranchiate (posses 4 gills), have radula with 9 teeth and 4 marginalia ("Lateradulata") and posses a large number of sticky finger-like arms that lack suckers. Reproduction is what is known as K-selected, with a few large yolk-rich eggs produced a number of times from which large well developed offspring come. Adults are long-lived.  
The palcephalopoda includes most of the Nautiloidea as originally defined but leaves out the Orthocerida which are combined with the Bactritida, Ammonoidea, and coleoids in the Neocephalopoda. Palcephalopods characteristically have cap-shaped apeces with a cicatrix ore something similar and are retrosiphonte, meaning the septal necks in the siphuncle point to the rear. The apex in neocephalopoda is bulbous and lacks a cicatrix. The only living palcephalopods are ''Nautilus'' and ''Allonautilus''. Both are tetrabranchiate (posses 4 gills), have radula with 9 teeth and 4 marginalia ("Lateradulata") and posses a large number of sticky finger-like arms that lack suckers. Reproduction is what is known as K-selected, with a few large yolk-rich eggs produced a number of times from which large well developed offspring come. Adults are long-lived.  
-
To what extent these features and characters can be extrapolated into the distant past is unknown. There is some question as to whether the palcephalopoda/neocephalopoda distinction really works with Paleozoic nautlloids other that the distinction between different embryonic forms.  
+
To what extent these features and characters can be extrapolated into the distant past is unknown. There is some question as to whether the palcephalopoda/neocephalopoda distinction really works with Paleozoic nautloids other that the distinction between different embryonic forms.  

Revision as of 00:37, 3 September 2009

Personal tools