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Phylum MOLLUSCA

Class CEPHALOPODA
Nautiloids, ammonites, squids cuttlefish and octopi
Subclass NAUTILOIDEA

  
This is the basal stock from which all other cephalopods arose. They are characterized by having essentially straight chamber separations [septa(e)].The nautiloids are still represented by the living genus Nautilus.

Subclass          
AMMONOIDEA
 The ammonites, are an extremely important group of Mesozoic molluscs.  Because they were wide ranging, they make excellent index fossils.  They are primarily classified on the basis of the shape of sutures of the septae (partitions) with the inside of the shell wall.

Cross-section of an ammonite showing septae.
 
 
 

These sutures are of three types:
a. goniatitic-sutures are not serrate

b. ceratitic-lobes are serrate

c. ammonitic-entire suture serrate.  Note that if the outer shell is present, the type of suture cannot be determined because the outer shell covers it.
    

Subclass COLEOIDEA
 These are the common modern cephalopods, mostly squids, cuttlefish, and octopi.  Being predators, they are adapted for speed.  The loss of external shell presumably allowed an increase in speed and was, therefore, advantageous.  Associated with this increase in speed is a tremendous development of the senses and of the brain. Of the two orders in this subclass, (the Octapoda and Decapoda) only the Decapoda are important in the fossil record.
Order DECAPODA
 This order is characterized by ten arms and an internal shell.  The best modern representative of this order is Sepia, the cuttlefish, whose shell is often found in parrots' cages.  In the fossil record, one group of decapods, the belemnoids have left as a common fossil, numerous cigar shaped internal skeleta.
Cross-section through a belemnite shell Hibolites hastatus,
Solnhofen Limestone, Germany
 
 
 

 

Order OCTOPODA
 This order is characterized by generally eight tentacles and a reduced to nonexistent internal shell. With no shell, they are very poorly represented in the fossil record.

 
 
 
 
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