Cephalopoda

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(Systematics: revised links Endoceratoidea and Actinoceratoidea)
(Subclass: Palcephalopoda: revised)
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====Subclass: Palcephalopoda====
====Subclass: Palcephalopoda====
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Mostly equivalent to the old term "Nautiloidea". Reproduction strategy mostly [[K-selected]] with a few well-developed large offspring and long-lived adults. Yolk-rich development (fewer larger eggs).  Radula with 9 teeth and 4 marginalia ("Lateradulata").  Four gills.  Early forms probably had ten arms but later many more arms developed.  The arms lack adhesive suckers. The [[phragmocone]] is well developed and large, originally slightly curved with the siphuncle was situated between the center and the ventral surface.  Siphuncle generally large with internal deposits, with many specialized forms developed that can be distinguished according to the structure of the siphuncle
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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 sicky 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.
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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.
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  '''Infraclass unnamed''' ("Ellesmeroceroidea"? or several infraclasses?)
  '''Infraclass unnamed''' ("Ellesmeroceroidea"? or several infraclasses?)

Revision as of 01:07, 17 August 2009

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