Cephalopoda

From Palaeos.org

Jump to: navigation, search
(eliminated extra references and speculative exobiology)
(Class Cephalopoda: revised palcephalopod-neocephalopod discussion)
Line 82: Line 82:
===Class Cephalopoda===
===Class Cephalopoda===
-
 
 
-
====Subclass: Palcephalopoda====
 
-
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.
+
====Palcephalopoda-Neocephalopoda ====
-
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.  
+
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.
 +
 
 +
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.  
 +
 
 +
Line 108: Line 110:
   Order [[Nautilida]] Agassiz, 1847
   Order [[Nautilida]] Agassiz, 1847
-
====Subclass: Neocephalopoda====
 
-
 
-
Evolved from Palcephalopoda.  Includes a number of lineages with reduced internal shells, and even some (e.g. octopoids and their relatives) that discarded their shell altogether.  Radula with seven teeth and two marginalia per row ("Angusteradulata").  Reproduction mostly strategy [[r-selected]] with many small [[plankton]]ic offspring, although some advanced forms (e.g. Octopoda) K-selected with yolk-rich development.  Mostly ten arms, which generally possess hooks (belemnites) or adhesive suckers (advanced Coloidea).  Siphuncle (in those types that retain their shell) thin and empty.  Additional layers on outside of shell.  Phragmocone (the shell) originally straight with the siphuncle situated at or near the center.  Later the position of the siphuncle shifted to the ventral surface, ([[Bactritida]]), the shell became coiled (Ammonoidea), internal or reduced or absent (Coleoidea). Most extinct forms presumably (like recent forms) with only two gills.
 
  '''Infraclass Orthoceratoidea''' Kuhn, 1940
  '''Infraclass Orthoceratoidea''' Kuhn, 1940

Revision as of 14:56, 22 April 2010

Personal tools