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.